Journal of Physics: Conference Series, 2702, 2024

DOI: https://iopscience.iop.org/article/10.1088/1742-6596/2702/1/012003/meta

Upward lightning requires a distinct approach compared to downward (cloud-toground) lightning. Some lightning strikes are triggered by objects on the ground itself. The occurrence of such strikes depends on various factors, including the object's geometric dimensions, structure, relative location within its environment, as well as the distribution and location of electrical charges within the thundercloud. This phenomenon takes place more and more often due to the spread of wind farms and higher buildings. In this article, simulation and calculation is carried out regarding the Gaisberg Tower in Austria which is actively used as a measurement and research site for lightning purposes. A finite element simulation is carried out to assess the electric field characteristics in the geometry. The close electric field measurement instruments are located 170 m away from the tower on an enclosure which must be considered during data analysis. The result of the created model is validated by former measurement data which confirms the arrangement of the model and creates the opportunity to directly transform the values of the electric field from the field mill to the tower during appropriate conditions.

Electric Power Systems Research 231, 110307, 2024

DOI: https://doi.org/10.1016/j.epsr.2024.110307

Flashes with positive polarity, representing about 10 % of all cloud-to-ground flashes, were analyzed for this publication. Data from positive cloud-to-ground flashes recorded in the Austrian Alps over five years from 2015 to 2022 were used. The measurements were performed to record ground truth flash data at 23 selected locations and to compare their characteristics with values from the literature. The used measurement system consists of a high-speed Video camera and an electric Field Recording System (VFRS). Data of both systems are correlated with Lightning Location System (LLS) data by using a GPS time stamp. The overall dataset showed 107 positive flashes, comprising 121 strokes (11 multi-stroke flashes). The percentage of positive single-stroke flashes is 90 % for VFRS data and 84 % for LLS data. The mean multiplicity for VFRS data is 1.1. The LLS estimated median return stroke peak current of all analyzed positive strokes is 47.4 kA. The mean duration for 67 continuing currents of the 121 strokes is 126.1 ms. The detection efficiency of correctly detected positive flashes and strokes is 96.3 % and 89.3 %, respectively. A better understanding of positive flashes in general and in the Alpine region in particular shall be provided by the presented results.

Electric Power Systems Research 229, 110199, 2024

DOI: https://doi.org/10.1016/j.epsr.2024.110199

Tall structures cause upward lightning (UL). It endangers and potentially damages tall wind turbines, which are a mainstay in the drive to reduce greenhouse gas emissions. Ground-truth lightning current measurements at the instrumented Gaisberg Tower (Austria) show that UL has no distinct annual cycle and that a conventional lightning location system (LLS) misses about half of UL events. The paper explores the possibility to diagnose UL only from larger-scale meteorological conditions of the thunderstorm environment in order to augment LLS data for damage risk assessment. It finds that wind-field thunderstorm environments account for nearly all UL in transition and cold seasons and still half in summer thus explaining the uniform annual UL distribution. A random forest model successfully diagnoses UL at the Gaisberg Tower using only meteorological variables. It performs best in winter and transitional seasons. Case studies show that the Gaisberg-trained model also succeeds in diagnosing UL at wind turbines in other regions, thus pathing the way for a better risk assessment of UL at wind turbines.

Hazards and Earth System Sciences Natural 24, 2511–2522, 2024

DOI: https://doi.org/10.5194/nhess-24-2511-2024

Evaluating the risk of lightning strikes to a particular structure typically involves adhering to the guidance outlined in IEC 62305-2. Among the multitude of factors influencing the overall risk, flash density emerges as a crucial parameter. According to its definition, each flash is assigned only one contact point to ground. Nevertheless, it is well known that, on average, flashes exhibit multiple ground termination points as shown by high-speed camera observations. In this research, lightning data collected by the European Cooperation for Lightning Detection (EUCLID) network are utilized in combination with a ground strike point (GSP) algorithm that aggregates individual strokes within a flash into ground strike points. This approach enables the examination of spatio-temporal patterns of GSPs across Europe throughout a decade, spanning from 2013 to 2022. Average GSP densities exhibit variations across the European continent, mirroring the observed patterns in flash densities. The highest densities are concentrated along the Adriatic Sea and the western Balkan region, reaching peak values of up to 8.5 GSPs km-2 yr-1. The spatial distribution of the mean number of ground strike points per flash reveals a noticeable increase in the Mediterranean, Adriatic, and Baltic Sea regions compared to inland areas. Moreover, it has been determined that the average number of GSPs per flash reaches its peak between September and November. Additionally, a daily pattern is discernible, with the lowest number of GSPs per flash occurring between 12:00 and 18:00 UTC (universal time coordinated). It is found that 95% of the separation distances between distinct GSPs are less than 6.7 km. Lastly, it is worth noting that the presence of the Alps has an impact on GSP behaviour, resulting in lower GSP counts in comparison to the surrounding areas, along with the shortest average distances between different GSPs.

Electric Power Systems Research, Vol. 217, 2023

DOI: https://doi.org/10.1029/2023JD039505

This paper presents an analysis of flashes and their Ground Strike Points (GSPs) by using Video and Field Recording System (VFRS) data and Lightning Location System (LLS) data from Austria. Analyses of the multiplicity of flashes and strokes per GSP have been performed. A comparison between strokes per flashes and strokes per GSPs has been conducted. By using the calculated strike point locations from the LLS, distances between different GSPs have been calculated. The used VFRS and LLS data set includes records from 2015, 2017, 2018 and 2021 recorded in the Austrian Alpine region. Atmospheric discharges have been recorded at 22 different measurement locations in Austria. For the present work a data set of 519 flashes including 1683 strokes were analyzed. Results for the mean multiplicity (3.3) revealed a lower value compared to multiplicities from other countries, but similar to the ones determined by former studies from Austria. Calculations of distances between the first GSP of a flash to the other GSPs of the same flash showed a median of 1.4 km, an arithmetic mean of 1.6 km and a geometric mean of 1.2 km. The maximum determined distance between two GSPs was 6.9 km for the analyzed data set.

Journal of Geophysical Research: Atmospheres, Vol. 128, 2023

DOI: doi.org/10.1029/2023JD039505

Upward lightning is rarer than downward lightning and requires tall (100+ m) structures to initiate. It may be either self-initiated or triggered by other lightning discharges. While conventional lightning location systems (LLSs) detect most of the upward lightning flashes superimposed by pulses or return strokes, they miss a specific flash type that consists only of a continuous current. Globally, only few specially instrumented towers can record this flash type. The proliferation of wind turbines in combination with damages from upward lightning necessitates an improved understanding under which conditions self-initiated upward lightning and the continuous-current-only subtype occur. This study uses a random forest machine learning model to find the larger-scale meteorological conditions favoring the occurrence of the different phenomena. It combines ground truth lightning current measurements at the specially instrumented tower at Gaisberg mountain in Austria with variables from larger-scale meteorological reanalysis data (ERA5). These variables reliably explain whether upward lightning is self-initiated or triggered by other lightning discharges. The most important variable is the height of the −10°C isotherm above the tall structure: the closer it is, the higher is the probability of self-initiated upward lightning. For the different flash types, this study finds a relationship to the larger-scale electrification conditions and the LLS-detected lightning situation in the vicinity. Lower amounts of supercooled liquid water, solid, and liquid differently sized particles and no LLS-detected lightning events nearby favor the continuous-current-only subtype compared to the other subtypes, which preferentially occur with LLS-detected lightning events within 3 km from the Gaisberg Tower.

12th Asia-Pacific International Conference on Lightning (APL), 2023

DOI: 10.1109/APL57308.2023.10182055

In order to evaluate the lightning risk to a particular structure, it is common practice to follow the guidelines set out in IEC 62305-2, i.e., the reference standard for lightning risk calculation. Amongst the various components that influence the total risk, the flash density is a key parameter. However, flashes have on average more than one ground termination point. This study seeks to ascertain whether existing ground strike point (GSP) algorithms estimate correctly the actual observed number of GSPs per flash based on observations made by high-speed cameras. In addition, lightning data as observed by the European Cooperation for Lightning Detection (EUCLID) network are used in combination with a particular GSP algorithm to retrieve the temporal behavior of GSPs in two topographically different regions in Europe, i.e., Austria and Belgium, over a ten-year period from 2012 to 2021. We find that although most GSP algorithms over-or underestimate to some extent the number of GSPs per flash, this number is fairly close to the observed value as derived from the ground-truth observations. Furthermore, it is found that the average number of GSPs per flash is highest during the summer months. Finally, a diurnal trend is visible where the number of GSPs per flash is lowest between 12 and 18 UTC (Universal Time Coordinated).

International Colloquium on lightning and Power Systems, International Symposium on Lightning Protection (CIGRE ICLPS - SIPDA), 2023

In this study, the open-source finite-difference time-domain (FDTD) solvers gprMax, Elecode and MEEP are investigated for their suitability to compute lightning electromagnetic field propagation. Several simulations are performed to reproduce the results of typical field propagation scenarios that can be found in the literature. The results of the presented solvers are validated through comparison with reference field results corresponding to propagation over perfectly conducting and lossy ground. In most of the tested scenarios, all solvers reproduce the reference fields with satisfactory accuracy. However, close attention must be paid to the proper choice of the spatial discretization to avoid artificial numerical dispersion, and the application of the simulation cell boundaries, which can cause significant impairment of the results due to undesired reflections. Some cases of inaccurate FDTD results due to improper choices of parameters are demonstrated. Further, the features, the performance and limitations, and the advantages and drawbacks of the presented solvers are highlighted. For familiarization with the solvers’ programmatical interfaces to initialize and run the simulations, the developed scripts are made available to the community in an openly accessible repository.

International Colloquium on lightning and Power Systems, International Symposium on Lightning Protection (CIGRE ICLPS - SIPDA), 2023

Thunderstorms and associated cloud-to-ground flashes have a significant effect on our infrastructure. Reliable assessments of these natural events is challenged by the unpredictable development of thunderstorms as well as changes in the climatic conditions. To assess the risk of thunderstorms for a specific area, weather radar data and lightning location data is used, giving an imprint about the current situation of the thunderstorm but is non-predictive regarding lightning strikes. In order to get a better insight into the distribution of electric charge in the cloud and, in the best case, to derive additional information for the prediction of lightning, an electric Field Meter (FM) network was installed in the surrounding of Graz Airport in 2022. Electric field meters provide information about the current electric charge state in a thunderstorm, even before the first stroke occurs. A first analysis of the recorded FM data was performed and is shown for one specific thunderstorm day. The correlation with lightning location system data and weather radar data provided additional information regarding the occurrence of lightning strikes. Thus, the potential for a possible short-term prediction of the first and last lightning strike can be stated by the prevailing conditions of the observed thunderstorm.

International Colloquium on lightning and Power Systems, International Symposium on Lightning Protection (CIGRE ICLPS - SIPDA), 2023

Bipolar flashes account for a small percentage of total Cloud-to-Ground (CG) flashes. For that reason, studies are rare compared to positive and negative CG flashes. Bipolar flashes analyzed in this paper are characterized by different polarities of the strokes grouped to a flash. For the present publication, data of two bipolar CG flashes are analyzed. The flashes were recorded in Austria in July 2015 and August 2018. To record the data, a high-speed Video camera and an electrical Field Recording System (VFRS) were used. To complete the data set the recorded VFRS data are also correlated with Lightning Location System (LLS) data. This data set allows an analysis and parametrization of each step of the bipolar flashes. In addition, a comparison with parameters from the literature is performed, to describe the basics of bipolar lightning and compare the results with national and international analyses. One of the two analyzed cases shows special characteristics concerning the polarity reversal from negative to positive return stroke peak current for two strokes following the same channel. The second flash shows a typical formation process of a bipolar flash concerning the attachment of CG stroke channels to two different areas in the cloud. The analyzed flashes were categorized as bipolar of Type 3 flashes according to Rakov [1] (polarity reversal between return strokes).

Journal of Geophysical Research: Atmospheres, Vol. 129, 2023

DOI: https://doi.org/10.1029/2023JD039505

Upward lightning (UL) has become a major threat to the growing number of wind turbines producing renewable electricity. It can be much more destructive than downward lightning due to the large charge transfer involved in the discharge process. Ground-truth lightning current measurements indicate that less than 50% of UL could be detected by lightning location systems (LLS). UL is expected to be the dominant lightning type during the cold season. However, current standards for assessing the risk of lightning at wind turbines mainly consider summer lightning, which is derived from LLS. This study assesses the risk of LLS-detectable and LLS-undetectable UL at wind turbines using direct UL measurements at instrumented towers. These are linked to meteorological data using random forests. The meteorological drivers for the absence/occurrence of UL are found from these models. In a second step, the results of the tower-trained models are extended to a larger study area (central and northern Germany). The tower-trained models for LLS-detectable lightning are independently verified at wind turbine sites in this area and found to reliably diagnose this type of UL. Risk maps based on cold season case study events show that high probabilities in the study area coincide with actual UL flashes. This lends credibility to the application of the model to all UL types, increasing both risk and affected areas.

Journal of Geophysical Research: Atmospheres, Vol. 127, Issue 6, 2022

DOI: https://doi.org/10.1029/2021JD036197

The amount of charge transferred to ground during long continuing currents in natural downward flashes can be obtained either through direct current measurements or using remote electromagnetic fields, if direct measurements are not feasible. In this study, measurements of a single-station E-field antenna were used to estimate charge transfer during continuing currents. With time-synchronized high-speed video recordings serving as ground-truth data for continuing current occurrence, we estimate transferred charge during long continuing currents in 140 natural downward flashes from electric field changes, assuming a simple monopole charge model. We present average parameters for the duration, transferred charge, amplitude, and the average amplitude on five segments along the channel. Further, we perform a simulation to investigate the estimation performance of the monopole charge model, when the cloud charge is spatially extended. Using an extended charge distribution model, we generate single-station remote field waveforms and infer the charge with the monopole model. Comparison of the known and inferred charge yields estimation errors depending on the leader orientation, its spatial extension, and the observation distance. A Monte Carlo approach is carried out to statistically evaluate parameter deviations. This novel estimation error analysis sheds light on the limitations of the applied simple techniques for charge transfer estimation of long continuing currents using single-station electric field measurements.

Atmosphere 13, 552, 2022

DOI: https://doi.org/10.3390/atmos13040552

Lightning causes significant damage and casualties globally by directly striking humans and livestock, by igniting forest fires, and by inducing electrical surges in electronic infrastructure, airplanes, rockets, etc. Monitoring the evolution of thunderstorms by tracking lightning events using lightning locating systems can help prepare for and mitigate these disasters. In this work, we propose to use Benford’s law to assess the quality of the data provided by lightning locating systems. The Jensen–Shannon and Wasserstein distances between the recorded data distribution and Benford’s distribution are used as metrics for measuring the performance of the lightning locating systems. The data are provided by the European lightning detection network (EUCLID) for the years from 2000 to 2020. The two decades consist of three time windows between which the lightning locating system underwent several upgrades to improve the detection of both positive and negative strokes. The analysis shows that the agreement with Benford’s law is consistent with the expected behavior caused by the applied upgrades to the system throughout the years. The study suggests that the proposed approach can be used to test the success of software and hardware upgrades and to monitor the performance of lightning locating systems.

Elektrotechnik & Informationstechnik (e&i), Vol. 139, Pages 335–343, 2022

DOI: https://doi.org/10.1007/s00502-022-01028-x

In this paper, the results of electromagnetic field computations using the finite-difference time-domain (FDTD) method are presented. Specifically, the propagation of an electromagnetic field caused by a lightning event on the Gaisberg mountain (Salzburg, Austria) was simulated. To initialize the FDTD simulation domain, digital elevation model (DEM) data of the real propagation paths from the Gaisberg to seven different sensors of the lightning detection network ALDIS were used. To model the lightning channel, the MTLE (modified transmission line model with exponential decay) was applied, and the lightning current measured at the top of the tower, that is, at the base of the lightning channel, was used as the current source in the simulation. This way, the electromagnetic fields were simulated for the different propagation paths, and the resulting peak values of the magnetic (H) fields at the ALDIS sensor locations were compared with the values actually measured by the ALDIS sensors. The results of the 3D FDTD computations correlate well with the values actually recorded by the sensors for a given event, while the cylindrical symmetrical 2D FDTD simulation is not capable of reproducing the values measured. This can be explained by the fact that the complex terrain of the Alpine region with its reflections and diffraction phenomena in valleys and on mountain ridges cannot be represented accurately in 2D FDTD simulations. Furthermore, by using a set of sensor values from 54 (normalized) events, we were able to verify that a strike on top of a mountain can lead to higher values compared to a strike on flat terrain. This is due to the specific topography from the strike point in the given direction of propagation. In general, rough terrain like the Austrian Alps has an attenuating effect on the propagating electromagnetic fields.

Elektrotechnik & Informationstechnik (e&i), Vol. 139, Pages 372–378, 2022

DOI: https//doi.org/10.1007/s00502-022-01021-4

In this paper, we present an optical lightning detection system, which was developed and set up in two steps at the University of the Federal Armed Forces Munich. The system is called OLDS (Optical Lightning Detection System), and it is based on a single video camera, which is installed in an axis to a convex mirror (positioned above). This configuration enables a 360-degree view. For lightning detection, the system makes use of the camera image and the duration of the thunder in order to determine the direction and the distance respectively. During the first step, the system was combined with a system for electric field measurements synchronized in time. By combining the two systems, the duration of the flash, the number of strokes, the time interval between two consecutive strokes, as well as the magnitude of the electric field were able to be determined. During the second step, we compared the data of the OLDS with the data of the European lightning detection network EUCLID. The average distance between the strike points and the OLDS was 5.6 km. The comparison showed that EUCLID detects 95% of all flashes and 94% of all strokes. EUCLID correctly classified 84% of the flashes as ground flashes, the remaining 16% were falsely classified as cloud-to-cloud flashes. The differences between the strike points calculated by OLDS and EUCLID respectively were relatively small, with 238 m (arithmetic mean) and 171 m (geometric mean).

Elektrotechnik & Informationstechnik (e&i), Vol. 139, Pages 344–351, 2022

DOI: https//doi.org/10.1007/s00502-022-01027-y

The present paper shows measurement data of cloud-to-ground flashes in the Austrian Alpine region with a focus on single-stroke flashes. The data were recorded over six thunderstorm seasons from 2009 to 2018 within the framework of a research project. The aim of the project was to generate a ground truth dataset of ground strikes of flashes. The used Video and Field Recording System (VFRS) consists of a high-speed camera and an electric field measurement system to be able to optically analyze the spatial and temporal sequences of lightning discharges and record the transient electric field. The VFRS measurements were performed at 33 different measurement locations in the Austrian Alpine region, and a total of 735 cloud-to-ground flashes with negative polarity were recorded on 61 days. For the evaluation, data from the Austrian Lightning Detection and Information System (ALDIS) are correlated with the VFRS data. The percentage of single-stroke flashes influences the multiplicity (strokes per flash), which is relevant, e.g., for the design of lightning protection measures of high voltage systems. The calculated percentages of single-stroke flashes for the present dataset (27%) are compared with previously published values from different countries (12% to 24%). In order to investigate the influence of the thunderstorm organization on the percentage of single-stroke flashes, weather radar data and wind measurements are additionally used and analyzed for the classification of the different thunderstorm types.

Monthly Weather Review, Pages 2213–2230, 2022

DOI: https://doi.org/10.1175/MWR-D-21-0149.1

A new lightning–flash and convective initiation climatology is developed over the Alpine area, one of the hotspots for lightning activity in Europe. The climatology uses cloud–to–ground (CG) data from the European Cooperation for LIghtning Detection (EUCLID) network, occurring from 2005 to 2019. The CG lightning data are gridded at a resolution of approximately 2 km and 10 min. A new and simple method of identifying convective initiation (CI) events applies a spatiotemporal mask to the CG data to determine CI timing and location.

Although the method depends on a few empirical thresholds, sensitivity tests show the results to be robust. The maximum activity for both CG flashes and CI events is observed from mid–May to mid–September, with a peak at the end of July; the peak in the diurnal cycle occurs in the afternoon. CI is mainly concentrated over and around the Alps, particularly in northern and northeastern Italy. Since many thunderstorms follow the prevailing mid–latitude westerly flow, a peak of CG flashes extends from the mountains into the plains and coastal areas of northeastern Italy and Slovenia. CG flashes and CI events over the sea/coast occur less frequently than in plains and mountains, have a weaker diurnal cycle, and have a seasonal maximum in autumn instead of summer.

Electric Power Systems Research, Vol. 213, 2022

DOI: https://doi.org/10.1016/j.epsr.2022.108626

We examined current waveforms for 58 upward flashes occurring in 2006–2014 initiated from the Gaisberg Tower located near Salzburg, Austria. The initial stage (IS) of these flashes comprised of relatively slowly varying “background” current, with faster, more impulsive current variations overlaid on this background current. In 46 of the 58 flashes (79%) the background continuing current was negative, and in the other 12 flashes (21%) it was bipolar. 1180 current pulses occurred during the IS of these 58 flashes, of which 708 (60%) were positive bipolar (positive initial polarity with a negative opposite polarity overshoot), 28 (2.4%) were positive unipolar (positive initial polarity with no opposite polarity overshoot), 440 (37%) were negative unipolar, and four (0.3%) were negative bipolar pulses. We found that bipolar current pulses only occurred in the IS at early times. We divided the IS current into two phases: (1) upward leader initiation and propagation phase (IPP) and (2) upward leader mature phase (MP). 901 or 76% (712 bipolar and 189 unipolar) pulses occurred during the IPP, and 279 or 21% (unipolar) pulses occurred during the MP. The median background-to-peak current was 134 A for IPP pulses and 687 A for MP pulses.

Electric Power Systems Research, Vol. 213, 2022

DOI: https://doi.org/10.1016/j.epsr.2022.108627

We examined the characteristics of electric field signatures occurring during the initial stage of 58 flashes measured simultaneously at near (170 m) and far (79 or 109 km) distances from the Gaisberg Tower located near Salzburg, Austria. Of the 340 field signatures measured at the near station, 68 (20%) were associated with current pulses occurring during the initiation and propagation phase (IPP) of the upward leader, and 272 (80%) were associated with pulses that occurred during the mature phase (MP) of the upward leader. Of the 68 field signatures of IPP pulses, 40 were associated with bipolar (IPP-B type) current pulses and 28 were associated with unipolar (IPP-U type) current pulses. Field signatures of IPP-B pulses were only detected at the near measurement station and appear to be associated with currents in relatively short (meter-scale) channel segments formed during the upward leader inception. At the far stations, field signatures of 84 IS pulses were recorded and analyzed. There was modest correlation between the background-to-peak current of IS pulses and near and far electric radiation field changes as well as between radiation field changes recorded at near and far distances.

36^th International Conference on Lightning Protection (ICLP), Cape Town, South Africa, 2022

DOI: https//10.1109/ICLP56858.2022.9942489

Upward lightning is rare, but destructive and not conned to the winter season as frequently presented in literature. This study nds the dominant thunderstorm types for upward lightning and the underlying meteorological settings for its initiation in the cold, warm and transition seasons. Further it assesses the ability to diagnose the upward lightning occurrence at the Gaisberg Tower (Austria) from meteorological conditions using random forest models. Results show that high shear and high wind speed thunderstorms with enhanced particle loadings dominate upward lightning initiation in the cold and in the transition seasons. In the warm season this dominance is reduced due to an increase in high CAPE thunderstorms associated with increased low-level moisture and higher-based cloud charge centers. The ability to diagnose upward lightning is highest in winter and when it occurs associated with the dominant wind-eld thunderstorm type in combination with enhanced cloud physics.

36^th International Conference on Lightning Protection (ICLP), Cape Town, South Africa, 2022

In this paper, lightning electromagnetic (EM) fields radiated by a lightning strike to the Gaisberg Tower in Salzburg, Austria, and propagating over irregular terrain are calculated using a 3D finite-difference time-domain (FDTD) method. With topographic height maps, 7 different propagation paths for a selected lightning strike to the Gaisberg tower are simulated and the resulting magnetic field (H-field) is compared with the received signal strength at seven sensors of the ALDIS (Austrian Lightning Detection and Information System) sensor network for that event.  The return stroke is represented by a transmission line (TL) model and the current waveform recorded at the top of the Gaisberg tower was used as an input to it. The results of the 3D-FDTD simulation correlate well with recorded H-fields at sensor sites in mountainous environments. The results of recent studies, which show that a lightning strike to a mountain gives rise to a field enhancement could be verified by comparing the FDTD results to a set of existing sensor measurements. Further, 3D-FDTD simulations of the Gaisberg region have been performed to investigate the lightning EM field radiation pattern of the mountain for first and subsequent return strokes (RS). The results show that the amplitude enhancement of subsequent RSs exhibits a significant direction- dependence, which is caused by the surface structure of the mountain. For first RS fields, a smaller enhancement and more homogeneous radiation pattern is observed.

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36^th International Conference on Lightning Protection (ICLP), Cape Town, South Africa, 2022

Information about lightning properties are important in order to advance the current understanding of lightning and consequently to improve lightning protection as well. Especially ground strike point (GSP) properties are helpful to improve the risk estimation for lightning protection. In this study, lightning properties of negative downward lightning flashes are analyzed. The high-speed video recordings are taken in different regions, including Austria, Brazil, South Africa and U.S.A., and are analyzed in terms of flash multiplicity, duration, and ground strike point properties. Although the results vary among the data sets, the analysis reveals that a third of the flashes are single-stroke events, while the overall mean number of strokes per flash equals 3.67. From the video imagery an average of 1.56 GSPs per flash is derived, with about 60% of the multiple stroke flashes striking ground in more than one place. It follows that a ground contact point is struck 2.35 times on average. Multiple-stroke flashes last on average 371 ms. Additionally, the observations are linked to the observations made by local a LLS. It follows that median values of the separation distance between the first stroke in the flash and subsequent ground strike points is found to vary between 1.3 km and 2.75 km.

Finally, it is observed that the median peak current of strokes to the first GSP in a flash is the highest compared to the median peak currents of strokes in subsequent GSPs. A similar trend is found for the peak current as a function of stroke order within a particular GSP.

36^th International Conference on Lightning Protection (ICLP), Cape Town, South Africa, 2022

Flashes with positive polarity, representing about 10 % of all cloud-to-ground flashes, were analyzed for this publication. Data of positive cloud-to-ground flashes recorded in the Austrian Alps over four years from 2015 to 2021 have been used. Measurements and analyses were carried out by Graz University of Technology in collaboration with the operator of the Austrian lightning location system ALDIS. The aim of the measurements was to collect and evaluate ground-truth data on lightning discharges in Austria. For this purpose, measurements were carried out at 22 selected locations distributed across the country. The used measurement system consists of a high-speed Video camera and an electric Field Recording System (VFRS). The camera records high-speed videos at 2000 frames per second and the electric field measurement system is used to record the transient electric field. Both systems are synchronized to GPS time and can therefore be correlated with Lightning Location System (LLS) data. Since 2021, the data have been recorded at Campus Inffeld of Graz University of Technology. The overall data set for measurements from 2015 to 2021 showed 90 positive flashes comprised of 104 strokes. The percentage of positive single stroke flashes for the VFRS data and the LLS data is 88 % and 81 %, respectively, for the merged dataset of all four years. For the analyzed VFRS data, the mean multiplicity for positive flashes is 1.2. The median return stroke peak current of all positive strokes in Austria is 43.6 kA (peak currents estimated by the LLS) for the merged dataset. The DE of correctly detected positive flashes and strokes is 97.8 % and 87.5 %, respectively. Results of this paper shall contribute to a better understanding of positive lightning flashes in general and in the Alpine region in particular.

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Engineering Geology, Vol. 311, 2022

DOI: https://doi.org/10.1016/j.enggeo.2022.106912

Ultralow frequency (ULF) to low frequency (LF) electromagnetic radiation represents one of the most promising effects of brittle rock strain and microcracking that might be potentially helpful for short term earthquake forecasting. In this study the results of a six month monitoring campaign are presented from Obir Cave in the eastern Alps. Direct experimental observations of electromagnetic radiation have been made using a customised broadband data logger installed next to the Obir Fault - this seismogenic fault near the Periadriatic Lineament is known to be related to at least three large prehistoric earthquakes. On the basis of these measurements it has been possible to characterise a number of distinct signals: artificial constant narrowband signals at discrete frequencies; short serial broadband impulses; high energy broadband impulses; and low energy broadband impulses. The narrowband artificial signals were removed from the electromagnetic radiation time series analysis so that the natural signals were enhanced and could be compared more easily to meteorological parameters and rock strain indicators. Critically, the high energy broadband impulses show a strong correlation with lightning activity across much of central Europe and the eastern Mediterranean while the low energy broadband impulses appear to be associated with local rock strain in and around Obir Cave. Unfortunately, it seems certain that some of the essential strain related impulses are likely to have been overprinted by the larger lightning related impulses.

Seismological Research Letters, Vol. 93 (6), Pages 3404–3421, 2022

DOI: https://doi.org/10.1785/0220220064

We obtain a large dataset of seismic data from the temporary seismic network AlpArray in Europe and a large dataset of lightning data from the lightning location system Austrian Lightning Detection and Information System and focus on the investigation of thunder signals recorded with seismic stations in a frequency range of 10–49 Hz if no other frequency band is specified. We try to establish whether important information about a lightning flash can be determined independent of optical and electrical measurements through the means of seismic analysis. Seismic data provide useful information on thunder and lightning, and we observe a correlation between lightning peak current and maximum ground displacement induced by the thunder for positive cloudto-ground flashes of lightning.

Electric Power Systems Research, Vol. 194, 2021

DOI: https://doi.org/10.1016/j.epsr.2021.107109

This study shows an analysis of data from measurements of natural cloud-to-ground lightning performed in Austria between 2009 and 2018. The measurement system consists of a high-speed Video and electric Field Recording System (VFRS). Over the whole period, 735 negative cloud-to-ground flashes were recorded at 33 different locations on 61 individual days. The measurement locations are scattered across the Austrian territory in Alpine and pre-Alpine terrain. Data from the Austrian Lightning Location System (LLS), ALDIS, are correlated with the collected VFRS ground truth data to complete the dataset. These datasets are used to analyze possible reasons for the detected variation of single-stroke flashes in Austria. The percentage of single-stroke flashes among all negative flashes is higher in this study (27 %) than in previous studies from different countries (12 to 24 %). A classification of thunderstorms does not show any dependency of the single-stroke flash occurrence on different thunderstorm types (based on radar data) or underlying meteorological characteristics (based on vertical wind shear computed from weather stations and radiosondes). In contrast, a possible dependency of the occurrence of single-stroke flashes on underlying terrain (Alpine versus pre-Alpine) is noted.

Journal of Geophysical Research: Atmospheres, Vol. 126, Issue 1, 2021

DOI: https://doi.org/10.1029/2020JD032825

We show for the first time that elves can be produced by an unusual small-scale continental spring-time thunderstorm. The storm occurred in Central Europe, covered a very small area of ∼50 × ∼30 km and lasted only for ∼4 h on April 2, 2017. The fraction of intense positive cloud-toground lightning strokes was unusually high, reaching 55%, with a mean peak current of 64 kA. The peak currents of return strokes (RS) associated with elves exceeded ∼300 kA. Elves and their causative RS have been observed with different optical and electromagnetic recordings. Signatures of ionospheric disturbances indicating the presence of elves were found in measurements of displacement currents, ionospheric reflections of sferics and man-made narrow-band transmissions. All these electromagnetic observations coincide with four optical detections of elves and strongly suggest the occurrence of two more elves later in the decaying phase of the storm. Surprisingly, the same electromagnetic measurements indicate that other strong strokes did not produce any elves. Our simulation results show that the formation of an elve is not only determined by the high-peak current of their causative strokes but that it is also controlled by the conductivity of the lightning channels and velocity of the current wavefront. We hypothesize that because of a lower conductivity of RS lightning channels and/or slower current waves only very strong strokes with peak currents above ∼300 kA might have been capable to produce observable elves during this thunderstorm.

Natural Hazards in Earth System Sciences, Vol. 21, Issue 6, 2021

DOI: https://doi.org/10.5194/nhess-21-1909-2021

Information about lightning properties is important in order to advance the current understanding of lightning, whereby the characteristics of ground strike points (GSPs) are in particular helpful to improving the risk estimation for lightning protection. Lightning properties of a total of 1174 negative downward lightning flashes are analyzed. The high-speed video recordings are taken in different regions, including Austria, Brazil, South Africa and the USA, and are analyzed in terms of flash multiplicity, duration, interstroke intervals and ground strike point properties. According to our knowledge this is the first simultaneous analysis of GSP properties in different regions of the world applying a common methodology. Although the results vary among the data sets, the analysis reveals that a third of the flashes are single-stroke events, while the overall mean number of strokes per flash equals 3.67. From the video imagery an average of 1.56 GSPs per flash is derived, with about 60% of the multiple-stroke flashes striking the ground in more than one place. It follows that a ground contact point is struck 2.35 times on average. Multiple-stroke flashes last on average 371 ms, whereas the geometric mean (GM) interstroke interval value preceding strokes producing a new GSP is about 18% greater than the GM value preceding subsequent strokes following a pre-existing lightning channel. In addition, a positive correlation between the duration and multiplicity of the flash is presented. The characteristics of the subset of flashes exhibiting multiple GSPs is further examined. It follows that strokes with a stroke order of 2 create a new GSP in 60% of the cases, while this percentage quickly drops for higher-order strokes. Further, the possibility of forming a new lightning channel to ground in terms of the number of strokes that conditioned the previous lightning channel shows that approximately 88% developed after the occurrence of only one stroke. Investigating the time intervals in the other 12% of the cases when two or more strokes re-used the previous lightning channel showed that the average interstroke time interval preceding a new lightning channel is found to be more than twice the time difference between strokes that follow the previous lightning channel.

Natural Hazards and Earth System Sciences, Vol. 21, Issue 6, 2021

DOI: https://nhess.copernicus.org/articles/21/1921/2021/

At present the lightning flash density is a key input parameter for assessing the risk of occurrence of a lightning strike in a particular region of interest. Since it is known that flashes tend to have more than one ground termination point on average, the use of ground strike point densities as opposed to flash densities is more appropriate. Lightning location systems (LLSs) do not directly provide ground strike point densities. However, ingesting their observations into an algorithm that groups strokes into respective ground strike points results in the sought-after density value. The aim of this study is to assess the ability of three distinct ground strike point algorithms to correctly determine the observed ground-truth strike points. The output of the algorithms is tested against a large set of ground-truth observations taken from different regions around the world, including Austria, Brazil, France, Spain, South Africa and the United States of America. These observations are linked to the observations made by a local LLS in order to retrieve the necessary parameters of each lightning discharge, which serve as input for the algorithms. Median values of the separation distance between the first stroke in the flash and subsequent ground strike points are found to vary between 1.3 and 2.75 km. It follows that all three of the algorithms perform well, with success rates of up to about 90% to retrieve the correct type of the strokes in the flash, i.e., whether the stroke creates a new termination point or follows a pre-existing channel. The most important factor that influences the algorithms’ performance is the accuracy by which the strokes are located by the LLS. Additionally, it is shown that the strokes’ peak current plays an important role, whereby strokes with a larger absolute peak current have a higher probability of being correctly classified compared to the weaker strokes.

IEEE Transactions on Electromagnetic Compatibility, Vol. 63, Issue 5, 2021

DOI:https://10.1109/TEMC.2021.3073787

In this article, numerical computations applying the finite-difference time-domain (FDTD) method are performed. Lightning electromagnetic fields radiated by a lightning strike to the Gaisberg Tower in Salzburg, Austria, and propagating over irregular terrain are calculated. The field propagation is computed with a three-dimensional (3-D) FDTD and, for comparison, a 2-D cylindrical symmetry FDTD.With topographic height maps, seven different propagation paths for a selected lightning strike to the Gaisberg tower are simulated and the resulting magnetic field (Hfield) is compared with the received signal strength at seven sensors of the ALDIS sensor network for that event. The return stroke is represented by a transmission line model and the current waveform recorded at the top of the Gaisberg tower was used. The results of the 3D-FDTD simulation correlate well with H-fields registered at sensor sites in mountainous environments. Hence, 3D-FDTD can represent complex local reflection phenomena correctly, which the 2D-FDTD cannot. The results of recent studies, which show that a lightning strike to a mountain gives rise to a field enhancement at the sensor site for a relatively flat propagation path, could be verified by comparing theFDTDresults with a set of existing sensor measurements. For the, otherwise, very hilly terrain conditions in Austria, the field enhancement caused by the mountain is attenuated along the paths to the respective sensors.

International Conference on Grounding & Lightning Physics and Effects (GROUND2020/21 & 9th LPE), Belo Horizonte, Brazil, June, 2021

We examine current and electric field waveforms for 58 negative upward flashes occurring in 2006-2014 initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the top of the tower using a 0.25 mΩ shunt. The initial stage (IS) of these flashes comprised of relatively slowly varying “background” current (having durations ranging from 74 to 691 ms), with faster, more impulsive current variations (pulses having durations ranging from 2.2 μs to 29 ms) overlaid on this background current. In 46 of the 58 flashes (79%) the background continuing current was negative, and in the other 12 flashes (21%) it was bipolar. 1180 current pulses occurred during the IS of these 58 flashes, of which 708 (60%) were positive bipolar (positive initial polarity with a negative opposite polarity overshoot), 28 (2.4%) were positive unipolar (positive initial polarity with no opposite polarity overshoot), 440 (37%) were negative unipolar, and four (0.3%) were negative bipolar pulses. We found that bipolar current pulses only occurred in the IS at early times; all bipolar pulses occurred within the first 15% of the IS duration in all flashes. We divided the IS current into two phases: (1) upward leader initiation and propagation phase (IPP) and (2) upward leader mature phase (MP). 901 (712 bipolar and 189 unipolar) pulses occurred during the IPP, and 279 unipolar pulses occurred during the MP. The median background-to-peak current was 134 A for IPP pulses and 687 A for MP pulses.

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35th International Conference on Lightning Protection (ICLP) & XVI International Symposium on Lightning Protection (SIPDA), 2021

DOI: https://doi.org/10.34726/hss.2020.65348

In this paper, numerical computations applying the finite-difference time-domain (FDTD) method are performed. Lightning electromagnetic fields radiated by a lightning strike to the Gaisberg Tower in Salzburg, Austria, and propagating over irregular terrain are calculated. The field propagation is computed with a 2D cylindrical symmetry FDTD, using the free and open source software package MEEP (MIT electromagnetic equation propagation), which is introduced to lightning research and described with respect to its functionality. As a radiation source for the simulation, a current waveform recorded at the top of the Gaisberg Tower and applying the MTLE model are used. The computed vertical E- field above ground is compared to the recorded and time correlated vertical E-field, measured with a flat plate antenna at a distance of 108.8 km (Neudorf).

35th International Conference on Lightning Protection (ICLP) & XVI International Symposium on Lightning Protection (SIPDA), 2021

DOI: https://10.1109/ICLPandSIPDA54065.2021.9627439

This paper presents an analysis of specific parameters of lightning discharges in the field of lightning research and observation in the Austrian Alps run by Graz University of Technology in correlation with detections from the Austrian Lightning Location System (LLS). Atmospheric discharges are observed at 21 different measurement locations in Austria by using a video and field recording system. This system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to record the electric field. The recorded data can be used to better understand the atmospheric discharges, especially in the alpine area. The ground truth data set of the measurement periods 2015, 2017 and 2018 have been correlated with LLS data to complete the data set. For this analysis the multiplicity of VFRS and LLS data have been compared. In addition, analyses regarding ground strike points (GSP) per flash and distances between GSPs of flashes have been carried out. The mean multiplicity of the analyzed ground truth data exhibited a lower value compared to multiplicities from other countries available in the literature, but similar ones determined by former studies from Austria. Data of GSP per flashes showed similar values like studies from Brazil and a first data set from Arizona but higher values compared to analyses of data from France and a second data set from Arizona. Calculations of distances between the first GSP of a flash to the other GSPs of the same flash showed a median of 1.4 km, an arithmetic mean of 1.6 km and a geometric mean of 1.2 km. The maximum determined distance between two GSPs was 6.9 km.

35th International Conference on Lightning Protection (ICLP) & XVI International Symposium on Lightning Protection (SIPDA), 2021

There is currently a gap between the existing engineering evaluation of the incidence of upward flashes on grounded structures and the up-to-date knowledge in lightning. This paper introduces a first attempt to update the assessment of the self-initiated upward lightning incidence by including the effects of terrain and seasonal variations using state-of-the-art evaluation procedures. Digital elevation map data are used to define terrain topography in calculations with the Self-consistent Leader Inception and Propagation Model –SLIM–. Historic atmospheric radio sounding measurements are also used to estimate the seasonal probability of thunderclouds generating background electric fields sufficient to initiate upward lightning. Three different case studies are considered: a wind turbine and its protective tower in Japan, Gaisberg Tower in Austria and a wind farm in Spain.

Earth and Space Science, Vol. 8, Issue 10, 2021

DOI: https://doi.org/10.1029/2021EA001914

Angle and amplitude estimation errors in magnetic direction finding, called site errors, are important sensor-specific errors in lightning location systems (LLS). They are known to be caused by nearby cables and overhead lines due to induced currents. Due to reflection and diffraction of electromagnetic waves, hills and mountains are also expected to generate these effects. In this paper, numerical computations applying the finite-difference time-domain (FDTD) method are performed to analyze the impact of hills or mountains on the angle and amplitude estimation of LLS sensors for typical first and subsequent return strokes (RS). The influence of hill size, distance of sensors to the hill, ground parameters and sensor bandwidth, are evaluated. The results show that on top of low ridges of only 125 m elevation, up to +/-10° angle site error and +50% amplitude site error occur. A key finding is that due to field attenuation caused by lossy ground and sensor bandwidth limitations, there is practically no difference in the angle site errors for first and subsequent RS. For two sensors, the average site errors obtained from real sensor measurements are compared to results from 3D-FDTD simulations, modelling the real terrain based on a digital elevation model (DEM). The simulation results are in good agreement with the observed, average angle site errors.

CIGRE - 3rd SEERC Conference, Viennna, Austria, 2021

The paper presents experiences on long term operation of enhanced real time powerline outage correlations with lightning events. The real time correlator service is implemented in the information system of lightning location system (LLS) SCALAR. The SCALAR system is covering the west Balkan region and it is part of the European LLS network EUCLID. EUCLID is a homogenous network and uses the same sensor technology across Europe with centralized data processing. LLS accuracy, detection efficiency, and lightning classification are the most important performance parameters and are important for the real-time correlation service. The high-quality data allows the implementation of real-time correlation in any country covered by the EUCLID network. Currently the real-time correlator service has been implemented on four power transmission networks, five power distribution networks, and one electrified 3kV DC railway network. Two decades (since 2000) of operation gave a considerable number of correlated events which have been analyzed in this paper. Additional ground truth data have been obtained from the power utilities and railways for advanced analyses. Many details about correlated lightning events such as lightning location, peak current, number of returned strokes and distances between them and powerline damages are shown in the paper. Different correlation dependencies such as temporal or permanent outages of powerlines on different voltage levels, powerline geometry, specific soil resistance and other parameters have also been considered in the analysis. The obtained data and the analysis results from the correlation process give us a better understanding and relationship between the incidence of lightning strikes and powerline outages.

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AGU, 2021

The classification of cloud-to-ground (CG) and cloud (IC) events detected by ground-based lightning location systems is one of the mostcritical outputs, both to day-to-day operational users as well as in research work. Although altitude is sometimes used as a means ofclassification, waveform information has been demonstrated to be more successful. We continuously engage in efforts to improve uponremaining inaccuracies in classification. This poster provides an update on our latest improvements as applied specifically in the U.S. NationalLightning Detection Network™ (NLDN™).

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Electric Power Systems Research (EPSR), Vol. 178, 2020

DOI: https://doi.org/10.1016/j.epsr.2019.106042

Lightning location systems geolocate lightning strokes. Given assumptions made in the geolocation models, errors in the reported locations can occur. Modelling these errors as a bivariate Gaussian distribution of historic stroke detections has found success in the form of confidence ellipses. However, the presence of outliers - strokes with large location errors - indicate that there is a better model for these errors. The Students’ t-distribution is a “heavier” tailed distribution. This paper investigates whether the bivariate Students’ t-distribution is a better model for such errors. A methodology for modelling and evaluating the distribution of location errors using maximum likelihood estimation, expectation-maximization and a Mahalanobis distance quality-of-fit test is described. This method is applied to stroke reports from the South African Lightning Detection Network and the Austrian Lightning Detection and Information System time-correlated with photographed lightning events to the Brixton Tower, South Africa and current measurements to the Gaisberg Tower, Austria respectively. In both cases, we find outliers in the distribution of location errors - even as the performance of the networks increase. Using the Mahalanobis test, we find the bivariate Students’ t-distribution to be a better statistical model for both the South African and the Austrian events.

Electric Power Systems Research (EPSR), Vol. 180, 2020

DOI: https://doi.org/10.1016/j.epsr.2019.106065

This paper presents recent ground truth data analyses in the Austrian Alps run by Graz University of Technology in cooperation with Austrian Lightning Detection and Information System (ALDIS). The project "Lightning Observation in the Alps - LiOn" was established in 2017 at the Institute of High Voltage Engineering and System Performance at Graz University of Technology. Atmospheric discharges are observed at different measurement locations in Austria by using a Video and Field Recording System (VFRS). This system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to measure the electric field. The recorded data can be used to better understand the atmospheric discharges, especially in the alpine area. For the present analysis a data set of the measurement periods 2015, 2017 and 2018 was used for validation of the Lightning Location System (LLS) data of ALDIS. In total 463 negative cloud-to-ground (CG) flashes and 1527 negative CG strokes were recorded in Austria during 51 days. The LLS performance parameter, location accuracy and detection efficiency and further the flash multiplicity and peak currents have been analyzed for these three years. Values for location accuracy are in the range of 90 m to 130 m. Flash detection efficiencies in the range from 96.08 % to 98.62 % and for the stroke detection efficiencies in the range from 76.36 %to 85.60 % have been determined. Mean multiplicity values determined with the VFRS data are comparable to the results of previous analyses in the Austrian Alps. The median values of negative stroke peak currents are around 10 % to 30 % lower for 2015, 2017 and 2018, than for detections of older VFRS measurements in the Alps.

IEEE Transactions on Electromagnetic Compatibility, Vol. 62, Issue 1, 2020

DOI: https://10.1109/TEMC.2019.2891898

In this paper, we present current pulses from upward lightning, which have been measured since 2011 at the new top structure of the Peissenberg Tower, Germany. The study comprises 38 negative and two positive flashes, which contained 199 current pulses. About 49 of them were return stroke current pulses, 133 of them were current pulses, which superimposed the initial continuous current (ICC-pulses), and 17 of them were M-component current pulses, which superimposed the continuing current of a preceding return stroke. The current pulses are used to evaluate the performance of the European lightning location system EUCLID. Fifty one (25.6%) out of the 199 current pulses were detected by EUCLID, 40 (81.6%) return stroke current pulses, and 11 (8.5%) ICC-pulses or M-component current pulses. The peak currents ranged from 0.1 to 40.8 kA. Two groups of current pulses could be identified. The first group is related to branches of nearby downward lightning which got in contact with the tower. Therefore, EUCLID reported much higher peak currents (more than 100%) compared to the peak currents measured at the Peissenberg Tower. The first group comprises the total of nine current pulses (six ICC-pulses, three return stroke current pulses). The second group of current pulses is related to upward lightning initiated from the top of the tower. The peak current inferred from EUCLID deviates much less from peak current measured at the Peissenberg Tower. The peak current was overestimated by about 20% by EUCLID. The second group comprises the total of 42 current pulses (4 ICC-pulses, 1 M-component current pulses, and 37 return stroke current pulses). The peak currents ranged from 3.1 to 40.8 kA, the geometric mean (GM) was 9.4 kA. About 30% of these events were misclassified as intra-cloud pulses by EUCLID. The GM of the location error was 161 m for all events, and 132 m considering only the return stroke current pulses.

Atmospheric Measurement Techniques (AMT), Vol. 13, Issue 6, Pages 2965–2977, 2020

DOI: https://doi.org/10.5194/amt-13-2965-2020

The Lightning Imaging Sensor (LIS) on the International Space Station (ISS), hereafter referred to as ISS-LIS, detects lightning from space by capturing the optical scattered light emitted from the top of the clouds. The ground-based European Cooperation for Lightning Detection (EUCLID) makes use of the low-frequency electromagnetic signals generated by lightning discharges to locate them accordingly. The objective of this work is to quantify the similarities and contrasts between these two distinct lightning detection technologies by comparing the EUCLID cloud-to-ground strokes and intracloud pulses to the ISS-LIS groups in addition to the correlation at the flash level. The analysis is based on the observations made between 1 March 2017 and 31 March 2019 within the EUCLID network and limited to 54∘ north. A Bayesian approach is adopted to determine the relative and absolute detection efficiencies (DEs) of each system. It is found that the EUCLID relative and absolute flash DE improves by approximately 10 % towards the center of the EUCLID network up to a value of 56.3 % and 69.0 %, respectively, compared to the averaged value over the full domain, inherent to the network geometry and sensor technology. In contrast, the relative and absolute ISS-LIS flash DE over the full domain is 48.4 % and 71.3 %, respectively, and is somewhat higher than the values obtained in the center of the EUCLID network. The behavior of the relative DE of each system in terms of the flash characteristics of the other reveals that the greater the value, the more likely the other system will detect the flash. For instance, when the ISS-LIS flash duration is smaller than or equal to 200 ms, the EUCLID relative flash DE drops below 50 %, whereas it increases up to 80 % for ISS-LIS flashes with a duration longer than 750 ms. Finally, the distribution of the diurnal DE indicates a higher DE for the ISS-LIS and a lower DE for EUCLID at night.

AGU Fall Meeting, 2020

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Earth, Planets and Space, 72, No. 28, 2020

DOI: https://doi.org/10.1186/s40623-020-01155-9

High electric fields that occur in thunderstorm clouds in the Earth’s atmosphere might accelerate energetic charged particles produced by cosmic rays. Such energetic particles, especially electrons, can cause additional ionization as they are multiplied and thus form avalanche of relativistic electrons. These relativistic electrons emit Bremsstrahlung in the X- or gamma-ray spectral ranges as they lose their kinetic energy via collisions. Thunderstorm ground enhancements (TGEs) of secondary cosmic ray fluxes recorded at the top of a sharp rocky mountain of Lomnický Štít in High Tatras (2634 m, Slovak Republic) are compared with simultaneous measurements of electric field at the mountain top and on its slope at the observatory of Skalnaté Pleso (1780 m). Results of measurements performed from May to September in 2017 and from May to October in 2018 are presented. The cosmic ray flux is measured by Space Environment Viewing and Analysis Network (SEVAN) and by neutron monitor with 1-s resolution. The TGEs that persisted usually several minutes were mainly detected in the SEVAN channel 1 which has the lowest energy threshold, about 7–8 MeV. A statistical analysis shows that these enhancements usually occurred (not only) during large values of vertical,
upward-pointing electric fields measured just above the detector. It is shown that the measurement of electric field at Skalnaté Pleso, distant about 1.86 km from the mountain top is also partly correlated with the enhancements and can provide additional useful information about the distance or dimension of charge structure and dynamics of electric field, especially on short time scales. The enhancements usually did not exceed several tens of percent of background values. However, events that exceeded the background values several times were also recorded. The most extreme event exceeded the background values about 215 times. This event was also detected by other SEVAN channels and by the neutron monitor (~ 130% enhancement), which indicates a possibility of photonuclear reactions. The enhancements were often terminated by a nearby lightning.

Electric Power Systems Research, Vol. 181, 2020

DOI: https://doi.org/10.1016/j.epsr.2019.106067

We present in this paper lightning current measurements, LMA (Lightning Mapping Array) data and fast antenna electric fields associated with upward flashes observed at the Säntis Tower during summer of 2017. The LMA network consists of six stations that were installed in the vicinity of the tower at distances ranging from 100m to 11 km from it. Out of 20 LMA recorded flashes here we analyze in detail three so-called ‘other-triggered flashes’, triggered by preceding activity. Based on the lightning activity derived from the European Lightning Detection Network (EUCLID) in an area within 30 km from the tower and within a 1-s time window before the start of the upward tower flashes, only one out of 20 flashes was classified as ‘other-triggered’(OT). However, the investigations based on the LMA data reveal that 3 more flashes of the 20 analyzed were preceded by nearby activity and should therefore be classified as OT flashes. We analyze conditions conducive to the OT flashes, such as the charge structure of the clouds, polarity of preceding leaders and level of activity of the storm. The LMA source active time period was on average seven times higher for the OT flashes than that for selfinitiated flashes.

EGU General Assembly, 2020

Upward lightning triggered by elevated objects, such as wind turbines (WT), may increase significantly the number of lightning strikes to these objects. In the recently publishes 2nd edition of the international standard IEC 61400-24 an environmental factor CDWL for winter lightning conditions was introduced to account for this additional lightning risk in the lightning exposure assessment of a WT. Values for CDWL should be 4 (in medium winter lightning activity areas) or 6 (high activity areas) or even higher in special cases. The main challenge is to get reliable data about the winter lightning activity for a given region and for first estimates maps of winter lightning activity for the continents are given in IEC 62400-24, Annex B.

A different approach is used in this contribution. As there is already a high number of WT installed in Europe, we have investigated the number (percentage) of existing WT that was at least struck one time in the winter periods of 2017/18 an 2018/19 based on data of the EUCLID lightning location system.

We have extracted the locations of 10.225 WT sites in Europe in the area from 45°N - 50°N and 10°W -30°E form OpenStreetMap database. Then we checked if there were any lightning strikes located by EUCLID within a 0.003° circular area (is about a 300 m radius) around each of these turbines during the cold season (October to April) in 2017/18 and 2018/2019, respectively. Out of the 10.225 WT 1.131 (11,1 %) and 913 (8,9 %) have been struck by lightning in cold season 2017/18 and 2018/19, respectively. It is worth noting, that only 101 WT (1%) were struck in both seasons, indicating that it is more a dependency on regional meteorological conditions changing from year to year, rather than on a specific group of WT. EUCLID detected flashes are likely to represent only about one half of the real occurring upward flashes from the WT. ICCOnly type upward lightning, which are discharges with current waveforms not followed by any return strokes are typically not detected by lightning location systems, and on instrumented towers this type of discharges makes up about 50% of all upward lightning. But there is a high chance, that a large fraction of this ICCOnly discharges were triggered by the same WT, where EUCLID detected some strokes.

In terms of dependency of the altitude of the WT site above sea level we observe a clear increase of probability of WT lightning with increasing altitude. About 10 % (29/315) of the 315 WT at altitudes up to 50 m ASL are struck by lightning increasing to almost 50 % (15/31) for WT at sites of 950 to 1000 m altitudes ASL. No clear trend is observed for higher altitudes, likely due to the low number of WT above 1000 m.

The obtained 10 % of the WTs triggering at least one upward lighting per cold season demonstrates the high probability of lightning to WT and emphasizes the need of proper protection of the WTs mechanical structure (rotor blades) as well as the entire electrical installation.

Journal of Climate, 2019

DOI: https://doi.org/10.1175/JCLI-D-18-0372.1

The climatology of (severe) thunderstorm days is investigated on a pan-European scale for the period of 1979–2017. For this purpose, sounding measurements, surface observations, lightning data from ZEUS (a European-wide lightning detection system) and European Cooperation for Lightning Detection (EUCLID), ERA-Interim, and severe weather reports are compared and their respective strengths and weaknesses are discussed. The research focuses on the annual cycles in thunderstorm activity and their spatial variability. According to all datasets thunderstorms are the most frequent in the central Mediterranean, the Alps, the Balkan Peninsula, and the Carpathians. Proxies for severe thunderstorm environments show similar patterns, but severe weather reports instead have their highest frequency over central Europe. Annual peak thunderstorm activity is in July and August over northern, eastern, and central Europe, contrasting with peaks in May and June over western and southeastern Europe. The Mediterranean, driven by the warm waters, has predominant activity in the fall (western part) and winter (eastern part) while the nearby Iberian Peninsula and eastern Turkey have peaks in April and May. Trend analysis of the mean annual number of days with thunderstorms since 1979 indicates an increase over the Alps and central, southeastern, and eastern Europe with a decrease over the southwest. Multiannual changes refer also to changes in the pattern of the annual cycle. Comparison of different data sources revealed that although lightning data provide the most objective sampling of thunderstorm activity, short operating periods and areas devoid of sensors limit their utility. In contrast, reanalysis complements these disadvantages to provide a longer climatology, but is prone to errors related to modeling thunderstorm occurrence and the numerical simulation itself.

IEEE Transactions on Electromagnetic Compatibility, Vol. 61, Issue 3, 2019

DOI: https://10.1109/TEMC.2019.2916047

We examined the occurrence characteristics of the lightning observed at the Gaisberg Tower (GBT) in the years 2000 to 2018 and analyzed current wave forms (measured using a 0.25-mΩ shunt) of upward flashes initiated from the tower. During this period, 865flasheswere recorded at the GBT, ofwhich823(95%)were upward and 4 (0.5%) were downward. For 18 flashes, the current wave forms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 kA, respectively. We expanded the traditional classification of bipolar flashes to include five categories. Of the 137 bipolar flashes, 45% were of Type 1S (single reversal of current polarity during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 [different polarities of current during IS and return stroke (RS)], 1.5% of Type 3 (RSs of different polarities), and 0.73% (one flash) of Type 4 (different polarities of RS and the following continuing current).

Electric Power Systems Research, Vol. 174, 2019

DOI: https://doi.org/10.1016/j.epsr.2019.04.023

In this paper, we present and discuss simultaneous records of current and wideband electric field waveforms at 380 km distance from the strike point associated with an upward bipolar flash initiated from the Säntis Tower. The flash contains 23 negative strokes and one positive stroke. The intervals between the groundwave and skywave arrival times are used to estimate ionospheric reflection heights for the negative return strokes using the so-called zero-to-zero and peak-to-peak methods. A full-wave, finite-difference time-domain (FDTD) analysis of the electric field propagation including the effect of the ionospheric reflections is also presented. FDTD simulation results are compared with the measured radiated electric field associated with the studied flash to evaluate the reference reflection height of the conductivity profile. It is also found that the ratio of the peak field to the current peak is about two times smaller for the positive pulse compared to negative pulses. This difference in the amplitudes can be attributed to a lower return stroke speed for the positive stroke compared to that for negative strokes, and also to the fact that the enhancement of the electric field due to the presence of the tower and the mountain might be more significant for negative pulses, which are characterized by faster risetimes compared to the positive one.

IEEE Transactions on Electromagnetic Compatibility, Vol. 61, Issue 3, 2019

DOI: https://10.1109/TEMC.2019.2913220

An unusual negative lightning flash was recorded at the Säntis Tower on June 15, 2012. The flash did not contain an initial continuous current typical of upward negative lightning, which is the most common type of event at the Säntis Tower. The flash contained four strokes, the last three of which were normal while the current associated with the first stroke resembled a Gaussian pulse with an unusually high peak value of 102.3 kA, a long rise- time of 28.4 µs, and a pulse width of 53.8 µs, which was followed by an opposite polarity overshoot with a peak value of 8.5 kA. Our current records suggest the involvement of a long upward con- necting positive leader in response to the approaching downward negative leader in the formation of this flash. Lightning location system (LLS) data indicate that a positive cloud-to-ground stroke occurred 1 ms prior to the first stroke of the flash. In this paper, we present a detailed description of the data associated with this event. Moreover, both a return stroke model and an M-component model are used to reproduce the far-field waveform of this bipolar stroke. The simulations result in a radiated electric field waveform that is similar to those of large bipolar events (LBEs) observed in winter thunderstorms in Japan. A sensitivity analysis of the used simulation models reveals that, by proper selection of the input parameters, all field waveform characteristics, except for the positive half-cycle width, can be made to fall in the range of LBE field characteristics reported in Japan.

11th Asia-Pacific International Conference on Lightning (APL), Hongkong, China, 2019

DOI: https://10.1109/APL.2019.8816002

We examined current waveforms of upward flashes initiated from the Gaisberg Tower in 2000 to 2018. Current was measured at the tower-top using a 0.25-mΩ shunt. During this period, 865 flashes were recorded at the Gaisberg Tower, of which 823 (95%) were upward and 4 (0.5%) were downward. For 18 flashes the current waveforms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 kA, respectively. We expanded the traditional classification of bipolar flashes to include five categories. Of the 137 bipolar flashes, 45% were of Type 1S (single current polarity reversal during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 (different polarities of current during IS and return stroke), 1.5% of Type 3 (return strokes of different polarities), and 0.73% (one flash) of Type 4 (different polarities of return stroke and the following continuing current).

11th Asia-Pacific International Conference on Lightning (APL), 2019

An important input parameter in lightning protection studies is the lightning flash density. Lightning Location Systems (LLS) do provide flash data, with a single location allocated to each flash. However, cloud-to-ground (CG) flashes are known to exhibit one or more ground strike points (GSP). Therefore, having a tool that is able to determine the different GSP within a single flash is of great importance to correctly investigate the potential risk of lightning damage. In this study a GSP identification algorithm, developed by Météorage, is tested against high-speed video measurements in order to validate the ability to reproduce the observed GSP in the field. The ground truth data were taken in Austria (2012, 2015), Brazil (2008), France (2013-2016), Spain (2017-2018), and USA (2015) and are correlated to operational LLS data in order to extract the location, peak current estimate and other parameters serving as input for the GSP algorithm. As a result, the validation of the GSP algorithm is based on 824 flashes with a total of 2413 strokes. Averaged over all the datasets the GSP algorithm is able to identify correctly new ground contacts (NGC) in 93% of the cases, whereas 82% of the strokes following previously existing channels (PEC) were captured accurately by the algorithm. It becomes clear that the actual performance of the algorithm depends on 1) the so-called distance parameter within the algorithm itself, i.e. the distance criterion to group individual strokes within a single GSP, and on 2) the location accuracy (LA) of the LLS.

11th Asia-Pacific International Conference on Lightning (APL), 2019

DOI: https://10.1109/APL.2019.8816037

In this paper we analyze the currents and electric fields of 5 negative return strokes and compare them with data from the European lightning location system (LLS) EUCLID. The return strokes were measured between May and August 2017 at the Peissenberg Tower, Germany. 1 out of the 5 return strokes was a first return stroke and the remaining 4 were subsequent return strokes. Their peak current Ip varied between 7.5 kA and 24.9 kA, the arithmetic mean value was 14.4 kA. We present high speed video images of the first return
stroke showing the extremely long upward connecting leader. The connecting leader produced a continuous current of approximately 30 A. The LLS detected 4 out of the 5 presented return strokes. We present the time errors of the individual sensors, which contributed to the localization. After an optimization process, the time errors were reduced and the location error was minimized. The location error varied between 101 m and 239 m. The peak current inferred by EUCLID was overestimated by 32.3% on average.

11th Asia-Pacific International Conference on Lightning (APL), 2019

DOI: https://10.1109/APL.2019.8815982

This paper presents an analysis related to the research project “Lightning Observation in the Alps” (LiOn) carried out by Graz University of Technology in cooperation with the Austrian Lightning Detection and Information System (ALDIS). Atmospheric discharges were observed at 21 different measurement locations in Austria by using a video and field recording system. The system consists of two main components, a high speed video camera and a flat plate antenna. The recorded ground truth data should help to get additional information about atmospheric discharge processes, especially in the alpine area. For this analysis, a dataset of negative cloud-to-ground strokes of the measurement periods 2015, 2017 and 2018 was used. In this analysis, we compare peak currents of first return strokes (LLS flash data) with peak currents of first strokes in a GSP. The GSPs are determined manually by using the video data. We especially considered the relation of first initial (FI) peak currents versus subsequent (SU) peak currents. The stroke peak current versus the multiplicity for flashes and strokes per GSP was investigated too. For all the analyses in this paper, the Austrian Lightnig Location System (ALDIS) provided the data for the stroke peak currents. In 42 % of the flashes, at least one SU peak current in a flash shows a greater value compared to the FI stroke in this flash. Checking the peak currents within GSPs reveals 37 % with at least one SU stroke with a larger peak current than the FI stroke in the same GSP. The analysis of the percentage of SU strokes with peak currents greater than the FI stroke reveals the same value of 14 % for flashes and per GSPs. Regarding the analysis of FI peak currents versus the multiplicity, the results show an increase for both analyses leading to higher peak currents for the FI strokes exhibiting a larger multiplicity.

AGU Fall Meeting, 2019

We examined current waveforms for 823 upward flashes occurring in 2000 to 2018 initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the tower-top using a 0.25 mΩ shunt. Of the upward flashes, 651 (79%) were negative (transferred negative charge to ground), 35 (4.3%) were positive (transferred positive charge to ground), and 137 (17%) were bipolar (transferred both negative and positive charge to ground during different stages of the flash). For positive flashes, the median initial stage (IS) current duration, peak current, and charge transfer were 96 ms, 3.2 kA, and 20 C, respectively. For bipolar flashes, the values of these parameters were 282 ms, 1.8 kA, and 36 C, respectively. Of the 137 bipolar flashes, 45% were of Type 1S (single reversal of current polarity during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 (different polarities of current during IS and return stroke), 1.5% of Type 3 (RSs of different polarities), and 0.73% (one flash) of Type 4 (different polarities of RS and the following continuing current). The median absolute value of charge transfer during the IS for Type 1S, 1M, 2, and 3 flashes were 38 C, 37 C, 95 C, and 39 C, respectively. The charge transferred during the IS of the Type 4 flash was 58 C. In this study, we will compare charge transferred during the IS of upward flashes with that during the post-IS period in which one or more returns strokes may occur. Additionally, we will analyze in detail the charge transfer characteristics of upward flashes with and without return strokes.

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CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Delft, Netherlands, 2019

This paper presents recent analyses of lightning observations in the Austrian Alps gathered by Graz University of Technology in cooperation with the Austrian Lightning Detection and Information System (ALDIS). The project “Lightning Observation in the Alps – LiOn” was initiated in 2017 to record ground truth data of atmospheric discharges in the Alpine region. Atmospheric discharges were observed at  21 different measurement locations using a Video and Field Recording System (VFRS). This system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to measure the electric field. Both, the camera and the electric field measurement system, are synchronized with GPS time and can therefore be correlated with Lightning Location System (LLS) data.  Two special cases have been recorded in the pre-alpine area during warm season thunderstorms of 2018. The first case shows a flash consisting of four downward strikes to two different wind turbines. The two wind turbines are part of a wind farm comprised 21 generators situated on a mountain ridge with a height of about 1450 m above sea level. The protocol data of the wind farm operator were correlated to VFRS and LLS data. The second case shows a flash consisting of three strokes, which hit a railway overhead line. The strokes caused flashovers on the insulators of three towers. The railway track is situated in build-up terrain; no train was on the section of the track in the moment of the lightning strike. The network protocol data of the Austrian railway operator and the related digital fault record of the installed distance protection relay was again correlated to VFRS and LLS data. Results of this report shall contribute to a better understanding of lightning processes in general and in the Alpine region in particular.

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21st International Symposium on High Voltage Engineering (ISH), Budapest, Hungary, 2019

In this paper, we analyse the currents and electric fields of 35 negative return strokes, which have been measured since 2012 at Peissenberg Tower, Germany. 27 were pure return strokes and 8 were return strokes with superimposed symmetrical Mcomponents. 2 out of this 8 were first return strokes. The measured peak currents ranged from 3.1 kA to 40.8 kA, the arithmetic mean value (AM) was 12.3 kA. Further we estimated the 10%-to-90% rise time, which ranged from 1.0 µs to 7.4 µs, the AM was 1.9 µs. The transferred charge varied from 0.1 C to 10.6 C, the AM was 1.0 C. The radiated electric field was measured in a distance of about 180 m to the tower. The electric field exhibits a first field change due to the descending leader. For the description of this first field change we introduced ∆E1. The values of ∆E1 varied from 0.8 kV/m to 10 kV/m, the AM was 2.8 kV/m. The first field change is immediately followed by a second field change of opposite polarity. We introduced ∆E2 to describe this field change, which is caused by the return stroke process. The values of ∆E2 varied from 1 kV/m to 14.2 kV/m, the AM was 3.5 kV/m. All analysed return strokes were detected by the lightning location system (LLS) EUCLID. The peak current inferred by EUCLID varied between 3.9 kA and 53.0 kA, the AM was 15.0 kA. 10 out of 35 detected return strokes were misclassified as cloud-tocloud discharge.

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International Symposium on Lightning Protection (XV SIPDA), São Paulo, Brazil, 2019

DOI: https://10.1109/SIPDA47030.2019.8951519

The present analysis shows data of measurements of natural cloud-to-ground lightning performed during warm season thunderstorms in the Alpine region of Austria. These measurements were performed during six years from 2009 to 2018 to generate a ground truth dataset of atmospheric discharges. The used measurement system consists of a high speed video and an electric field recording system (VFRS), to observe the optical properties of lightning discharges and to record the transient electric field. Measurements have been conducted at 33 different measurement locations and 735 negative cloud-to-ground flashes have been recorded during 61 days spread over the whole measurement period. Data of the Austrian Lightning Location System (LLS), ALDIS (Austrian Lightning Detection and Information System), is correlated with the VFRS ground truth data to complete the dataset. These datasets are used to analyze possible reasons for the detected variation of single stroke flashes in the Alpine region of Austria. The calculated values for single stroke flashes of the present analysis are also compared to previously published values (12 – 24 %) from different countries. Additionally radar data and wind measurements have been analyzed to classify thunderstorm types in order to investigate the effect of thunderstorm organization on their lightning characteristics. Compared to values from the literature the percentage of negative single stroke flashes in this study is higher (27 %). Results of this report shall contribute to a better understanding of atmospheric discharges in general and their behavior in the Alpine region in particular.

International Symposium on Lightning Protection (XV SIPDA), São Paulo, Brazil, 2019

In this paper the results of a simple model for the calculation of continuing currents (CCs) and the corresponding transferred charge in natural negative and positive downward lightning are presented. The model takes into consideration the height of the charge center in the cloud, the distance of the measurement system to the lightning strike point and the true, undistorted E-field at the location of observation. The evaluation is based on ground truth data from field campaigns in 2015 and 2017 in Austria, where E-fields and simultaneous high speed video recordings of the lightning channels were obtained. 76 cases of negative and 12 cases of positive downward lightning containing long continuing currents (CC duration > 40 ms) were analysed. Parameters such as duration mean and median values of the amplitude and transferred charge of CCs were evaluated and compared to other studies. Further, the continuing current waveforms were divided into five equal segments of which the results on the statistics are presented by means of boxplots.

13. VDE Blitzschutztagung, Aschaffenburg, Germany, 2019

This paper introduces a newly developed optical lightning detection system (OLDS). The OLDS is based on a video system using a camera with a 360° panorama view and a frame rate of 60 frames per second. The OLDS locates the lightning from the direction of the illuminated lightning channel and the travel time of the thunder sound. The analysis is based on 37 flashes containing 52 strokes, which occurred during the time period from 9th of May 2018 to 13th of August 2018. The results are compared to data obtained by the European lightning location network EUCLID. The comparison revealed a flash detection efficiency of 95% and a stroke detection efficiency of 94% for EUCLID. EUCLID classified the fraction of 84 % correctly as cloud-to-ground lightning and only 16 % of the events were miss-classified as cloud-tocloud lightning. On average (arithmetic mean), the flashes detected by the OLDS contained 1.4 return strokes with 1.2 different striking points. The average striking distance was about 5.6 km. The OLDS located the lightning with a deviation of 238 m (arithmetic mean) and 171 m (geometric mean) compared to the data of EUCLID.

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Journal of Geophysical Research: Atmospheres, Vol. 124, Issue 24, Pages 14198-14219, 2019

DOI: https://doi.org/10.1029/2018JD030014

In this paper, a full-wave two-dimensional Finite-Difference-Time-Domain model is developed to evaluate the propagation effects of lightning electromagnetic fields over mountainous terrain in the Earth-ionosphere wave guide. In the model, we investigate the effect of the Earth-ionosphere wave guide structure and medium parameters, including the effect of the ionospheric cold plasma characteristics, the effect of the Earth curvature, and the propagation effects over mountainous terrain. For the first time, the obtained results are validated against simultaneous experimental data consisting of lightning currents measured at the Säntis Tower and electric fields measured in Neudorf, Austria, located at 380-km distance from the tower. It is shown that both the time delays and amplitudes of the lightning electromagnetic fields at 380-km distance can be strongly affected by the ionospheric electron density profile, the mountainous terrain, and the Earth curvature. After taking into account the effect of the irregular terrain between the Säntis Tower and the field measurement station, the vertical electric fields calculated by using our modelare found to be in good agreement with the corresponding measured cases occurred in both daytime and nighttime. The ideal approximation used in either the classical solutions or the simplified models might lead to inaccuracies in the estimated reflection height. Furthermore,we discuss the sensitivity of our results by considering different return stroke models, as well as different typical values of the return stroke speed and of the ground conductivity.

Electric Power Systems Research, Vol.  157, Pages 211–226, 2018

DOI: https://doi.org/10.1016/j.epsr.2017.12.008

Field observations have shown that the frequency of dangerous lightning events to wind turbines, caculated according to the IEC standard 61400-24:2010, is grossly underestimated. This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to offshore wind power plants. Three different farms are used as case studies.The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis shows that only a small fraction of damages observed in the analysed farms can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunder-storms in the area of the analysed farms can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can be triggered even under low thunder-cloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the case studies.

Atmosphere, Vol. 9, Issue 1, 2018

DOI: https://doi.org/10.3390/atmos9010020

We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of upward-initiated lightning from Gaisberg Tower, in Austria, which is located at an altitude of 1300 m. This is the first observation of X-ray emissions from upward lightning from a tower top located at high altitude. Measurements were carried out using scintillation detectors installed close to the tower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of three flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to the observations from downward natural or triggered lightning, X-rays were observed only within 10 µs before the subsequent return stroke. This shows that X-rays were emitted when the dart leader was in the vicinity of the tower top, hence during the most intense phase of the dart leader. Both the detected energy and the fluence of X-rays are far lower compared to X-rays from downward natural or rocket-triggered lightning. In addition to the above 108 flashes, an interesting observation of X-rays produced by a nearby downward flash is also presented. The shorter length of dart-leader channels in Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI:https://10.1109/ICLP.2018.8503409

This article deals with the measurement of lightning current on high structures and, in particular, wind turbines. It offers a synopsis of the special lightning current characteristics for high building structures. The special requirements for lightning current measurements on high objects and, especially, wind turbines, are derived from this. It presents a measurement system for determining lightning currents in wind turbines based on Rogowski coils. In order to verify this lightning current recording unit, comparison measurements were carried out at the lightning measurement stations Gaisberg (Austria) and Peissenberg (Germany). First measurements have also been carried out on wind turbines.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI: https://10.1109/ICLP.2014.6973308

Lightning flashes are still a major cause of injury, fire, mechanical destruction and, above all, surges. Time and again there are reports of extremely high lightning currents which, of course, can cause considerable damage and destruction. In some cases, peak values of over 300 kA are mentioned. This throws up questions because the “classical” lightning statistics (e.g., CIGRE and IEC [9, 11]) do not recognize such values. These extreme lightning currents are, as a rule, identified using the data provided by lightning detection systems.
This article will examine such extreme lightning currents. The necessary fundamentals of lightning detection will be explored as well as the limits when verifying extreme values. “Classical” lightning statistics and further studies of extreme lightning currents will be discussed.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI:https://10.1109/ICLP.2018.8503436

This paper investigates the statistical distribution that best describes Lightning Location System location errors. It is generally assumed that the bivariate Gaussian distribution is a good description but the presence of outliers calls for a heaviertailed distribution. The bivariate Students’ t-distribution is proposed. Using Maximum Likelihood Estimation, Mahalanobis quality-of-fit tests and ground-truth events to the Brixton tower, South Africa and Gaisberg tower, Austria, we are able to show that the bivariate Students’ t-distribution is a better fit than the bivariate Gaussian distribution.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI: https://10.1109/ICLP.2018.8503422

Ball lightning (BL) research lacks instrumental records and field experiments. Today, widely available networks for lightning detection offer a chance to assess alleged BL reports with located lightning and its physical parameters. This assessment was realized with BL case data and EUCLID lightning data for Germany, Austria, the Czech Republic, and Switzerland. According to the initial flash hypothesis, the electromagnetic pulse of a detected stroke near the geocoded BL position could be the triggering event of the BL process. The lightning location system (LLS) provides stroke location, time, polarity (positive/negative), and peak current in kA. In this assessment, 34 BL case reports over the period from 1994 to 2016 suggested a physical link of the stroke with maximum peak current in kA to the subsequent BL process. No electromagnetic field values for individual strokes can be computed with the kA peak current. As a first step in this analysis, geocoded distances of less than 1 km between stroke location and BL observation site were called „close event“ (CE) and distances 1-10 km „distant event“ (DE). For cloud-ground (CG) strokes the stroke-to-BL distance values were obtained. LLS-provided coordinates for intra-cloud (CC) strokes were also used to obtain a distance, although CC do not have a defined striking point. 28 BL cases occurred in the summer months, 6 cases in winter. Their time pattern follows the diurnal thunderstorm frequency in Central Europe. 19 cases were from Germany, 10 from Austria, 3 from the Czech Republic and 2 from Switzerland. 19 of the correlated EUCLID strokes were positive and 15 negative. Positive strokes ranged from 4 to 370 kA, negative from -3 to -37 kA. 28 were classified by the LLS as CG, 6 as CC strokes. The mean value of the semi-major axis of the EUCLID location confidence ellipse was 0.45 km. The BL-to- stroke distance in km for CG events had a mean of 0.42 km and a range of 0.0-0.8 km for CE cases, and a mean of 5.7 km and a range of 1.4-10 km for DE cases. The case statistics of this sample showed smaller BL-stroke distances for higher kA peak currents for both negative and positive CG events. This result leads to the interpretation that most DE strokes are uncorrelated with BL since their physical effects are marginal. Replications with more BL cases will show whether the pattern holds.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI: https://10.1109/ICLP.2018.8503310

Upward lightning flashes that originate from tall grounded structures have been classified as either ‘self-triggered’
or ‘other-triggered’ based on preceding lightning events in their vicinity, i.e. strokes or pulses from cloud-to-ground (CG) or intracloud (IC) discharges occurring around the tower. A total of 165 upward flashes recorded between March 2015 to July 2016 at the Säntis Tower in the eastern Swiss Alps are analyzed considering the related meteorological conditions. The results are compared to similar previous studies to reveal some general ideas on how meteorological conditions influence the initiation of selfand other- triggered upward flashes.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI:https://10.1109/ICLP.2018.8503277

This paper introduces a newly developed optical lightning measurement system (OLMS). The OLMS is based on a video system using a camera with a 360° panorama view and a frame rate of 60 frames per second. The OLMS locates the lightning from the direction of the illuminated lightning channel and the travel time of the thunder sound. The analysis comprises 34 flashes containing 144 strokes, which occurred during the time period from 30th of July 2017 to 24th of August 2017. The results are compared to data obtained by the European lightning location network EUCLID. The comparison revealed a flash detection efficiency of 100 % and a stroke detection efficiency of 93.1 % for EUCLID. With EUCLID, the fraction of 92.5 % was classified correctly as cloud-to-ground lightning and the minor fraction of 7.5 % was miss-classified as cloud-to-cloud lightning. On average (arithmetic mean), the negative flashes contained 4.3 return strokes with 1.7 different striking points. For multi-stroke flashes with return strokes using the same lightning channel, EUCLID located the individual striking points with a median deviation of 57.3 m. The average striking distance was about 10 km. Due to the far distance the thunder sound could often not be separated from the thunder sound of other lightning. Therefore, the OLMS located the lightning with a relatively high deviation compared to the data of EUCLID. The arithmetic (geometric) mean value of the deviation was 1330 m (670 m).

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI: https://10.1109/ICLP.2018.8503297

This paper presents the current analysis in the section of lightning research and observation in the Austrian Alps run by Graz University of Technology in correlation with Austrian Lightning Location System detections. The project “Lightning Observation in the Alps – LiOn” was created in 2017 at the Institute of High Voltage Engineering and System Performance at Graz University of Technology. Atmospheric discharges are observed at 19 different measurement locations in Austria by using a video and field recording system. These system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to measure the electric field. The recorded data can be used to better understand the atmospheric discharges, especially in the alpine area. For this analysis a data set of the measurement periods 2015 and 2017 was used for validation of the Lightning Location System data of the Austrian Lightning Detection and Information System. The LLS performance parameter location accuracy and detection efficiency for these two years have been analyzed. Values for location accuracy are in the range of 111 m and 139 m for 2015 and 2017. For flash detection efficiency 96.08 % and 98.49 % and 85.60 % and 76.34 % for the stroke detection efficiency have been calculated for 2015 and 2017 respectively.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI:https://10.1109/ICLP.2018.8503322

We present in this paper lightning current measurements and LMA (Lightning Mapping Array) data associated with upward flashes observed at the Säntis Tower during Summer 2017. The LMA network consists of six stations located in the vicinity of the tower at distances ranging from 100 m to 11 km from it. 20 flashes simultaneously recorded by the current measurement system and the LMA are analyzed. Based on the lightning activity derived from the European Lightning Detection Network (EUCLID) in an area within 30 km from the tower and in a 5-second time window before the start of the flash, all the 20 flashes were classified as ‘self-triggered’ (ST). However, the investigations based on the LMA data reveal that 3 of the flashes were preceded by nearby activity and should be therefore classified as ‘other-triggered’ (OT) flashes. The results suggest that the number of OT flashes inferred from LLS data can be underestimated.

34^th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018

DOI:https://10.1109/ICLP.2018.8503374

We examine current and electric field waveforms for 58 negative upward flashes occurring in 2006-2014 initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the top of the tower using a 0.25 mΩ shunt. Electric field was measured simultaneously at near (170 m from the tower) and far (79 km from the tower in 2006 - 2007 and 109 km in 2008 - 2014) distances. The initial stage (IS) of these flashes comprised of relatively slowly varying “background” current (having durations ranging from 74 to 691 ms), with faster, more impulsive current variations (pulses having durations ranging from 4.0 μs to 29 ms) overlaid on this background current. In 45 of the 58 flashes (78%) the background continuing current was negative, and in the other 13 flashes (22%) it was bipolar. 1180 current pulses occurred during the IS of these 58 flashes, of which 709 (60%) were positive bipolar (positive initial polarity with a negative opposite polarity overshoot), 27 (2.3%) were positive unipolar (positive initial polarity with no opposite polarity overshoot), 435 (37%) were negative unipolar, and nine (0.76%) were negative bipolar pulses. The median peak current for the 718 bipolar pulses was 0.16 kA and that for the 462 unipolar pulses was 0.20 kA. The 1180 IS current pulses produced 341 detectable electric field change signatures at the near station and 109 at the far stations.

25^th International Lightning Detection Conference and 7^th International Lightning Meteorology Conference (ILDC/ILMC), Ft. Lauderdale, Florida, USA, 2018

Upward lightning initiated from tall objects is classified in two distinct groups. Self-initiated lightning occurs when the slow charge buildup produced by cloud electrification is sufficient to start the upward propagating leader. Nearby-lightning-triggered flashes are initiated by the rapid field changed caused by nearby lightning activity, either to ground or inter-cloud. In this paper we analyzed 307 upward flashes initiated from the Gaisberg Tower in Austria and based on measured electric fields at close distance to the tower 80 % were classified as self-initiated and 20 % as nearby-lightning-triggered. Some storms produced exclusively one type of discharges. In case of self-initiated lightning the median height of the -10°C isotherm was in the range of 3000 m, whereas in case of nearby-lightning-triggered lightning the isotherm was at a height of 4800 m.

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25^th International Lightning Detection Conference and 7^th International Lightning Meteorology Conference (ILDC/ILMC), Ft. Lauderdale, Florida, USA, 2018

The response of local lightning to local surface temperature is examined near the Säntis mountain (2502 m ASL) in the eastern Swiss Alps during the first of the two hiatuses in global warming covering the time periods 1940-1972 and 1998-2014. The mountain summit was used since 1881 as a telegraph and meteorology station and since 1955, 3 different towers have been installed at the top of the mountain. In order to take the effect of the presence of a tower and its height into consideration, and also to be able to compare the trend of data inside and outside the hiatuses, the analysis period (1931-1994) is subdivided into four different time intervals. Depending on the availability of the data, the combination of surface air temperature and number of thunderstorm days obtained from MeteoSwiss were used to investigate the sensitivity of lightning activity to changes in surface air temperature at monthly and yearly timescales. The results show a clear difference between the hiatus and post-hiatus intervals and seem to support the idea that the lightning activity is correlated with the surface air temperature.

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XVI International Conference on Atmospheric Electricity, Nara, Japan, 2018

This study is based on a three-dimensional (3D) full-wave FDTD approach with a topographic map including the Säntis Tower located in the Swiss Alps and the nearby sensor sites belonging to lightning location systems (LLSs). In the analysis, the considered seven sensors are divided into two different groups to represent different sensor geometrical distributions, and six different onset time estimation methods proposed in the literature are used to evaluate the time of arrival of a measured signal utilized by the ToA technique to determine the stroke location. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity, and that the location error associated with the ToA technique highly depends on the used onset time estimation technique and the geometrical distribution of the sensor sites.

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Natural Hazards and Earth System Sciences, Vol. 17, Issue 3, Pages 305–314, 2017

DOI: https://doi.org/10.5194/nhess-17-305-2017

This study develops methods for estimating lightning climatologies on the day^-1 km^-2 scale for regions with complex terrain and applies them to summertime observations (2010–2015) of the lightning location system ALDIS in the Austrian state of Carinthia in the Eastern Alps.

Generalized additive models (GAMs) are used to model both the probability of occurrence and the intensity of lightning. Additive effects are set up for altitude, day of the year (season) and geographical location (longitude/latitude). The performance of the models is verified by 6-fold cross-validation.
The altitude effect of the occurrence model suggests higher probabilities of lightning for locations on higher elevations. The seasonal effect peaks in mid-July. The spatial effect models several local features, but there is a pronounced minimum in the north-west and a clear maximum in the eastern
part of Carinthia. The estimated effects of the intensity model reveal similar features, though they are not equal. The main difference is that the spatial effect varies more strongly than the analogous effect of the occurrence model.
A major asset of the introduced method is that the resulting climatological information varies smoothly over space, time and altitude. Thus, the climatology is capable of serving as a useful tool in quantitative applications, i.e. risk assessment and weather prediction.

10^th Asia-Pacific International Conference on Lightning (APL), Thailand, Krabi, 2017

In this paper we provide details about a performance parameter of the EUCLID lightning location system (LLS) called the percentage of outliers. The term outlier means an event (CG stroke or IC pulse) located by the LLS on a wrong place. In this study we use data from weather radar networks in two regions of the EUCLID network (Belgium and Austria) to distinguish between outlier and non-outlier. It is shown that the percentage of outliers is sensitive to changes in the network and also changes related to the location algorithm itself. The overall percentage of outliers for both regions is between 0.8% and 1.9% for a distance to the nearest precipitation of 2km.

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4^th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

We examine current and electric field waveforms produced by lightning strikes initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the top of the tower and electric field measured simultaneously at close (170 m from the tower), and far (79 or 109 km from the tower) distances. In this preliminary study, we establish the criteria for characterizing current and electric field pulses that occur during the initial stage of upward lightning flashes (including those at the initiation of the initial stage) based on the characteristics of the measurement system used to record the current and electric field waveforms and the occurrence context of the pulses in the flash. Of the seven negative upward flashes analyzed in this study, two flashes had bipolar IS background current, which were first negative, followed by brief (duration < 2 ms) positive current, and then negative again. The initial stage background current was negative in the other five flashes. Overall, 71% of the pulses occurring during the initial stage were positive bipolar, 2% were positive unipolar, and 27% were negative unipolar. No negative bipolar pulses were found. The total duration of unipolar pulses ranged from 4.9 to 702 μs and that of bipolar pulses ranged from 4.1 to 197 μs.

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4^th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

We present a 4-stroke negative lightning flash recorded at the Säntis Tower for which the current waveform associated with the first return stroke was unusual and resembled a Gaussian pulse. Such current pulses could be the sources of similarly-shaped electric field waveforms that are attributed to LBEs. Correlated data form the EUCLID lightning detection network show that this flash was preceded by a positive flash located 0.8 km from the tower and 1 ms prior to its first stroke. We also present simulation results of the radiated electric fields considering two different models for the LBE and we show that the simulated waveforms agree well with the experimentally observed characteristics of the radiated fields associate with LBEs.

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CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

This work aimed at analysing the occurrence of Intense Cloud-to-Ground (ICG) in Western Europe, including a large part of maritime areas, defined as lightning flashes exhibiting at least one return stroke peak current larger than 200 kA based on lightning data collected by EUCLID between 2007 and 2016. As expected, the rate of ICG is low in average, about 0.18 % of the total Cloud-to-Ground (CG), but because of a pronounced seasonal trend it can increase up to 1.5% in winter. Around 70% of ICG occurring over the Atlantic Ocean and the Mediterranean Sea are of negative polarity whereas, in around the same proportion, they are positive over the continental regions. The geographical distribution of ICG shows a clear enhancement of ICG occurrences during winter time along coastal areas exhibiting elevated terrain, in northern Spain and western Italy and in Balkans. In these regions ICG are mainly located in land and surprisingly their polarity is negative on the contrary to the general trend stating most ICG are positive on the Continent. The discrepancies observed in the geographical, seasonal and polarity distributions are thought to be related to the different type of thunderstorms occurring across Europe and particularly oceanic and Mediterranean winter and continental deep-convective clouds. Finally, some high-density areas along Italian or Balkan coastlines can reach up to 0.45 ICG/km²/year, both polarities combined.

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CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

In this paper we provide details about a performance parameter of the EUCLID lightning location system (LLS) called the percentage of outliers. The term outlier means an event (CG stroke or IC pulse) located by the LLS on a wrong place. In this study we use data from weather radar networks in two regions of the EUCLID network (Belgium and Austria) to distinguish between outlier and non-outlier. It is shown that the percentage of outliers is sensitive to changes in the network and also changes related to the location algorithm itself. The overall percentage of outliers for both regions is between 0.7% and 1.9% for a distance to the nearest precipitation of 2km.

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CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

Since 2008, measurements of natural cloud-to-ground (CG) lightning were performed during warm season thunderstorms in the Alpine region of Austria to generate a ground truth data set of lightning strikes. Those measurements were performed with a mobile high speed video and an electric field recording system (VFRS) to observe the optical properties of lightning discharges and to record the ambient electric field. In 2015, the VFRS`s high speed camera was upgraded in order to significantly increase the optical and temporal resolution of the video data. Due to the upgrade it was possible to record a high quality data set during 20 thunderstorm days at 15 different sites between May and August 2015.
For this paper these data sets of the Alpine region are used to analyze possible reasons for the detected variation of single-stroke flashes. The ground truth data sets are also compared to formerly published values from different countries. To provide additional information, data of the Austrian Lightning Location System (LLS), ALDIS (Austrian Lightning Detection and Information System), is compared to the VFRS ground truth data, operated by Graz University of Technology, to analyze the reasons for the varying amount of single-stroke flashes in the considered region. Thunderstorm types are classified with radar data and with wind measurements in order to investigate the effect of thunderstorm organization on their lightning characteristics.
Compared to values from the literature the percentage of single-stroke flashes in this study present a higher value for negative flashes (26 %) and values in the same range for positive flashes (89 %).
Results of this report shall contribute to a better understanding of the lightning process in general and the behavior of thunderstorms in the Alpine region in particular.

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Electric Power Systems Research (EPSR) Vol. 153, Pages 38-45, 2017

DOI: https://doi.org/10.1016/j.epsr.2016.07.014

Rubinstein et al. (2012) [1] examined a method to compare electric fields from lightning discharges measured with analogue integrators of different time constants. The time constant distorts the waveforms and has to be corrected numerically. We have extended the work of Rubinstein et al. (2012) [1] by considering the antenna characteristics in the system equations. In this paper we focus on the compensation of the integrators time constant and present some cases and results after applying the method. Furthermore we discuss the importance of any existing offset errors. A simple approach for handling the offset will be presented. Examples and determination of continuing currents are given in section IV. Advantages of the compensation method are mentioned in the conclusion. Together with Appendix A (system equations) this paper can be seen as a reference to a profound understanding of the measurement methods of E-fields with flat plate antennas for lightning researchers.

4^th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

This paper is dedicated to the question of measuring lightning current events on tall objects such as wind turbines. Because of their height, location at the open or uplands area, the probability of lightning strike increases significantly. Modern wind power plants with total height up to 200 m are able to trigger upward or ground-to-cloud flashes, especially during winter season, which are different from downward flashes. In general, upward lightning is critical for the air-termination system of wind turbine with regard to transferred charge, which can easily exceed the value of 300 As specified for lightning protection level I (LPL I) in the international standard IEC 62305 [1]. For proper operation and efficient maintenance regimes measurement of the lightning events on wind turbines is needed. The measuring principle, based on Rogowski coil sensors is presented in this paper. The data obtained from the measuring system allow to evaluate the effects of lightning strikes on wind turbines. Some specifics during the measuring of lightning events on tall objects are discussed as well. In particular, the peak value of lightning impulse currents, to be able to be measured, should be greater than 200 kA specified again for LPL I. Also upward lightning may have only a long duration initial continuing current (ICC-only), which is difficult to be detected by LLS and to be measured. Both of these lightning parameters have different effects on components of wind turbines, which are discussed in detail in the paper.

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4^th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

Upward lightning initiated from tall objects has gained considerable interest in the past years. Modern wind turbines are reaching total heights of 200 m and more and these structures are often exposed to this type of discharge. The data set collected by directly measured lightning current waveforms at the Gaisberg Tower (GBT) in Austria is used to evaluate the seasonal variations in the occurrence of upward lighting and variations in their current parameters. Initial continuing currents with superimposed pulses (ICCP) transfer the highest amounts of charge and occur mostly during non-convective season. All flashes with transferred charge exceeding 300 As occurred during non-convective season and were mostly of negative polarity. All days, where more than 10 flashes were recorded at the GBT within 24 hours, belong to the non-convective season.

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4^th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

A new approach is presented to evaluate lightning incidence for the cases when wind turbines are located close to each other and in complex terrain. Lightning incidence to a wind turbine park consisting of 16 wind turbines and located on the Mont-Crosin Mountain in Switzerland is analysed. A significant increase in the number of upward flashes is observed following the construction of the new wind turbines and replacement of the old wind turbines in the area of Mont Crosin.

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International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

Although there are some data on lightning attachment to tall towers (height over 60 m), there are no observational data of lightning attachment to common structures or buildings (under 60 m) that are present in almost every city. In this paper we analyze current measurements of upward leaders induced by a downward negative lightning flash that struck a building located in São Paulo, Brazil. The attachment process was recorded by two high-speed cameras running at 37,800 and 70,000 images per second, two current sensors and an electric field sensor.

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International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

DOI: https://10.1109/SIPDA.2017.8116921

In this paper, we present and discuss simultaneous records of current and wideband electric field waveforms at 380 km distance from the strike point associated with an upward bipolar flash initiated from the Säntis Tower. The flash contains 23 negative strokes and one positive stroke. The height of the ionospheric reflection for the positive pulse was inferred to be about 94.9 km, a value which is significantly higher than for negative pulses of this same flash, which range from 73 to 81 km. It is also found that the ratio of the peak field to the current peak is about two times smaller for the positive pulse compared to negative pulses. This difference can be attributed to a lower return stroke speed for the positive stroke compared to that for negative strokes, and also to the fact that the enhancement of the electric field due to the presence of the tower and the mountain might be more significant for negative pulses, which are characterized by faster risetimes, than for the positive pulse.

Electric Power Systems Research (EPSR), Vol. 153, Pages 10-18, 2017

DOI: https://doi.org/10.1016/j.epsr.2017.05.010

Current generation and current dissipation return stroke models are engineering models based on the theory associated with the propagation of current pulses along transmission lines undergoing corona. However, neither of these models incorporates the complete theory associated with the phenomenon. One can make the physical scenario complete by combining the current generation concept with the current dissipation concept. In this paper how this can be done is demonstrated by creating a return stroke model which is a combination of these two model types. The new model encompasses the full theory associated with the pulse propagation along transmission lines under corona. The paper provides a full description of the model together with a description of the spatial and temporal variation of the return stroke current and the electric and magnetic fields generated at different distances as predicted by the model.

Journal of Geophysical Research: Atmospheres, Vol. 122, Issue 21, Pages 11,760-11,775, 2017

DOI: https://doi.org/10.1002/2017JD027520

In this paper, we analyze the location error of time of arrival (ToA)-based lightning location
systems (LLSs) caused by propagation effects over mountainous terrain around the Säntis Tower located in the Swiss Alps. The study is based on a full-wave three-dimensional (3-D) finite difference time domain approach using the topographic map including the Säntis Tower and the nearby sensors belonging to LLSs. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity and that the location error associated with the ToA technique depends strongly on the used onset time estimation technique. The evaluated location errors associated with amplitude thresholds of 10% and 20% and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest (about 300mor less). Finally, we assess the accuracy of two simplified methods (terrain envelope method and tight-terrain-fit method) to account for the location error due to propagation over mountainous terrain. These two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a nonflat terrain by using available topographic data. In addition, a possible real-time location error compensation algorithm using the elongated propagation path method to improve the location error of the LLSs in mountainous regions is presented and discussed.

Atmospheric Measurement Techniques, Vol. 10, Issue 11, Pages 4561-4572, 2017

DOI: https://doi.org/10.5194/amt-10-4561-2017

Lightning data as observed by the European Cooperation for Lightning Detection (EUCLID) network are used in combination with radar data to retrieve the temporal and spatial behavior of lightning outliers, i.e., discharges located in a wrong place, over a 5-year period from 2011 to 2016. Cloud-to-ground (CG) stroke and intracloud (IC) pulse data are superimposed on corresponding 5 min radar precipitation fields in two topographically different areas, Belgium and Austria, in order to extract lightning outliers based on the distance between each lightning event and the nearest precipitation. It is shown that the percentage of outliers is sensitive to changes in the network and to the location algorithm itself. The total percentage of outliers for both regions varies over the years between 0.8 and 1.7 % for a distance to the nearest precipitation of 2 km, with an average of approximately 1.2 % in Belgium and Austria. Outside the European summer thunderstorm season, the percentage of outliers tends to increase somewhat. The majority of all the outliers are low peak current events with absolute values falling between 0 and 10 kA. More specifically, positive cloud-to-ground strokes are more likely to be classified as outliers compared to all other types of discharges. Furthermore, it turns out that the number of sensors participating in locating a lightning discharge is different for outliers versus correctly located events, with outliers having the lowest amount of sensors participating. In addition, it is shown that in most cases the semi-major axis (SMA) assigned to a lightning discharge as a confidence indicator in the location accuracy (LA) is smaller for correctly located events compared to the semi-major axis of outliers.

10^th Asia-Pacific International Conference on Lightning (APL), Thailand, Krabi, 2017

Lightning Location Systems (LLS) are the most common way to geolocate lightning. The performance characteristics of the Austrian Lightning Detection and Information System (ALDIS) as an Austrian ground-based LLS are determined by their ability to detect lightning flashes and strokes as well as their ability to geolocate lightning events accurately and provide relevant information such as time, peak current, number of strokes, etc. The performance of LLS is very important for LLS operators as well as for users to get the right information about lightning. During the years the configuration of ALDIS in terms of hardware and software changed several times resulting in continuous performance improvements. Comparing ALDIS data with data of a Video and Field Recording System (VFRS) is a ground-truth data method that can be used to validate the performance characteristics of LLS for a large region. The VFRS consists of a high speed camera, an electric field sensor and a GPS clock for time synchronization. Performance characteristics of ALDIS for cloud-to-ground lightning were obtained by comparing ALDIS data with VFRS data recorded during a Lightning Observation Project in 2015. VFRS data were recorded between May and August 2015 during 20 days. In the recorded data, 153 negative, 28 positive and 6 bipolar flashes could be identified. The most important performance parameters of LLS are the Detection Efficiency (DE) of cloud-to-ground (CG) flashes and strokes and the Location Accuracy (LA) of the located strokes. Our analyses showed that the DE for negative flashes and strokes are 96,1% and 87,9% respectively and 100% and 91,2% for positive flashes and strokes, respectively. The median LA could be identified to be 111 m.

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

DOI: https://10.1109/SIPDA.2017.8116925

Transient measurements in several substations in the Austrian transmission system were performed in the last years. Based on these measurements it was possible to detect signal sequences which show direct and nearby lightning discharges as well as the coupling effect to phases after lightning strikes to the ground wire. These transient voltages were measured by using resistive-capacitive voltage dividers to generate a high quality dataset. The high bandwidth of the RC-divider and its resulting frequency stability is needed to analyzed such transient events. A correlation between the measurements and the data of the Austrian Lightning Location System was performed. In this paper, three different representative measurements of atmospheric discharges to or close to transmission lines will be presented.

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

DOI: https://10.1109/SIPDA.2017.8116896

This work compares the classification accuracy (CA) of two algorithms applied to data from the EUCLID lightning location system (LLS). As CA we call the accuracy of a LLS to correctly distinguish between cloud-to-ground (CG) and intra cloud (IC) discharges. The ground-truth data, used for this evaluation, was taken from optical and electric field data measured in various regions in Austria (2012 and 2015) and France (2014). The data set contains CG and IC discharges of positive and negative polarity. The data set was split up into further sub-categories as long as the number of data was still sufficient to give reasonable results. For a coarse overview of the algorithm performances, the total CA was first calculated for each year and country for both polarities. Furthermore, for the class of CG discharges, the CA of first return strokes, the CA of subsequent return strokes (with and without respect to the polarity) as well as the CA of IC events with respect to their polarity was evaluated. Specifically the subdivision into classes of amplitudes of the peak currents for different events can give further insight to the performance of the algorithms. For that reason the total CA, the CA of negative and positive events and CG and IC was analyzed.
The evaluation shows that in combination with the new sensor data format LS the new algorithm exhibits an improvement of 2% at the CA. In combination with the old data format IMPACT, the new algorithm performs worse than the old one. In total, IC events have been classified much better by the new algorithm, irrespective of the sensor data format. CG discharges on the other hand show a worse CA throughout all years for the new algorithm.

Upward lightning initiated from tall objects is classified in two distinct groups. Self-initiated lightning occurs when the slow charge buildup produced by cloud electrification is sufficient to start the upward propagating leader. Nearby-lightning-triggered flashes are initiated by the rapid field changed caused by nearby lightning activity, either to ground or inter-cloud. In this paper we analyzed 307 upward flashes initiated from the Gaisberg Tower in Austria and based on measured electric fields at close distance to the tower 80 % were classified as self-initiated and 20 % as nearby-lightning-triggered. Some storms produced exclusively one type of discharges. In case of self-initiated lightning the median height of the -10°C isotherm was in the range of 3000 m, whereas in case of nearby-lightning-triggered lightning the isotherm was at a height of 4800 m.

Natural Hazards and Earth System Sciences, Vol. 16, Issue 2, Pages 595-605, 2016

DOI: https://doi.org/10.5194/nhess-16-595-2016

In this paper we present a performance analysis of the European lightning location system EUCLID for cloud-to ground flashes/strokes in terms of location accuracy (LA), detection efficiency (DE) and peak current estimation. The performance analysis is based on ground truth data from direct lightning current measurements at the Gaisberg Tower (GBT) and data from E-field and video recordings. The E-field and video recordings were collected in three different regions in Europe, namely in Austria, Belgium and France. The analysis shows a significant improvement of the LA of the EUCLID network over the past 7 years. Currently, the median LA is in the range of 100m in the center of the network and better than 500m within the majority of the network. The observed DE in Austria and Belgium is similar, yet a slightly lower DE is determined in a particular region in France, due to malfunctioning of a relevant lightning location sensor during the time of observation. The overall accuracy of the lightning location system (LLS) peak current estimation for subsequent strokes is reasonable keeping in mind that the LLS-estimated peak currents are determined from the radiated electromagnetic fields, assuming a constant return stroke speed. The results presented in this paper can be used to estimate the performance of the EUCLID network related to cloud-toground flashes/strokes for regions with similar sensor baselines and sensor technology.

Natural Hazards and Earth System Sciences, Vol. 16, Issue 2, Pages 607–616, 2016

DOI: https://doi.org/10.5194/nhess-16-607-2016

Cloud-to-ground (CG) lightning data from the European Cooperation for Lightning Detection (EUCLID) network over the period 2006–2014 are explored. Mean CG flash densities vary over the European continent, with the highest density of about 6 km^-2 yr^-1 found at the intersection of the borders between Austria, Italy and Slovenia. The majority of lightning activity takes place between May and September, accounting for 85% of the total observed CG activity. Furthermore, the thunderstorm season reaches its highest activity in July, while the diurnal cycle peaks around 15:00 UTC. A difference between CG flashes over land and sea becomes apparent when looking at the peak current estimates. It is found that flashes with higher peak currents occur in greater proportion over sea than over land.

Journal of Geophysical Research: Atmospheres, Vol.121, Issue 2, Pages 595–606, 2016

DOI: https://doi.org/10.1002/2015JD024259

In this paper, we present a performance analysis of the European Cooperation for Lightning Detection (EUCLID) lightning detection network using data obtained on lightning currents measured at the Säntis Tower (located in northeastern of Switzerland) from June 2010 to December 2013. In the considered period of analysis, a total number of 269 upward negative flashes were recorded at the Säntis Tower. The performance of the EUCLID lightning detection network is evaluated in terms of detection efficiency, location accuracy, and peak current estimates for upward flashes. Excluding flashes containing only an initial continuous current with no superimposed pulses exceeding 2 kA, the flash detection efficiency for upward flashes is estimated to be 97%. The recorded flashes contained a total of 2795 pulses (including return strokes and International Conference on Communications pulses characterized by risetimes lower than 8μs and peaks greater than 2kA). The overall pulse detection efficiency was found to be 73%. For pulses with peak values higher than 5 kA, the pulse detection efficiency was found to be about 83%. Peak current estimates provided by the EUCLID network were found to be significantly larger than their directly measured counterparts. This overestimation might be attributed to the enhancement of the radiated electromagnetic fields associated with the presence of the tower and themountain. Themedian of the absolute distance error, defined as themedian distance between the Säntis Tower location and the EUCLID’s stroke locations, was found to be 186m, the majority of large location errors being associated with measured current peaks lower than 10 kA. The analysis revealed also that the location accuracy of the EUCLID network improved significantly in 2013 as a result of an upgrade in the location algorithms to take into account propagation effects.

Journal of Geophysical Research: Atmospheres, Vol. 121, Issue 2, Pages 895–911, 2016

DOI: https://doi.org/10.1002/2015JD024234

In this paper, we analyze the propagation effects on lightning-radiated electromagnetic fields over mountainous terrain by using a three-dimensional (3-D) finite difference time domain (FDTD) method. We also discuss the time delay error in the time-of-arrival (ToA) technique currently used to locate lightning in detection networks, specifically. Furthermore, the accuracy of different approximate methods presented in the literature is discussed and tested by using our 3-D FDTD method. It is found that (1) the time delays and amplitudes of the lightning-radiated electromagnetic fields can be significantly affected by the presence of a mountainous terrain and associated diffraction phenomena; (2) for a finitely conducting ground, the time delay shows a slight increase with the increase of the observation distance, but the time delay resulting from the finite ground conductivity appears to be smaller than that caused by the mountainous terrain; and (3) the timing error associated with the ToA technique depends on the threshold times. Threshold times of 10% and 20% of the peak provide very similar results compared to those corresponding to the peak of the first derivative of the magnetic field, and the threshold time exceeds 50% of the initial rising amplitude of the signal. Furthermore, we have assessed the accuracy of two simplified methods (terrain-envelope method and tight-terrain fit method) to account for the time delays resulting from the propagation in a mountainous terrain. It is found that both methods result in time delays that are in reasonable agreement but always overestimating the results obtained using the full-wave 3-D FDTD approach for the perfectly conducting ground. These two methods represent interesting alternatives to account for the time delay over a nonflat terrain using the terrain model.

Journal of Geophysical Research: Atmospheres, Vol. 121, Issue 8, Pages 3929-3953, 2016

DOI: https://doi.org/10.1002/2015JD023745

In the framework of the European HyMeX project, a field campaign devoted to the study of electrical activity during storms took place in the South of France in 2012. An acoustic station composed of four microphones and four microbarometers was deployed within the coverage of a Lightning Mapping Array network. On the October 26 2012, a thunderstorm passed just over the acoustic station. Fifty six natural thunder events, due to cloud-to-ground and intra-cloud flashes, were recorded. This paper studies the acoustic reconstruction, in the low frequency range from 1 to 40 Hz, of the recorded flashes and their comparison with detections from electromagnetic networks. Concurrent detections from the EUCLID European lightning location system were also used. Some case studies show clearly that acoustic signal from thunder comes from the return stroke but also from the horizontal discharges which occur inside the clouds. The huge amount of observation data leads to a statistical analysis of lightning discharges acoustically recorded. Especially, the distributions of altitudes of reconstructed acoustic detections are explored in detail. The impact of the distance to the source on these distributions is established. The capacity of the acoustic method to describe precisely the lower part of nearby cloud-to-ground discharges, where the Lightning Mapping Array network is not effective, is also highlighted.

24^th International Lightning Detection Conference and 6^th International Lightning Meteorology Conference (ILDC/ILMC), San Diego, California, USA, 2016

In this study, cloud-to-ground (CG) and cloud-to-cloud (CC) stroke data are superimposed on corresponding quantitative precipitation estimations (QPE) derived from radar observations in order to extract the percentage of lightning outliers, i.e. ‘fake’ or ‘ghost’ strokes, based on the distance between each lightning event and the nearest precipitation. Applying this to a large dataset from 2006-2015 it is possible to analyze the behavior of outliers over time with respect to the performance of the EUCLID network. We find that the introduction of the newest sensor technology has a positive impact on the occurrence of outliers over the years with a clear drop from 2011 onwards. Outside the European summer thunderstorm season the percentage of outliers tends to increase somewhat. This increase results from an underestimation of the precipitation by the radar at the outer radar observation boundary. The latter in its turn could be due to the fact that in general winter storms are less vertically developed compared to summer storms. In addition, it is shown that the majority of the semi-major axis (SMA) assigned to a lightning discharge is much smaller for non-outlier events compared to the SMA of outliers retrieved by this method.

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24^th International Lightning Detection Conference and 6^th International Lightning Meteorology Conference (ILDC/ILMC), San Diego, California, USA, 2016

In this paper we want to shed some light on the relation between the DE and the average number of sensors reporting (ANSR). For this purpose we use a simple detection efficiency (DE) model. We validate the simple DE model with real data from the EUCLID network and show that even with such a simple model the agreement between the modeled and the observed ANSR is reasonable. We further show that observed ANSR cannot be used to estimate the DE for networks containing only a few sensors and networks with large sensor baselines. In such networks, more advanced analysis and modeling of the full NSR distribution is necessary. In general, we suggest that the probability of DE given a certain ANSR is a more reasonable way to describe the ANSR-DE relationship.

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CIGRE C4 International Colloquium on EMC, Lightning and Power Quality
Considerations for Renewable Energy Systems, 2016

Cloud-to-ground (CG) flashes in the Alpine Region show some specific characteristics not being observed in other regions of the world. One of them is the observation of a 10% to 15% higher percentage of negative single stroke flashes during warm-season thunderstorms. Such a high percentage of single stroke negative CG flashes is a unique observation and deserves a scientific investigation. Besides that, a lack of information to confirm or refute a geographical dependence of the majority of lightning parameters is currently present and most of the lightning parameters for lightning protection were determined more than 35 years ago. As the design of lightning protection systems is based on lightning parameters, further research on their regional suitability is of interest for the society. In the course of this research project, root causes for the high percentage of negative single stroke flashes in the Alpine Region are closely investigated and a detailed study on flash and stroke parameters is performed. On-site time correlated high-speed-videos in conjunction with E-field measurements during different storms are conducted and sites with orographically exposed areas such as mountain tops and mountain crests are the main observation areas. Additionally wind turbines, antennas and transmission line towers under alpine conditions are in the focus. Lightning location data and meteorological data are used as supplementary data in order to base the analysis on comprehensive flash and storm information. The captured high resolution video and E-field data in conjunction with lightning location data and meteorological data will allow to identify the reasons for the deviating CG flash conditions as well as to extract properties and behavior of natural lightning in general. Investigations within the context of the research will provide comprehensive flash and stroke parameters considering alpine orography and exposed infrastructure. Results of this project will give an elaborated data set to estimate the hazard potential due to negative CG lightning and will contribute to a better understanding of the discharge behavior of lightning in general.

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International Colloquium on Lightning and Power Systems (CIGRE), Bologna, Italy, 2016

In this paper we show the first performance analysis of the BLIDS/EUCLID lightning location system (LLS) with data from direct current measurements at the Peissenberg Tower. We evaluate the performance of the BLIDS/EUCLID LLS in terms of detection efficiency (DE), location accuracy (LA) and peak current estimation. The flash/stroke DEs determined in this paper (100%/81%) are in good agreement to the results determined at the Gaisberg Tower in Austria. We further show that at the Peissenberg Tower all strokes greater than 10 kA were detected by the LLS.

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33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

DOI: https://10.1109/ICLP.2016.7791501

In this paper we show an analysis regarding the Peak-to-Zero (PTZ) time, the rise time, and the peak current for strokes to tall objects. It was previously shown that field pulses radiated by return strokes to the Gaisberg Tower (GBT) exhibit a smaller PTZ time than natural CG strokes [1]. We compare field data (LLS sensor reported field parameters) from strokes to the GBT to field data from strokes detected during winter thunderstorms. We show that winter thunderstorms exhibit smaller PTZ times independent if the strike is to a tall object or not. We also present evidence that the PTZ time does not depend on the lightning channel length, using the altitude of the -10° C isotherm as a proxy of the channel length. The -10° C isotherm is often assumed to be the altitude of the negative charge center. The results obtained in this study indicate that the hypothesis of an influence of the channel length on the resulting PTZ time is not valid.

33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

The question of interest is the charge transfer of lightning flashes. On instrumented towers, it is possible to measure these values. In many countries there is no equipment installed on the towers in order to measure the charge transfer. The other fact is that there are just a few cases of downward flashes occurring in towers around the world. In this paper, a method for estimation of charge transfer will be present. We used data from Brazil, USA and Austria for the analysis and comparison.

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33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

DOI: https://10.1109/ICLP.2016.7791448

Wind turbines are very exposed structures to lightning due to their height and installation in unshielded areas and are therefore expected to influence the local lightning activity. However, it is unknown in what quality and quantity lightning is influenced. The following paper presents a study conducted at wind farms in Germany showing to what extent lightning activity is influenced by wind turbines. Conducting the analysis all lightning strokes of more than 50 wind farms over the period of 10 years were analyzed before and after the construction of the considered wind farms using the lightning detection system BLIDS [1].
Thereby both onshore and offshore wind farms are investigated. Examining lightning density, amplitudes and probability distributions of the considered wind farms a significant increase in lightning frequency was observed inside the impact area of the wind turbines both for onshore and offshore wind farms. With regard to amplitudes of negative lightning strokes no specific conclusion could be drawn for onshore wind turbines while amplitudes at offshore wind farm increased extremely.
The data obtained during current initial investigation can be used for further study to estimate the average number of lightning flashes to a single or a group of wind turbines. That can help to design a sufficient lightning and overvoltage protection system for wind turbines. However, one should take into account that detection efficiency (DE) of lightning detection systems for ground-to-cloud flashes (upward lightning) in special cases is reduced compared to cloud-to-ground flashes (downward lightning). In [2] a DE of 43 % is reported and explained by the fact that the ground-to-cloud lightning currents often are an initial continuous current only (ICCOnly), free from any superimposed impulse currents and therefore cannot be detected by lightning detection system at all. However this type of flashes could pose a risk to wind turbines because of their enormous high transfered charge values which can easily exceed 300 As. This specific type of ground-to-cloud flashes cannot be considered in this study because of reason mentioned above.

33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

DOI:https://10.1109/ICLP.2016.7791514

Severe weather events are responsible for hundreds of fatalities and millions of euros of damage every year in the Mediterranean basin. Lightning activity is a characteristic phenomenon of severe weather and often accompanies torrential rainfall, which under certain conditions like terrain type, slope, drainage and soil saturation can generate flash flood. Therefore, the improvement in forecast skill for those high impact weather events is one of the main challenges in early warning systems. On the line of this need the behavior of the Lightning Potential Index (LPI) is evaluated in different case studies involving complex terrain. Such index represents a measure of the potential for charge generation and separation that lead to total lightning occurrence in clouds (both IC and CG).

24^th International Lightning Detection Conference and 6^th International Lightning Meteorology Conference (ILDC/ILMC), San Diego, California, USA, 2016

DOI: https://doi.org/10.1002/2017JD027520

In this paper, we analyze the location error of Time of Arrival (ToA)-based lightning location systems (LLSs) resulting from propagation over mountainous terrain. For the analysis, we have considered the region around the Säntis Tower, located in the Swiss Alps. The study is based on a full-wave finite-difference time-domain (FDTD) approach and the two-dimensional (2D) topographic maps along the direct path between the Säntis Tower and nearby sensor sites. The accuracy of the ToA lightning location results associated with times of arrival determined 1) as the time intercepts of straight lines passing through the peak of the return stroke pulse and different amplitude threshold crossing points, 2) as the time of the peak of the first derivative of the field and 3) as the time of occurrence of the peak value of the field are evaluated by using our full-wave FDTD method. The evaluated location errors associated with amplitude threshold crossing points of 10% and 20% of the initial rising amplitude of the field were found to be the lowest.

33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

DOI: https://10.1109/ICLP.2016.7791360

Upward lightning flashes from towers are sometimes preceded by other lightning activity in the vicinity of the tower. This observation has led to a classification of upward tower lightning into self-initiated, which are not preceded by nearby lightning activity, and other-triggered, for which one or more CG or IC flashes occur within a given preceding interval and within a given distance from the tower. The causality relation between other-triggered flashes and the preceding activity has not been established. In this paper, we hypothesize that at least some of the activity prior to other-triggered tower flashes can be explained as being due to chance rather than causality.

33^rd International Conference on Lightning Protection (ICLP), Estoril, Portugal, 2016

Most of what is known about the electric current of downward flashes and striking distance of lightning protection systems come from information gathered on tall towers. There are no observational data of lightning attachment to common structures or buildings (under 60 m) that are present in almost every city. In order to study lightning strikes to common buildings, several instruments were installed in and around two identical buildings located in São Paulo city, Brazil. This paper describes the setup of electric field sensors, current transformers, X-ray sensors, high-speed video and standard video cameras. Some of the data already obtained in its first two months of operation are also shown.

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24^th International Lightning Detection Conference and 6^th International Lightning Meteorology Conference (ILDC/ILMC), San Diego, California, USA, 2016

We present simultaneous current and wideband electric field waveforms at 380 km associated with upward flashes initiated from the Säntis Tower. To the best of the authors’ knowledge, the presented dataset in this study includes the first simultaneous records of lightning currents and associated fields featuring ionospheric reflections, and the longest distance at which lightning fields have been measured simultaneously with the current. Electric field data are used to evaluate ionospheric reflection characteristics during day and night times using the so-called zero-zero and peak-peak methods. During daytime, the estimates for the ionospheric reflection height is about 80 km, corresponding to the D layer. The estimated height at night time is about 90 km, corresponding to the E layer. Finally, we present a full-wave, finite-difference time-domain (FDTD) analysis of the field propagation including the effect of the ionospheric reflection and compare the results with experimental data.

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24^th International Lightning Detection Conference and 6^th International Lightning Meteorology Conference (ILDC/ILMC), San Diego, California, USA, 2016

Lightning incidence analysis is required for the design of adequate lightning protection systems for wind turbines. We present a method to estimate the number of upward flashes for multiple wind turbines located in mountainous areas. The proposed method is applied to the case of the Mont Crosin wind turbine park located in the Jura mountains in the Northwestern part of Switzerland. A significant increment of the number of upward flashes was observed with the installation of new power units.

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World meeting on Lightning (WOMEL), Cartagena, Colombia, 2016

With the installation of the Austrian Lightning Detection & Information System (ALDIS) in 1991 a new area of lightning research started. Austria has a rather complex terrain and the south eastern regions of the country together with Northern Italy and Slovenia show one of the highest flash density values observed in all Central Europe. Performance evaluation of lightning location system (LLS) and validation of the data provided by the LLS became the main focus of the research activities from the very beginning. As ground truth reference lightning current measurements on an instrumented tower (Gaisberg Tower) and GPS time synchronized video and field recordings are used. This paper is an overview of some of the main research results collected and published in the last 25 years by the ALDIS research team. Although lightning research in general has made significant progress in the last decades, supported by new and improved observation technologies (e.g. high speed cameras or 3D-lightning location systems) there is still a number of open questions. Some of the topics waiting for better understanding or experimental validation are briefly discussed at the end of this paper.

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International Conference on Environment and Electrical Engineering (EEEIC), 2015

In recent years the number of wind turbines installed in Europe and other continents has increase dramatically. Appropriate lightning protection is required in order to avoid costly replacements of lightning damaged turbine blades, components of the electronic control system, and/or temporary loss of energy production. Depending on local site conditions elevated objects with heights of 100 m and more can frequently initiate upward lightning. From the 100 m high and instrumented radio tower on Gaisberg in Austria more than 50 flashes per year are initiated and measured. Also lightning location systems or video studies in Japan [1], [2] or in the US [3] show frequent occurrence of lightning initiated from wind turbines, especially during cold season. Up to now no reliable method exists to estimate the expected frequency of upward lightning for a given structure and location. About half of the flashes observed at the GBT are of ICCOnly type. Unfortunately this type of discharge is not detected by lightning location systems as its current waveform does not show any fast rising and high peak current pulses as typical for first or subsequent return strokes in downward lightning (cloud-toground). Nevertheless some of this ICCOnly type discharges transferred the highest amount of charge, exceeding the 300 C specified in IEC 62305 for lightning protection level LPL I.

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Atmospheric Measurement Techniques, Vol. 8, Issue 2,  AMT, 8, 649–669, 2015

DOI: https://doi.org/10.5194/amt-8-649-2015

The PEACH project (Projet en Electricité Atmosphérique pour la Campagne HyMeX – the Atmospheric Electricity Project of the HyMeX Program) is the atmospheric electricity component of the Hydrology cycle in the Mediterranean Experiment (HyMeX) experiment and is dedicated to the observation of both lightning activity and electrical state of continental and maritime thunderstorms in the area of the Mediterranean Sea. During the HyMeX SOP1(Special Observation Period) from 5 September to 6 November 2012, four European operational lightning locating systems (ATDnet, EUCLID, LINET, ZEUS) and the HyMeX lightning mapping array network (HyLMA) were used to locate and characterize the lightning activity over the northwestern Mediterranean at flash, storm and regional scales. Additional research instruments like slow antennas, video cameras, microbarometer and microphone arrays were also operated. All these observations in conjunction with

operational/research ground-based and airborne radars, rain gauges and in situ microphysical records are aimed at characterizing and understanding electrically active and highly precipitating events over southeastern France that often lead to severe flash floods. Simulations performed with cloud resolving models like Meso-NH and Weather Research and Forecasting are used to interpret the results and to investigate further the links between dynamics, microphysics, electrification and lightning occurrence. Herein we present an overview of the PEACH project and its different instruments.

Examples are discussed to illustrate the comprehensive and unique lightning data set, from radio frequency to acoustics, collected during the SOP1 for lightning phenomenology understanding, instrumentation validation, storm characterization and modeling.

Earth and Space Science, Vol. 2, Issue 4, Pages 65-93, 2015

DOI: https://doi.org/10.1002/2014EA000051

Ground-based and satellite-based lightning locating systems are the most common ways to detect and geolocate lightning. Depending upon the frequency range of operation, LLSs may report a variety of processes and characteristics associated with lightning flashes including channel formation, leader pulses, cloud-to-ground return strokes, M-components, ICC pulses, cloud lightning pulses, location, duration, peak current, peak radiated power and energy, and full spatial extent of channels. Lightning data from different types of LLSs often provide complementary information about thunderstorms. For all the applications of lightning data, it is critical to understand the information that is provided by various lightning locating systems in order to interpret it correctly and make the best use of it. In this study, we summarize the various methods to geolocate lightning, both ground-based and satellite-based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately with high detection efficiency and with low false detections and report various features of lightning correctly. Different methods or a combination of methods may be used to validate the performance characteristics of different types of lightning locating systems. We examine these methods and their applicability in validating the performance characteristics of different LLS types.

Journal of Atmospheric and Solar-Terrestrial Physics (JASP), Vol. 125-126, Pages 38-49, 2015

DOI: https://doi.org/10.1016/j.jastp.2015.02.008

We examine properties of pulses superimposed on the slowly varying initial-stage current in upward flashes initiated from the Gaisberg Tower (GBT), Austria, based on simultaneous measurements of currents, electric field changes, and high-speed video images. These pulses, often referred to as initial continuous current (ICC) pulses, are associated with the M-component mode of charge transfer to ground, if only one branch of the upward lightning channel is active. However, due to multiple branches formed by an upward leader from the tall tower, ICC pulses are often associated with a downward leader/return-stroke process in a decayed (new) channel branch that is connected to another, continuous current carrying channel, with the connection point being some tens to a few hundreds of meters of the tower top. We call this scenario mixed mode of charge transfer to ground, which optically appears as a re-illuminated (previously luminous) or newly illuminated branch connecting to the already luminous channel attached to the tower. If the connection point were a kilometer or more above the tower top (inside the cloud), the resultant ICC pulse measured at the tower top would appear as a “classical” M component, and if it were very close to the tower top (say, within a few meters), the ICC pulse would be characteristic of a return stroke. In contrast to tower-initiated lightning, ICC pulses in rocket-triggered lightning (at least in Florida and China), usually involve only one channel below the cloud base and hence are associated predominantly with the M-component mode of charge transfer to ground. In our data set, ICC pulses associated with the mixed mode of charge transfer to ground exhibit shorter risetimes, larger peaks, and shorter half-peak widths than “classical” M-components, as previously reported for lightning initiated from tall objects by Miki et al. [2005]. We found that the mixed mode of charge transfer to ground can also occur in M-components following return strokes in upward lightning.

9th Asia-Pacific International Conference on Lightning (APL), Nagoya, Japan, 2015

Upward initiated lightning from tall structures has become a major topic in lightning research and lightning protection. Wind turbines of heights of 150 m and more are frequently initiating upward lightning and these discharges may cause severe damage. Upward initiated lightning shows a wide variety of waveform characteristics and does often not contain any return strokes. Lightning location systems (LLS), such as the EUCLID network, are typically detecting return strokes and therefore performance of LLS in detecting upward lightning is very different from the performance to detect downward lightning. Analyzing the lightning data collected at the Gaisberg Tower (GBT) in Austria from 2000 – 2013 we determine a flash detection efficiency (DE) of 43 %. Different from natural CG, lightning where it is mostly the small peak current events that are not located, the low DE of upward lightning is determined by the occurrence of ICCOnly type discharges which are not detected at all. Some of these not located ICCOnly discharges showed a total charge transfer exceeding 300 As and in case of wind turbines those flashes have certainly the potential for severed blade damage. As a result of the significantly shorter peak-to-zero times of the radiated fields from return strokes to the GBT 31% of these return strokes were classified as IC discharges by the LLS.

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9th Asia-Pacific International Conference on Lightning (APL), Nagoya, Japan, 2015

The Austrian lightning location system ALDIS (Austrian Lightning Detection and Information System) has been in operation for more than 20 years. During this time the system has almost continuously been upgraded and improved. This paper gives an overview of the used methods to evaluate the location accuracy, the main improvements in the network during the last 10 years and their resulting impact on the location accuracy of the network.

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International Symposium on Lightning Protection (XIII SIPDA), Balneário Camboriú, Brazil, 2015

Rubinstein et al. [1] presented the measurement of the electric field strength during lightning discharge with analogue integrators as amplifiers and their numerical correction of the time constant that is needed by means of stability of the integrator. We have extended his work by incorporating the antenna characteristic into the system equations. In this paper we focus on the compensation of the integrator time constant (Eq. (13), [1]). We also defined the parameter ka which was introduced in Eq. (6) of [1]. Further, we analyzed and present results of that compensation method for time synchronized E-field measurements with two different integrators, so called E-slow and E-fast, recorded in Sao Paulo City on March 1st, 2014. In this context we discovered the importance of offset errors that exist. It will be discussed, why the offset of the system has a large influence while using the method of compensation and a simple approach for handling the offset will be presented. Additionally we show that this compensation method can be used to determine continuing currents by applying this method to fast E-fields. To verify this we used a sample of recorded current and fast E-field at Gaisberg Tower in Salzburg, Austria. The advantage of the compensation method for E-fast in comparison to an almost ideal integrator (E-slow) regarding the gain and quantization noise will be mentioned in the conclusion. The Appendix A contains the description of the system in the Laplace domain with useful simplifications, and shows the Bode plots.

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International Symposium on Lightning Protection (XIII SIPDA), Balneário Camboriú, Brazil, 2015

Since 2012 upward flashes have been observed in two locations in Sao Paulo City: Jaraguá Peak and Paulista Avenue. TV and Radio towers are located on the top of a steep hill called Jaraguá Peak. Paulista Avenue is a very busy complex of several buildings with tall towers on top. Upward flashes were registered from towers for both locations. For one of the events we observed upward leaders from both locations even though they are 11 kilometers apart. In order to understand which meteorological and terrain conditions are propitious for upward leader initiation, 83 flashes were analyzed and the results are presented in this paper. An analysis of the mountain profile and a comparison between Jaraguá Peak and other towers around the world used in lightning incidence studies is shown in this paper. The analysis is done by using 3 methods to calculate the effective height of the towers proposed by the literature.

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32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI: https://10.1109/ICLP.2014.6973097

During the last years the ground strike points of a flash got more attention because it was realized that risk estimation should not be performed with flash densities but with ground strike point densities. In countries with a lightning location system (LLS) the ground flash densities are normally derived from the LLS data. Recently an effort has been made to derive also ground strike points and ground strike point densities from LLS data [1], [2]. Detection efficiencies (DE) for flashes are well understood and often used to correct the ground flash densities. In this paper we show that also ground strike point densities determined with a LLS exhibit a DE. We further present a theoretical estimation of this DE, which we validate with real data from video and E-field measurements.

XV International Conference on Atmospheric Electricity (ICAE), Norman, Oklahoma, USA, 2014

During the HyMeX (Hydrology cycle in the Mediterranean Experiment) SOP1 (Special Observation Period 1; September-November 2012) campaign, the New Mexico Tech Lightning Mapping Array in conjunction with four European operational lightning detection networks (ATDNET, UKMO; EUCLID; LINET, nowcast; ZEUS, NOA) recorded the total lightning activity over South-Eastern France. We present here observations collected during three different weather situations: one isolated thunderstorm occurring on the 5th of September, a multi-cellular system on the 24th of September (HyMeX IOP6 case) and the 14 November tornadic cell. So far the analysis of the lightning data has been focusing on some specific parameters or features like flash density, convection surge or intra-cloud ratio. We first briefly describe the instrumentation operated during the field campaign and the methodology applied to analyze the data. Some properties of the lightning activity (e.g. flash rate, intra-cloud ratio, altitude of flash triggering) are then discussed according to the type and the stage of convective clouds and related to the properties of the parent clouds as derived from concurrent radar and satellite observations. Further investigations on relating the lightning activity to the cloud properties are currently underway and results will be presented during the conference.

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XV International Conference on Atmospheric Electricity (ICAE), Norman, Oklahoma, USA, 2014

The PEACH (Projet en Electricité Atmosphérique pour la Campagne HyMeX) project is the Atmospheric Electricity component of the decadal HyMeX (Hydrology cycle in the Mediterranean Experiment) project. PEACH aims at measuring and analyzing the lightning activity and electrical state of thunderstorms over the Mediterranean Sea for the HyMeX Community. During the SOP1 (Special Observation Period 1; September-November 2012), records of four European operational lightning detection networks
(ATDNET, UKMO; EUCLID; LINET, nowcast; ZEUS, NOA) and the NMT Lightning Mapping Array were used to document the total lightning activity over South-Eastern France. Other research instruments such as electric field sensors (ALDIS; LA; NMT), video cameras (ALDIS; ONERA), micro-barometer and microphone arrays (CEA) were deployed to characterize the properties of the lightning flashes as well as the electrical state of parent thunderclouds. All these observations are used to describe the evolution of the electrical activity during the life cycle of SOP1 storms in conjunction with microphysics and kinematics description of the parent storms as derived from groundbased radar, ground-based and airborne in situ observations. Cloud models (WRF; MESO-NH with electrification and lightning schemes) are used to interpret the observational-based results. We will first present an overview of the observations collected during the SOP1. Results of flash-scale, storm-scale and regional-scale analysis will then be discussed. We will also introduce some of the products that will be made available to the HyMeX Community. Finally we will discuss on the next steps of the PEACH project. The French MISTRALS program and the ANR IODA-MED project support the PEACH project. Additional supports came from Université de Toulouse, the French LEFA-IDAO program, GOES-R Visiting Program.

8th HyMeX Workshop, 2014

During HyMeX (Hydrology cycle in the Mediterranean Experiment) SOP1 (Special Observation Period 1), a number of observational systems such as a Lightning Mapping Array (HyLMA), Operational Lightning Locating Systems (OLLSs), Slow Antennas (SLAs), a Fast Antenna as a part of the Video and Field Recording System (VFRS) and an Electric Field Mill (EFM) were operated in order to record the electrical state of thunderstorms and the lightning activity. This unique lightning dataset in Europe provides key information to investigate the properties of the lightning flashes. Each observation system/instrument itself has its advantages, disadvantages as well as limits. Taking simultaneous records of different systems/instruments for the same lightning flash into account, a more precise interpretation of each flash component can be done and several uncertainties, caused by the limits of the systems/instruments, can be eliminated. The temporal and spatial development of lightning discharges based on HyLMA, OLLS, SLA, VFRS and EFM data are investigated and interpreted. To start the analysis we chose the 24^th September IOP6 case which was characterised by a remarkable and well-sampled lightning activity over the HyMeX SOP1 Cevennes-Vivarais domain. The analysis procedures, the current state of the analysis as well as some interesting cases will be discussed.

8th HyMeX Workshop, 2014

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8th HyMeX Workshop, 2014

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8th HyMeX Workshop, 2014

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European Geosciences Union, General Assembly (EGU), 2014

Narrow ∼(1–2 s) infrasound pulses that followed, with ∼11 to ∼50 s delays, rapid changes of electrostatic field were observed by a microbarometer array in the Czech Republic during thunderstorm activity. The angles of arrival (azimuth and elevation) were analyzed for selected distinct events. Comparisons of distances and azimuths of infrasound sources from the center of microbarometer array with lightning locations determined by EUCLID lightning detection network show that most of the selected events are most likely associated with intra-cloud (IC) discharges. Preceding rapid changes of electrostatic field, potential association of infrasound pulses with IC discharges, and high elevation angles of arrival for near infrasound sources indicate that an electrostatic mechanism is probably responsible for their generation. It is discussed that distinguishing of the relative role of thermal and electrostatic mechanism is difficult, and that none of published models of electrostatic production of infrasound thunder can explain the presented observations precisely. A modification of the current models, based on consideration of at least two charged layers is suggested. Further theoretical and experimental investigations are however needed to get a better description of the generation mechanism of those infrasound pulses.

23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

In this paper we analyze LLS located discharges (return strokes and superimposed ICC pulses) to the Gaisberg Tower (GBT) in terms of their location accuracy and assigned confidence ellipse (often also called error-ellipse). From 2000 to 2013 EUCLID (ALDIS) located 681 return strokes and 779 ICC pulses in upward initiated flashes from the GBT. We found that for 49 % of the return strokes and for 48 % of the ICC pulses the true tower location was inside the 50% confidence ellipse assigned by the LLS to the LLS estimated striking point. After implementation of several improvements in the location algorithm median location accuracy for strokes to the GBT is in the range of 100 m. For the most recent years (GBT data since 2010) we observe a significantly higher than expected percentage of GBT location being included in the assigned confidence ellipse. Most likely this is a result of the discretization of the length of the semi-major axis of confidence ellipse in 100 m steps, which is in the same range as the median location accuracy.

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European Geosciences Union, General Assembly (EGU), 2014

Parameters of upward initiated lightning from tall objects are gaining increasing interest in recent years. This is a result of the installation of tall objects (wind turbines, radio towers, etc.) experiencing a high number of lightning flashes. Lightning current waveforms of flashes initiated from the Gaisberg Tower (GBT) in Austria are continuously measured since 1998. On average this radio tower (tower height 100 m) located on a small mountain (1287 m ASL) next to the city of Salzburg triggers about 60-70 flashes per year. More than 50% of the triggered flashes occurred during cold season (similar to so-called winter lightning in Japan) and more or less independent of the overall thunderstorm activity in Austria. Up to now, more than 800 flashes have been recorded by employing a 0.25 m shunt at the tower top. Compared to inductive sensors, with their limited lower bandwidth, a shunt allows correct measurement of the slowly varying, low amplitude initial continuing current (ICC). ICCs are lasting for several hundreds of milliseconds and they are the main contributors to the transferred charge by a flash. Maximum transferred charge in a single flash was 783 C and this negative flash was recorded during cold season on October15th, 2012. Contrary to observations in winter lightning in Japan, where upward initiated flashes with very large charge transfer are predominantly bipolar, at the GBT 7 out of the 10 flashes exceeding a charge transfer of 300 C were negative, 2 were positive and 1 bipolar.

Most of the flashes (93%) triggered by the GBT are of negative polarity, initiated by a positive upward propagating leader. But also positive (4%) and bipolar (3%) flashes are observed at the GBT. In case of negative lightning flashes, 45% of current records exhibit an ICC only, 23% of flashes has pulses of peak current exceeding 2 kA superimposed on the ICC, and return strokes following the ICC after a period of “no current” is observed in 32% of the events. Median peak current of return strokes (N=913) is 9.2 kA and similar to values observed in triggered lightning and to lightning location system peak current estimates for subsequent strokes in cloud-to-ground lightning. Detectability of upward flashes by typical lightning location systems strongly depends on the presence of fast rising current pulses, either as return strokes or superimposed pulses on the ICC. In addition to the current records, corresponding vertical electric fields at close distance (170m) and far distance (about 100 km) are measured with flat-plate antennas. Upward initiated lightning often shows extensive branching and this is assumed to be the reason for the observation of rather complex overall current waveforms. In the presentation we will provide a review of the statistical analyses of the lightning parameters. Records of the E-fields at a distance of about 100 km and radiated by the return strokes to the tower show a significantly shorter peak-to-zero time (10 µs) than typically observed in cloud-to-ground lightning (30 – 40 μs).

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32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI: https://10.1109/ICLP.2014.6973112

Austria is one of the European countries with relatively high lightning activity [1] and lightning is a main cause of transmission line outages. In this presentation we have analyzed in detail lightning caused outages in the grid of the Austrian transmission system operator Austrian Power Grid (APG). 222 outage related lightning flashes were identified in the database of the Austrian lightning location system ALDIS as these flashes were located within a corridor of ±1 km along the considered lines and within a time window of ±1 second of the recorded outage time. These 222 flashes (203 of negative and 19 of positive polarity) are analyzed in terms of their local occurrence along the transmission lines and their peak currents. Tower footing resistances, which are affecting the occurrence of back flashover failures, were found to be log-normal distributed with a median of 4.4 Ω and a σln = 1.1. Median peak current of negative and positive flashes causing transmission line outages is -11.3 kA and 37.9 kA, respectively, and median distance of flash locations to the line is 322 m.

IEEE Transaction on Electromagnetic Compatibility, Vol. 56, Issue 1, Pages 149–158, 2014

DOI:https://10.1109/TEMC.2013.2266932

In this paper, we discuss the use of the electromagnetic time reversal (EMTR) method to locate lightning strikes. After a brief description of the EMTR and its application to lightning location, we mathematically demonstrate that the time-of-arrival method can be seen as a subset of EMTR. We propose three different models of backpropagation to address the issue of EMTR not being invariant for lossy media. Two sets of simulations are carried out to evaluate the accuracy of the proposed methods. The first set of simulations is performed using numerically generated fields and the proposed algorithm is shown to give very good results even if the soil is not perfectly conducting. In particular, we show that considering a model in which losses are inverted in the back propagation yields theoretically exact results for the source location. We show also that a lack of access to the complete recorded waveforms may lead to higher location errors, even though the computed errors are found to be within the range of performance of current lightning location systems (LLS). A second set of simulations is performed using the sensor data reported by the Austrian LLS. The locations obtained by way of the proposed EMTR method using only the available sensor data (amplitude, arrival time, and time-to-peak), are observed to be within a few kilometers of the locations supplied by the LLS.

32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI:https://10.1109/ICLP.2014.6971989

Cloud-to-ground (CG) lightning data from the European Cooperation for Lightning Detection (EUCLID) network over the period 2006-2012 are explored.

CIGRE International Colloquium on Lightning and Power systems, Lyon, France, 2014

In this paper we present a performance analysis of the European lightning location system EUCLID in terms of location accuracy (LA), detection efficiency (DE) and peak current estimation. The performance analysis is based on ground truth data from direct lightning current measurements at the Gaisberg Tower (GBT) and data from E-field and video recordings. The E-field and video recordings were taken in three different regions in Europe, in Austria, in Belgium and in France. The analysis shows a significant improvement of the LA over the past seven years. Currently the median LA is in the range of 100 m. The observed DE in Austria and Belgium is similar yet a slightly lower value is found in France because during the measurement period in France a nearby lightning location sensor was out of order. The accuracy of the lightning location system (LLS) peak current estimation for subsequent strokes is reasonable keeping in mind that the LLS estimated peak currents are determined from the radiated electromagnetic fields assuming a constant return stroke speed.

The results presented in this paper can be used to estimate the performance of the EUCLID network for regions with similar sensor baseline and sensor technology.

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32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI: https://10.1109/ICLP.2014.6973346

In this paper, we present an analysis of the initiation of upward lightning flashes for the Gaisberg and the Säntis Towers. The results are compared with previous studies. It is found that the majority of upward lightning discharges from both towers are initiated without any preceding lightning activity. We show also that the results of the presented studies on the initiation of upward flashes from tall structures might be affected by the selected parameters of the study.

Electric Power Systems Research, Vol. 113, Pages 15–24, 2014

DOI: https://doi.org/10.1016/j.epsr.2014.03.011

We derive electric field expressions, associated with lightning strikes to a tall object, using the monopole (Continuity Equation) technique which is distinctly different from the traditional dipole (Lorentz Condition) technique. Expressions to calculate the charge density along the tall object and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall object and a series point current source placed at the object top, are also derived. These expressions are used to calculate the very close-range electric fields in the monopole (Continuity Equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz Condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI: https://10.1109/ICLP.2014.6973303

Meteorological parameters associated with the initiation of upward lightning discharges from a tall tower were investigated. The parameters, including temperature, wind speed, air pressure, and relative humidity, were measured at the Gaisberg Tower top and at a distance of 170 m from the tower. A comparison of parameters associated with self-initiated upward flashes (initiated without any nearby preceding lightning activity) and nearby-lightning-triggered upward flashes (triggered by nearby preceding lightning activity) was made. It shows that at the Gaisberg Tower (GBT), relatively low ambient temperature facilitates to initiate upward flashes without any nearby preceding lightning activity. Relatively high ambient temperature (8.5 °C to 15.5 °C) facilitates to initiate upward positive stepped leaders, rather than the common continuous leaders, in the self-initiated upward negative flashes. No reliable correlation between wind speed and upward initiation of lightning was found.

32^nd International Conference on Lightning Protection (ICLP), Shanghai, China, 2014

DOI: https://10.1109/ICLP.2014.6973283

Ground-based or satellite-based lightning locating systems are the most common way to geolocate lightning. Depending upon the frequency range of operation, such systems can also report a variety of characteristics associated with lightning events (channel formation processes, leader pulses, cloud-to-ground return strokes, M-components, ICC pulses, and cloud lightning pulses). In this paper, we summarize the various methods to geolocate lightning, both ground-based and satellite-based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately and report various features such as lightning type and peak current. We examine the various methods used to validate the performance characteristics of different types of lightning locating systems.

23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

This paper deals with performance evaluation of the European lightning location system EUCLID in France during the HyMeX [1] Special Observation Period 1 (SOP1) in 2012. Beside other instruments a Lightning Mapping Array (HyLMA) and a mobile Video and Field Recording System (VFRS) was deployed in the south of France. The data of those independent systems are used to determine the performance of the EUCLID lightning location system (LLS) in terms of detection efficiency (DE) and location accuracy (LA) for both CG and IC flashes.

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23rd International Lightning Detection Conference and 5th International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2014

Lightning locating systems (LLS) can provide flash data derived from individual return stroke based on a flash grouping algorithm. However the latter considers negative cloud-to-ground (CG) flashes striking the ground in a unique point represented by the location of the first return stroke. According to video observations flashes have often different ground strike points. This can be a limitation in some engineering applications like the lightning risk assessment where the actual number of ground contacts is an important parameter. To get around this limitation Météorage has developed an algorithm allowing the identification of the location of the ground strike points (GSP) based on a statistical clustering (‘k-means’) method. The effectiveness of this algorithm, using operational LLS data, is tested on a total of 227 negative CG flashes observed with high speed video cameras in Austria and in France, in 2012 and 2013 respectively. The comparison between GSP computation and video observations reveals a GSP detection efficiency (DE) of about 95%. In addition the algorithm is able to discriminate between strokes creating a new ground contact (NGC) or using a pre-existing channel (PEC) in 83% out of the 767 observed strokes. The analysis shows that the limitation of the model is highly depending on the DE and location accuracy (LA) of the LLS collecting the data. Nevertheless, the fairly good results obtained with the GSP identification algorithm permits to build from existing VLF/LF LLS lightning data a hierarchical interlocked data structure composed of chronological events starting with the flash as the root event which is composed of GSPs being containing themselves strokes. This new dataset describes in a more complete way some lightning parameters related to a flash (e.g. flash multiplicity and number of ground strike points per flash) and their individual relationship, giving room to the improvement of engineering and research applications.

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Journal of Geophysical Research, Vol. 119, Issue 2, Pages 445-460, 2014

DOI: https://doi.org/10.1002/2013JD020854

The initial breakdown (IB) stage of lightning flashes typically occurs in the first 20ms of a flash and
includes a series of IB pulses often detected with electric field change sensors. There is disagreement about the percentage of negative cloud-to-ground (CG) flashes that begin with IB pulses. This study includes new data on IB pulses in 198 CG flashes in Austria (latitude ~48°N), Florida, USA (~29°N) and South Dakota, USA (~44°N) with, respectively, 100%, 100%, and 95% of the flashes having IB pulses. The data indicate that the amplitude of the largest IB pulse, range normalized to 100 km, is often weak,< 0.5 Vm
1,with the lower latitude having a greater percentage (36%) of these weak maximumIB pulses than the higher latitude (11%). Since sensor noise levels are often larger than this value, detection of smaller amplitude IB pulses may be difficult. A similar result is seen in the amplitude ratio of the largest IB pulse to the first return stroke: at the lower latitude, 50% of flashes had a ratio<0.1 versus 8% of flashes at the higher latitude. However, comparisons of the amplitude ratios from
Austria (~48°) and South Dakota (~44°) do not support a simple latitude dependence. The data also show that 5–10% of IB pulses occur more than 100ms before the first return stroke. These findingsmay explain why some previous studies found percentages <100%. Overall, the results indicate that all negative CG flashes probably begin with IB pulses.

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

DOI: https://10.1109/SIPDA.2013.6729206

In this paper we present a detailed evaluation of the performance of the Austrian Lightning location system (LLS) ALDIS regarding detection of positive flashes. Additional to the performance of the LLS some parameters of positive flashes are determined. We also present detailed information about two positive flashes with a subsequent stroke following the same channel to ground as the first stroke.

Atmospheric Research, Vol. 129–130, Pages 110–116, 2013

DOI: https://doi.org/10.1016/j.atmosres.2012.08.014

We compare in this paper direct measurements obtained on the towers on San Salvatore Mountain (Switzerland) and on the Gaisberg Mountain (Austria). They are situated in similar topographical environments but in different lightning activity zones. Direct measurements of lightning currents on these towers have revealed a major difference in terms of the number of downward flashes. While measurements made by Berger and co-workers revealed a significant number of downward flashes on the two towers on San Salvatore Mountain, more recent observations on the Gaisberg and Peissenberg towers were essentially composed of upward flashes. We use in this paper a new method to estimate the proportion of upward/downward flashes to a given tower, based on the data from lightning location systems. The analysis using the proposed method explains the discrepancy in terms of the measured number of downward flashes in the Gaisberg and San Salvatore towers. The analysis presented reveals also that in the evaluation of the percentage of upward flashes initiated from a tall structure, different parameters should be carefully examined, namely (i) the value of the ground flash density, (ii) the topographical conditions, and (iii) the presence of other tall structures in the region from which upward flashes might be initiated.

12^th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

DOI:https://10.1109/SIPDA.2013.6729246

CIGRE TB 549 (2013) is an update on previous CIGRE documents on the subject, published in Electra more than three decades ago. Lightning parameters needed in different engineering applications are reviewed. New experimental data, as well as the old data, are evaluated. Additional lightning parameters, previously not considered by CIGRE, are included. Possible geographical and seasonal variations in lightning parameters are examined. Specific applications are considered and recommendations are made.

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

DOI: https://10.1109/SIPDA.2013.6729216

The distribution of lightning flash density over large areas depends on many factors. In this paper, we analyze one of them, namely the terrain elevation. Some previous studies suggest that the lightning flash density decreases above a certain altitude. We show that this conclusion could be affected by the used method of analysis and we suggest two improved methods. The lightning flash density distribution over Switzerland and Austria was used as an example to test the proposed methods. The results of the application of both methods suggest that the lightning flash density grows over the whole altitude range.

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

DOI: https://10.1109/SIPDA.2013.6729189

We calculate vertical electric field and azimuthal magnetic field at different elevation angles and distances associated with lightning strikes a tall object. Simple and exact expressions for electromagnetic fields are derived when the current reflection coefficient at tall object top is zero and return stroke propagation speed in the lightning channel is equal to the speed of light. Further, we investigate the effects of current reflection coefficient at tall object top being not zero and the propagation speed is less than the speed of light (e.g., one half) on electromagnetic fields. Interestingly, we find that the vertical electric field has its largest peak value either at the smallest elevation angle or at the largest elevation angle. While for the azimuthal magnetic field, we note that its largest peak value is always at the smallest elevation angle or relatively small elevation angles.

Journal of Geophysical Research: Atmospheres, Vol. 118, Issue 19, Pages 10,653-10,664, 2013

DOI: https://doi.org/10.1002/jgrd.50805

Narrow (~1–2 s) infrasound pulses that followed, with ~11 to ~50 s delays, rapid changes of electrostatic field were observed by a microbarometer array in the Czech Republic during thunderstorm activity. A positive pressure fluctuation (compression phase) always preceded decompression; the compression was usually higher than the decompression. The angles of arrival (azimuth and elevation) were analyzed for selected distinct events. Comparisons of distances and azimuths of infrasound sources from the center of microbarometer array with lightning locations determined by the European Cooperation for Lighting Detection lightning detection network show that most of the selected events can be very likely associated with intracloud (IC) discharges. The preceding rapid changes of electrostatic field, their potential association with IC discharges, and high-elevation angles of arrival for near infrasound sources indicate that an electrostatic mechanism is probably responsible for their generation. It is discussed that distinguishing the relative role of thermal and electrostatic mechanism is difficult and that none of the published models of electrostatic production of infrasound thunder can explain the presented observations precisely. A modification of the current models, based on consideration of at least two charged layers, is suggested. Further theoretical and experimental investigations are however needed to get a better description of the generation mechanism.

11 Höfler´s Days, 2013

In Europe, several Lightning Location Systems (LLS) are operated in order to monitor the lightning activity and to gather information on lightning discharges inside a certain area. For operators of LLS as well as for users of lightning location data, information on the performance of their particular LLS is important. In the past, several studies on the performance of LLS were done with cross comparison of different LLS data sets, but such cross comparisons unfortunately do not provide clear results. Another approach to determine the performance of LLS is a comparison of LLS data with so called “ground truth data”. Such ground truth data are for example “natural lightning to instrumented towers” (e.g. Gaisberg tower), “artificial rocket triggered lightning” or “video and E-field measurements of natural lightning discharges”. Each of these methods has its advantages and disadvantages. In this paper we describe combined video and E-field measurements, the used Video-Field Recording System (VFRS) and our approach to gather ground truth data with such a VFRS. Based on the comparison of VFRS Data with LLS we show the Detection Efficiency (DE) for flashes and for strokes as well as the Location Accuracy (LA) of the Austrian Lightning Detection and Information System (ALDIS) for the southern and eastern part of Austria.

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Journal of Atmospheric and Oceanic Technology, Vol. 30, Issue 5, Pages 942–951 , 2013

DOI: https://doi.org/10.1175/JTECH-D-12-00162.1

This study reports results from electric field measurements coupled to high-speed camera observations of cloud-to-ground lightning to test the performance of lightning location networks in terms of its detection efficiency and location accuracy. The measurements were carried out in August 2011 in Belgium, during which 57 negative cloud-to-ground flashes, with a total of 210 strokes, were recorded. One of these flashes was followed by a continuing current of over 1 s—one of the longest ever observed in natural negative cloud-toground lightning. Lightning data gathered from the lightning detection network operated by the Royal Meteorological Institute of Belgium [consisting of a network employing solely Surveillance et Alerte Foudre par Interfe´rome´ trie Radioe´ lectrique (SAFIR) sensors and a network combining SAFIR and LS sensors], the European Cooperation for Lightning Detection (EUCLID), Vaisala’s Global Lightning Detection network GLD360, and the Met Office’s long-range Arrival Time Difference network (ATDnet) are evaluated against this ground-truth dataset. It is found that all networks are capable of detecting over 90% of the observed flashes, but a larger spread is observed at the level of the individual strokes. The median location accuracy varies between 0.6 and 1 km, except for the SAFIR network, locating the ground contacts with 6.1-km
median accuracy. The same holds for the reported peak currents, where a good correlation is found among the networks that provide peak current estimates, apart from the SAFIR network being off by a factor of 3.

European Geosciences Union, General Assembly, Vol. 15, Pages 6893, 2013

CEA (Commissariat à l’Energie Atomique et aux Energies Alternatives) installed two acoustic networks in South-East of France. The first one was located in Observatoire Haute Provence in the framework of the ARISE European design project (arise-project.eu). It was composed of 4 microbarometers set in an equilateral triangle. The other network was located close to Uzès in the frame of the HyMeX (www.hymex.org) Special Operational Period from August 27th to November 16th. It was composed of 4 microbarometers and 4 microphones. From August to November 2012, several thunderstorms occurred close to these stations. The global thunder spectrum from 0.01 Hz to 250 Hz is studied using the microbarometer and microphone recordings. A very large thunderstorm, in August 30th-31st, produced more than 100 sprites over the south-western part of the Mediterranean Sea. Ten of them were dancing sprites. This kind of sprites can produce infrasound. This experiment offers a unique to triangulate sprites with infrasound measurements. Lastly, a tornado occurred in October 14th. This event will be analyzed with the data of both networks.

Journal of Geophysical Research: Atmospheres, Vol. 118, Issue 19, Pages 11,098-11,106, 2013

DOI: https://doi.org/10.1002/jgrd.50804

Bipolar lightning is usually defined as a lightning flash where the current waveform exhibits a polarity reversal. There are very few reported cases of cloud-to-ground (CG) bipolar flashes using only one channel in the literature. Reports on this type of bipolar flashes are not common due to the fact that in order to confirm that currents of both polarities follow the same channel to the ground, one necessarily needs video records. This study presents five clear observations of single-channel bipolar CG flashes. High-speed video and electric field measurement observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high-speed cameras, we suggest that positive leader branches which do not participate in the initial return stroke of a positive cloud-to-ground flash later generate recoil leaders whose negative ends, upon reaching the branch point, traverse the return stroke channel path to the ground
resulting in a subsequent return stroke of opposite polarity.

Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 92, Pages 37–42, 2013

DOI: https://doi.org/10.1016/j.jastp.2012.09.008

Positive flashes correspond to approximately only 10% of the total number of flashes produced by a thunderstorm. However, strokes with high peak currents and long continuing currents are usually present in positive flashes. Therefore, positive flashes are responsible for more intense damage than the negative ones. Positive flashes often are preceded by significant and long duration intracloud (IC) discharge activity. We observe in detail the electric field variations produced by 80 cloud-to-ground lightning flashes in 9 different storms in S. Paulo, Brazil during the summers of 2009–2011. Intracloud discharges preceding the positive cloud-to-ground flashes and some characteristics of the electric field changes produced by the return stroke that occurred at ranges of 3–80 km from the site of the electric field measurements were analyzed. All flashes presented breakdown pulses prior to the return stroke. The mean time interval between the preliminary breakdown pulse (PBP) and return stroke was 157 ms. The pulse train duration have a mean value of 3.1 ms. Only 6 out of 80 cases analyzed did not present pulse trains but only one single bipolar breakdown pulse before the return stroke. In 95% of cases the initial breakdown pulse presented the same initial polarity of the succeeding return stroke. Time interval between pulses in a pulse train had a mean value of 280 μs. The mean values of pulse width is 25.2 μs. The mean values of zero-to-peak risetimes and of the 10–90% risetimes for 72 return strokes electric field waveforms are 9.5 and 5.7 μs respectively. The AM value of peak amplitudes of the positive return strokes fields normalized to 100 km is 17.0 V/m.

Journal of Geophysical Research: Atmospheres, Vol. 117, D6, 2012

DOI: https://doi.org/10.1029/2011JD017135

This paper presents some parameters of negative cloud-to-ground lightning flashes in terms of frequency distribution. All data are based on so-called “accurate-stroke-count studies” from different climatological regions in the world and were already published in the literature with the exception of our measurements. We used GPS synchronized data from two digital high-speed cameras (at 1–8,000 frames/sec). The parameters considered in this study are: (1) continuing current duration, (2) time intervals between strokes, (3) number of strokes per flash and (4) total flash duration. The analysis includes Berger’s data of Monte San Salvatore (Switzerland), which is the basis for lightning protection standards. The comparison suggests that despite of overall agreement of those parameters that some of them, currently used in protection standards, should be revised in order to be more realistic.

Journal of Geophysical Research: Atmospheres, Vol. 117, D6, 2012

DOI: https://doi.org/10.1029/2011JD016903

We report the measured current characteristics of positive lightning discharges to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. On the basis of the recorded current waveforms, a total of 26 flashes consisting of initial stage only were identified as upward positive discharges initiated by an upward negative leader from GBT. They accounted for 4% (26/652) of the total flashes recorded at the GBT. Nineteen (73%) out of the 26 positive flashes occurred during nonconvective season (September–March). Median values of flash peak current, flash duration, charge transfer, and action integral were determined as 5.2 kA, 82 ms, 58 C, and 0.16 x 103 A2 s, respectively. Current pulses of high repetition rate superimposed on the initial portion of initial continuous current are inferred to be associated with the upward negative stepped leader process. The weighted arithmetic means of leader pulse peak current, leader pulse duration, leader interpulse interval, and leader pulse charge are 3 kA, 31 µs, 32 µs, and 42 mC, respectively. On the basis of an assumed stepped leader speed in the range of 8 x 104 to 4.5 x 105 m/s an upward negative stepped leader channel charge density of 15–87 mC/m, a leader length of 168–945 m, and an average leader step length of 2.4–13.3 m were estimated. The upward negative stepped leader channel charge density and length are significantly larger and smaller than their counterparts in downward negative stepped leaders, respectively, while the upward leader step length is consistent with previous studies. Possible reasons for this are discussed.

31^st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

In this paper, we discuss the use of the Electromagnetic Time Reversal (EMTR) method to locate lightning discharges. The main problem of EMTR is that losses are not invariant under time reversal. In this paper, we propose 3 different models of back-propagation to fix this problem. Simulations are made to evaluate the accuracy of the proposed methods. It is shown that by assuming a lossless backpropagation model, the resulting location errors are in the order of some hundreds of meters. It is also shown that a theoretically exact estimate can be obtained considering a back-propagation over a fictitious 'inverted-loss' ground.

Journal of Geophysical Research: Atmospheres, Vol.117, D8, 2012

DOI: https://doi.org/10.1029/2011JD017269

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lightning location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, most of them (87% or 179/205) were initiated from the tower top without any nearby preceding lightning activity (called "self-initiated"), whereas 26 upward flashes (13%) were initiated from the tower top with immediately preceding nearby lightning activity (called "nearby-lightning-triggered"), including 15 positive ground flashes, one negative ground flashes, and 10 cloud discharges. The possible reasons for self-initiated upward flashes dominating at the GBT could be the field enhancement due to the Gaisberg Mountain above the surrounding terrain and low altitude of charge region during non-convective season (September to March), since we note that self-initiated lightning at the GBT occurred predominantly (79% or 142/179) during non-convective season. On the other hand the majority (85% or 22/26) of nearby-lightning-triggered upward flashes at the GBT occurring during convective season (April to August) and 80 nearby-lightning-triggered upward flashes out of 81 upward flashes observed at the ten tall towers in Rapid City in South Dakota of USA occurring during summer seasons, could be due to the result of high altitude of charge region. The triggering flashes were detected to be within 1 and 18 km distance and the time intervals between them and upward lightning initiation are in the range of 0.3 to 90.7 ms.

Theoretical and Applied Climatology, 111, Pages 183-193, 2013

DOI: https://doi.org/10.1007/s00704-012-0653-7

Besides human-caused fires, lightning is the major reason for forest fire ignition in Austria. In order to analyse the causes of ignition and to characterise lightning-induced forest fires, fire records were compared with the real appearance of lightning events by using the Austrian Lightning Detection and Information System for the period from 1993 to 2010. A probability was estimated for each forest fire being caused by lightning by using a decision tree and decision matrices based on flash characteristics (e.g. amplitude, time, location). It could be shown that 15 % of documented forest fires were
lightning-caused. Nearly all lightning-caused fires were found during the summer months, whereas almost 40 % of all fires occurring from June to August were naturally caused. Most lightning-caused fires took place in the south and east of Austria. Lightning fires were more frequent at higher altitudes and primarily affected conifer forests. The median burned area was lower than that for anthropogenic forest fires.

IEEE Transactions on Electromagnetic Compatibility, Vol. 54, No. 4, 2012

DOI: https://10.1109/TEMC.2011.2177840

We present measurements of nearby vertical and horizontal electric fields from leaders and return strokes associated with lightning strikes to the 100-m-tall Gaisberg Tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20m from the tower's vertical axis. Simultaneously with the fields, return-stroke currents were also measured at the top of the tower. The measured data are used to test engineering and electromagnetic models for the return stroke. In general, the agreement between measured waveforms and model-predicted ones is satisfactory.

Electric Power Systems Research (EPSR) Vol. 82, Issue 1, Pages 34-43, 2012

DOI: https://doi.org/10.1016/j.epsr.2011.08.011

The paper presents the characteristics of a system suitably developed to measure lightning current waveforms on the Säntis Tower in Switzerland. The paper first describes the configuration of the system then presents the characteristics of the measurement equipment, in terms of bandwidth and accuracy, inferred by means of experimental tests. Finally, we present and discuss examples of waveforms measured using the system. Keywords: lightning current, elevated struck objects, Rogowski coils, lightning current parameters.

31^st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

DOI:https://10.1109/ICLP.2012.6344342

We derive expressions to calculate the charge density along the tower and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall tower and a series point current source placed at the tower top. These expressions are used to calculate the very close-range electric fields in the monopole (continuity equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

31^st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

In this paper, we discuss the use of the Electromagnetic Time Reversal (EMTR) method to locate lightning discharges. The main problem of EMTR is that losses are not invariant under time reversal. In this paper, we propose 3 different models of back-propagation to fix this problem. Simulations are made to evaluate the accuracy of the proposed methods. It is shown that by assuming a lossless backpropagation model, the resulting location errors are in the order of some hundreds of meters. It is also shown that a theoretically exact estimate can be obtained considering a back-propagation over a fictitious 'inverted-loss' ground.

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Journal of Geophysical Research: Atmospheres, Vol. 118, Issue 19, Pages 11,098-11,106, 2013

31^st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

DOI: https://doi.org/10.1002/jgrd.50804

Bipolar lightning is defined as a lightning event where the current waveform exhibits a polarity reversal within the same flash. There are very few reported cases of cloud-to-ground (CG) bipolar flashes using only one channel in the literature. Reports on this type of bipolar flashes are not common due to the fact that in order to confirm that currents of both polarities follow the same channel to the ground one necessarily needs video records. This study presents two clear observations of single-channel, bipolar CG flashes. High-speed video and electric field measurements observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high-speed cameras [1-3] we will suggest that recoil leaders occurring after the positive stroke may generate a subsequent stroke with an opposite polarity.

CIGRE C4 Colloquium on Power Quality and Lightning, Sarajevo, Bosnia and Herzegovina, 2012

In this paper we present a detailed performance evaluation of the Austrian Lightning Location System ALDIS in terms of detection efficiency and location accuracy based on ground truth measurements. The ground truth measurements used in this evaluation are video and E-field measurements.

151 out of the 154 negative cloud to ground flashes and 449 out of 540 strokes were detected by the lightning location system (LLS). This results in a flash detection efficiency of 98% and a stroke detection efficiency of 83%. Only two additional cloud to ground strokes (0.4%) were detected but misclassified as cloud discharges. The analysis of 37 flashes which exhibit at least two strokes within one lightning channel results in a median location accuracy of 368 m (STD = 650 m).

The results of those measurements are in good agreement to performance evaluation done with data from current measurements at the Gaisberg Tower in Austria.

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International Conference on Grounding and Earthing and 5^th International Conference on Lightning Physics and Effects (GROUND/LPE), Bonito, Brazil, 2012

Positive flashes correspond to approximately only 10% of the total number of flashes produced by a thunderstorm. However, strokes with high peak currents and long continuing currents are usually present in positive flashes. Therefore, positive flashes are responsible for more intense damage than the negative ones. Positive flashes often are preceded by significant and long duration intracloud (IC) discharge activity. We observe in detail the electric field variations produced by 80 cloud-to-ground lightning flashes in 9 different storms in S. Paulo, Brazil during the summers of 2009 to 2011. Preliminary breakdown pulses (PBP) preceding positive cloud-to-ground flashes and some characteristics of the electric field changes produced by the return stroke that occurred at ranges of 3 km to 80 km from the site of the electric field sensor were analyzed. All flashes presented PBP prior to the return stroke. The mean time interval between the PBP and return stroke was 157 ms. The pulse train duration have a mean value of 3.1ms. Only 6 out of 80 cases analyzed did not present pulse trains but only one single bipolar breakdown pulse before the return stroke. In 95% of cases the initial breakdown pulse presented the same initial polarity of the succeeding return stroke. Time interval between pulses in a pulse train had a mean value of 280 μs. The mean values of pulse width is 25.2 μs. The mean values of zero-to-peak risetimes and of the 10-90% risetimes for 72 return strokes electric field waveforms are 9.5 and 5.7 μs respectively. The arithmetic mean value of peak amplitudes of the positive return strokes fields normalized to 100 km is 17.0 V/m.

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22^nd International Lightning Detection Conference and 4^th International Lightning Meteorology Conference (ILDC/ILMC), Broomfield, Colorado, 2012

Since 1998 direct measurements of lightning current are performed at the Gaisberg Tower in Austria [1]. One goal of those measurements is to validate the most important performance parameters of the Austrian lightning location system ALDIS, namely the detection efficiency, the location accuracy and accuracy of the peak current estimate. Location accuracy analysis resulted in a median of about 350m [2] which is in agreement with model-based estimates for the location accuracy, but basically this value is only valid for the location of the Gaisberg Tower. In order to evaluate performance in a variety of locations, we developed a portable GPS synchronized video- and field measurement system [3-5]. From the data recorded by this system we can also infer the detection efficiency and the location accuracy of the LLS. Further it is also possible to use these data to evaluate the type-categorization (cloud-to-ground vs. cloud pulse) assigned by the LLS.

In this study we will focus on the parameter location accuracy. We will present a methodology that allows us to determine the location accuracy of the LLS from video records, and we give some theoretical background about comparing location accuracy estimates from tower (triggered lightning) measurements and video records. Finally we will compare results obtained during a combined video and field measurement campaign performed at 13 different locations in the eastern part of Austria in 2009 and 2010 with results from data measured at the Gaisberg Tower.

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22^nd International Lightning Detection Conference and 4^th International Lightning Meteorology Conference (ILDC/ILMC), Broomfield, Colorado, 2012

In this paper, we report results from electric field measurements, coupled to high-speed camera observations to test the performance of lightning location networks in terms of its detection efficiency and location accuracy. The measurements were carried out during August 2011 in Belgium, during which 57 negative cloud-to-ground flashes, with a total of 210 strokes, were recorded. Data from the Belgian lightning network, the European Cooperation for Lightning Detection EUCLID and Vaisala's Global Lightning Detection network GLD360 are evaluated against this ground-truth data set.

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3rd International Symposium on Winter Lightning (ISWL), Sapporo, Japan, 2011

In this paper, we report the measured current characteristics of positive lightning discharges observed from the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Based on the recorded current waveforms, a total of 26 flashes were identified as upward positive discharges initiated by an upward negative leader from the top of the GBT, consisting of initial stage current only, without any leader-return-stroke sequences. No downward positive flashes containing microsecond-scale current waveforms of return strokes were observed. The occurrence of upward positive flashes accounts for 4% (26/652) of the total number of recorded flashes at the GBT during the 10-year observation period. Nineteen (73%) out of the 26 flashes occurred during non-convective or cold season (September- March). Median values of flash peak current, flash duration, flash charge transfer, and flash action integral were determined as 5.2 kA, 82 ms, 58 C and 0.16×103 A2s, respectively. Narrow current pulses of high repetition rate, different from normal initial continuous current (ICC) pulses typical for upward negative flashes, were found during the initial portion of the ICC for these upward positive flashes. Based on simultaneous current and electric field measurements at a distance of 170 m from the GBT these pulses are inferred to be associated with the stepping process of the upward propagating negative leaders initiated from the tower top.

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3rd International Symposium on Winter Lightning (ISWL), Sapporo, Japan, 2011

First instrumentation for lightning measurements at the Gaisberg Tower (GBT) has been installed in 1998. During the 10-years period from 2000 – 2009 a total of 652 lightning events have been recorded. Fast majority of the lightning to the GBT was upward initiated and only very few records are candidates for downward flashes when the recorded current waveform is used for discrimination between upward and downward lightning. Most of the lightning to the GBT (about 60%) is observed during cold season, comparable to winter lightning in Japan. 3% (21/652) of the flashes were bipolar and 4% (26/652) were positive. Maximum measured charge transfer to ground in a single flash was 546 C. 10 out of the 652 flashes (1.5 %) transferred charge values exceeding 300 C and all those events occurred during cold season. Median peak current of return strokes following the initial continuing current is 9.2 kA and similar to values observed in triggered lightning and to peak current estimates for subsequent strokes from lightning location systems.

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3^rd International Symposium on Winter Lightning (ISWL), Sapporo, Japan, 2011

Recent high-speed video experiments have indicated that positive cloud-to-ground lightning (+CG) might present M components during their continuing current period [1]. As only optical data were available, their results consisted mainly on occurrence- and time-related parameters and their statistical distribution. In the present work we address this issue by extending those investigations through the addition of simultaneous slow and fast electric field data (obtained through the use of capacitive antennas) to the high-speed camera recordings (obtained by two different cameras, Red Lake Motion Scope 8000S and Photron Fastcam 512 PCI, operating at frame rates ranging from 1000 or 8000 frames per second). Through the use of an algorithm previously developed by the authors [1-2] we were able to plot luminosity-versus-time curves of each continuing current recorded by the cameras. Once an individual M component is identified in this luminosity data, it is possible to find the electric field change it has produced and that could be measured by the antennas. By using a simple electrostatic model it is possible to estimate the peak current and total charge transfer to ground of each M component observed from the slow electric field data. These intensity-related parameters can be also be correlated to occurrence- and time-related parameters such as duration, elapsed time since the return stroke and time interval between successive M components, making it possible to see at which periods of a continuing current M components can be more or less intense. This type of data is very relevant for both engineering applications (such as EMC studies [3]) and scientific research (especially sprite initiation [4] and the bidirectional leader model for lightning [5]).

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3rd International Symposium on Winter Lightning (ISWL), Sapporo, Japan, 2011

Past results from streak photographs by Berger and Vogelsanger [1] show that there may be no steps on the positive leader propagation. Steps in positive leader propagation have never been observed by high speed cameras with frame rates up to 8000 frames per second [2]. However some other studies found pulses on the electric field during the progression of the leader toward ground that could indicate the presence of steps in the leader propagation. Hojo et al. [3] observed electric field leader pulses in 26-30% of their cases, with mean time interval of about 17µs (ranging from 3 µs  to 31 µs ). Proctor [4] observed two out of 175 flashes in South Africa that show pulses on the leader. Cooray and Lundquist [5] reported step pulses within a mean time interval of 26µs for positive lighting in Sweden. This work presents high-speed video camera and electric field measurements of downward leaders in positive cloud-to-ground lightning in Brazil. Several cases were analyzed and the percentage of leaders containing pulses is reported.

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14th International Conference on Atmospheric Electricity (ICAE), Rio de Janeiro, Brazil, 2011

The aim of this investigation is to analyze the phenomenology of positive and negative (stepped-, dart- and recoil) leaders observed in natural lightning from simultaneous high-speed video recordings and electric field measurements. For that intent we have used two different high-speed cameras (obtained by two different cameras, Red Lake Motion Scope 8000S and Photron Fastcam 512 PCI, operating at frame rates ranging from 1000 or 8000 frames per second) and two different types of electric field sensors (slow and fast) in addition to data from a lightning locating system (BrasilDat). All the instruments were GPS time synchronized in order to avoid ambiguities in the analysis and allow us to estimate the peak current of and the distance to each flash that was detected by BrasilDat. From these data it is possible to calculate the two-dimensional speed of each observed leader, allowing us to obtain its statistical distribution along with its correlation to other characteristics of the associated flash, such as return stroke peak current, interstroke interval and presence and duration of continuing current after the return stroke initiated by the leader. Also, the availability of electric field data makes it possible to correlate it to the optical characteristics of each leader type, providing us new insights on the microphysics of these phenomena. In the analyzed dataset, the speeds of positive leaders and negative dart-leaders follow a lognormal distribution at the 0.05 level (according to the Shapiro-Wilk test), while the negative stepped leaders have a similar distribution even though according to a relatively weaker test (Kolmogorov-Smirnov). Unfortunately, due to the range of frame rates reached by the used cameras, only the minimum propagation speeds could be estimated in the recoil leaders that were observed.

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14th International Conference on Atmospheric Electricity (ICAE), Rio de Janeiro, Brazil, 2011

Positive flashes are usually composed of a single stroke. A large fraction of the positive cloud-to-ground (+CG) flashes (81%) produces just a single-stroke, and the average multiplicity is only 1.2 strokes per flash. Almost all (~95%) subsequent strokes in multiple-stroke +CG flashes create a new ground termination [Saba et. al. 2010]. In the present work we combine high-speed video recordings (obtained by two different cameras, Red Lake Motion Scope 8000S and Photron Fastcam 512 PCI, operating at frame rates ranging from 1000 or 8000 frames per second) with fast electric field measurements (obtained through the use of flat plate antennas) to investigate the characteristics of the electric field changes produced by leader pulses and return stroke in positive flashes.

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11th International Symposium on Lightning Protection (SIPDA), Fortaleza, Brazil (invited), 2011

DOI: https://10.1109/SIPDA.2011.6088435

Current parameters of tower triggered lightning are of interest for the lightning protection of high objects, such as wind turbines. Initiation conditions of upward lightning are still subject of ongoing research and probably needs consideration of local conditions. In this paper results of measurements of upward lightning from the Gaisberg Tower (GBT) in Austria are summarized and compared with some other measurements on towers or rocket-triggered lightning. Upward lightning initiated by a positive upward leader is the most common type (93%) of lightning observed at the GBT, nevertheless 3% of the discharges are bipolar and 4% of the tower flashes are lowering positive charge to ground. Positive flashes are initiated by a negatively charged leader and short current pulses associated with the stepping leader process are observed.

11th International Symposium on Lightning Protection (SIPDA), Fortaleza, Brazil. (invited), 2011

DOI: https://10.1109/SIPDA.2011.6088458

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lighting location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, we find that most of upward flashes (179/205 or 87%) are initiated at the tower top without any nearby preceding discharge activity, 26 flashes (13%) are initiated by nearby triggering lightning discharges, including 10 positive cloud-to-ground lightning, 1 negative cloud-to-ground lightning, and 15 cloud discharges. The possible reasons for self-triggered upward flashes dominating at the GBT would be the field enhancement due to the Gaisberg Mountain above 800 m of the surrounding terrain of the city of Salzburg and low cloud base during cold season.

11th International Symposium on Lightning Protection (SIPDA), Fortaleza, Brazil, 2011

DOI: https://10.1109/SIPDA.2011.6088468

In this paper, direct lightning current measurements obtained on the Säntis Tower from June 1st, 2010 to May 31st, 2011 are used to evaluate the ability of the EUCLID lightning detection network to detect this type of lightning triggered by a tall tower in terms of detection efficiency, location accuracy and peak current estimates. The Säntis Tower is a 124-meter tall tower sitting on the top of the Säntis Mountain (2500 m) in Switzerland. The tower has been instrumented to measure waveforms of the lightning current and of its time derivative. In the considered period, 57 flashes were recorded at the Säntis Tower out of which 15 were of positive polarity. The data have been correlated to EUCLID data by comparing the time-stamps provided by the GPS time references. The flash detection efficiency for negative flashes is estimated to be 93%. The median value of the location error is 126 m. The EUCLID peak current estimates were on average significantly larger than the measured current. The measurements include four typical positive flashes, which were successfully detected by EUCLID. The location errors for the positive events ranged from 1 to 3 km, with a median of 959 m.

11th International Symposium on Lightning Protection (SIPDA), Fortaleza, Brazil, 2011

DOI:https://10.1109/SIPDA.2011.6088450

Comparison measurements with a mobile lightning current detection system and measuring station used for scientific purposes were carried out. The mobile system including sensors, data acquisition and transmission are described. The lightning current parameters recorded during comparative measurements are analyzed. The results are in line with the lightning current statistics. Mainly upward initiated flashes were measured.

11th International Symposium on Lightning Protection (SIPDA), Fortaleza, Brazil, 2011

DOI: https://10.1109/SIPDA.2011.6088466

A new method to estimate the number of upward flashes from tall structures is presented. The method is based on the analysis of the data provided by lightning location systems (LLS) and thus could be applied for any tall structure located in the region covered by a LLS. About 80 tall objects in Europe with heights ranging from 100 m to 300 m were selected for the analysis. LLS data for a period of 10 years on flashes within circles of 8 km around each object were exported from the EUCLID network database and analyzed. The number of upward flashes for each considered structure was estimated and the obtained results were compared with those calculated using the empirical formula of Eriksson. For towers located on hilly terrain, the physical height of the structure was replaced by ist effective height determined according to the IEC recommendation. The obtained results follow the trend predicted by Eriksson’s formula. However, significant dispersion is observed. This dispersion might be attributed essentially to meteorological and geological factors associated with different objects.

7th Asia-Pacific International Conference on Lightning (APL), Chengdu, China. (invited), 2011

DOI: https://10.1109/APL.2011.6110121

First instrumentation for lightning measurements at the Gaisberg Tower (GBT) has been installed in 1998. During the 10-years period from 2000 – 2009 a total of 652 lightning events have been recorded. Vast majority of the lightning to the GBT was upward initiated and only one record is a candidate for a downward flash when the recorded current waveform is used for discrimination between upward and downward lightning. 3% (21/652) of the flashes were bipolar and about the same number of 4% (26/652) were positive. 73% of positive and 63% of bipolar lightning was triggered by the GBT during cold season in Austria.

7th Asia-Pacific International Conference on Lightning (APL), Chengdu, China, 2011

DOI: https://10.1109/APL.2011.6110212

In this paper, we discuss properties of initial continuous current (ICC) pulses superimposed on the slow varying initial stage current in upward flashes initiated from the Gaisberg Tower (GBT) based on simultaneous current, near/far electric field measurements. ICC pulses are usually associated with M-component mode of charge transfer to ground, if only one branch of the lightning channel is active. However, there are several cases in which ICC pulses involve a predominantly leader/return-stroke mode of charge transfer to ground in a new channel that connects to the old channel carrying continuous current. With the help of high-speed video camera records, we find that this mixed mode of charge transfer for ICC pulses, leader-return stroke in one channel and continuous current in another channel, occurring simultaneously during the initial stage current in upward lightning from tall towers is very common. This is in contrast to triggered-lightning where usually only one channel is dominant and ICC pulses are associated predominantly with M-component mode of charge transfer. Also, ICC pulses with mixed-mode of charge transfer have shorter risetimes, larger peaks, and shorter half-peak widths.

7th Asia-Pacific International Conference on Lightning (APL), Chengdu, China, 2011

We compare in this paper direct measurements obtained at Tower 1 on Mount San Salvatore (Switzerland) and the Gaisberg Tower (Austria). They are situated in similar topographical environment but in different lightning activity
zones. Direct measurements of lightning currents on these two towers have revealed a major difference in terms of the number of downward flashes. While Berger and co-workers obtained a significant number of downward flashes, more recent observations on Gaisberg and Peissenberg Towers were essentially composed of upward flashes. We use in this paper a new method to estimate the proportion of upward/downward flashes to a given tower, based on data from lightning location systems. The analysis using the proposed method explains the discrepancy in terms of the measured number of downward flashes in Gaisberg and in Monte San Salvatore.

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Union Radio-Scientifique Internationale (URSI), General Assembly, Istanbul, 2011

DOI: https://10.1109/URSIGASS.2011.6051177

We study a penetration of whistlers to the ionosphere. We developed an automatic method for assigning causative lightning to fractional-hop whistlers observed on the DEMETER satellite. Processing data from 364 passes of the satellite over Europe, we found that at nighttime, a mean whistler intensity is approximately three times larger than at daytime. A maximum of whistler intensity is shifted approximately one degree from the satellite magnetic footprint owing to the oblique propagation. Calculations of wave attenuation made using IRI2007 and MSIS models show very similar result in a ratio of nighttime and daytime intensities.

Journal of Geophysical Research, Vol. 116, Issue D13, 2011

DOI: https://doi.org/10.1029/2011JD015634

We analyze current records for 21 upward initiated bipolar lightning flashes observed at the Gaisberg Tower (GBT) in Austria from 2000 to 2009. A bipolar lightning flash occurrence of 3% (21/652) is found during the 10‐year observation period. Thirteen (62% or 13/21) of them occurred in nonconvective season (September–March). On the basis of the classification suggested by Rakov and Uman (2003), 13 (62%) of the 21 bipolar flashes belong to Type 1 associated with a polarity reversal during the initial stage (IS) current, five belong to Type 2 associated with different polarities of the IS current and the following return strokes, one belongs to Type 3 associated with
return strokes of opposite polarity following the IS, and two of them are not assigned.
We also find that the initial polarity reversal from negative to positive occurs more often (76% or 16/21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean (GM) and arithmetic mean (AM) of the total absolute charge transfer are 99.5 C and 125 C, with the GM and AM total flash duration of 320 ms and 396 ms, respectively. From simultaneous current and high‐speed video measurements of one bipolar flash, within the field of view, the positive charge was transferred along one branch initially, followed by the negative charge transfer after cessation of the luminosity for 142 ms, while the other two branches connected to the main channel always contributed to the negative charge transfer during the whole process.

Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, 2011

A field mill (FM) has been permanently operated at a distance of about 170m from the Gaisberg Tower (GBT) in Austria since several years. The electric field measurements suffer from field enhancement due to its location on a 4-m tall metal platform near the tower which itself is located on a 1280 meter high mountain. A special measuring campaign was conducted to determine the fair-weather atmospheric electric field at the Gaisberg Mountain on June 24th, 2010. The main objectives of this campaign were to calibrate the field mill in order to infer the relation between the electric fields at the tower tip and the ground level measured by the field mill under thunderstorm conditions. Besides the permanent field mill near the tower, two Campbell Scientific CS100 electric field meters were used during this campaign, and distances between each other were determined by using the Global Positioning System (GPS). Overall we determined an enhancement factor of 2.75 due to the mountain itself with reference to the mountains surrounding terrain. A field enhancement factor of 7.81 was obtained for the permanently installed field mill at the measurement platform next to the GBT with reference to the undisturbed electric field at the mountain top close to the platform at ground level. The electric field near the tower (distances about the tower height of 100 m) was smaller than the field measured at larger distance from the Tower. This observation was possibly caused by a shadowing effect of the tower.

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Elektrotechnik und Automation (ETZ), Nr. 78, 7, 2011

Weltweit wurden in den vergangenen Jahrzehnten Blitzstrommessungen zur wissenschaftlichen Grundlagenforschung an verschiedenen Blitzmess-stationen durchgeführt. Bedingt durch den wissenschaftlichen Ansatz dieser Untersuchungen sind deren Messsysteme technisch sehr aufwendig und somit nur bedingt für einen mobilen Einsatz an wechselnden Standorten geeignet. Das in diesem Beitrag vorgestellte mobile Blitzstromerfassungssystem erlaubt hingegen Messungen an unterschiedlichen Standorten und Anlagen, zum Beispiel Windenergieanlagen, Telekommunikationstürmen oder hohen Bürogebäuden. Die Erfassung und Bewertung von tatsächlich stattgefundenen Blitzeinschlägen in solchen Anlagen ermöglicht es beispielsweise, die Notwendigkeit von Wartungsmaßnahmen nach Blitzeinschlägen zu bewerten.

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ECSS, 2011

In 2001 several countries (Austria, France, Germany, Italy, Norway and Slovenia) started a cooperation called EUCLID (EUropean Cooperation for LIghtning Detection). It is the goal of this cooperation to provide European wide lightning data with nearly homogeneous quality. Subsequently also Spain, Portugal, Finland and Sweden joined EUCLID. The EUCLID cooperation is special in the sense that the individual partners are highly motivated to run their individual networks with state-of-the-art lightning sensors. All the partners employ dedicated technicians to supervise and maintain the network and to react fast in case of sensor or communication problems. As of August 2011 the EUCLID network employs 142 sensors, 4 LPATS III, 13 LPATS IV, 1 SAFIR, 16 IMPACT, 42 IMPACT ES/ESP and 66 LS7000 sensors (oldest to newest), all operating over the same frequency range with individually-calibrated gains and sensitivities.  Data from all of these sensors are processed in real-time using a single common central processor, which also produces daily performance analyses for each of the sensors. This assures that the resulting data are as consistent as possible throughout Europe. In fact, the Europe-wide data produced by EUCLID is frequently of higher quality than the data produced by individual country networks, due to the implicit redundancy produced by shared sensor information.  Further, this cooperation also acts as a platform for exchange of knowledge related to lightning location technology. Since the beginning of the cooperation the performance of the EUCLID network has been steadily improved, e.g. with improved location algorithms, with newer sensor technology and by adapting sensor positions because of bad sites.  Over the next 1-2 years, at least 15 of the older sensors are expected to be upgraded to the newest (LS700x) instruments.

To compare the estimated performance to real values several measurement campaigns were carried out in Europe, e.g. in Slovenia where LLS data was compared to data from GPS synchronized flash counters installed on mobile phone towers [1] – [2], in France where video surveys were used to determine the actual network performance of the French lightning location system [3]. In the presentation detailed performance results from a comparison with direct lightning current measurements on a tower and from video and E-field measurements in Austria will be provided.

REFERENCES

1. Djurica V., Kosmač J.: LLS Accuracy improvements by measurements collected by the RLDN, 19th International Lightning Detection Conference, Tucson, USA, 2006.
2. Djurica V., G. Milev , J. Kosmač: Lightning location networks performance validation with RLDN, 16th International Symposium on High Voltage Engineering, South Africa, 2009.
3. Berger, G. and S. Pedeboy: Comparison between real CG flashes and CG flashes detected by a lightning detection network, ICOLSE, Blackpool, 2003.

Meteorologentag, 2011

Kurzfassung:

Das österreichische Blitzortungssystem ALDIS ist seit fast 20 Jahren in Betrieb und während dieser Zeit wurde das System immer wieder verbessert. Speziell das Upgrade der Sensoren im Jahr 2005 und die Verbesserungen im Ortungsalgorithmus der letzten Jahre machen eine neuerliche Überprüfung der Performance notwendig.

Die wichtigsten Performance Parameter eines Blitzortungssystems sind die Detection Efficiency, die Ortungsgenauigkeit, die Genauigkeit der Blitzstromamplituden-bestimmung und die Zuverlässigkeit der Klassifizierung der einzelnen Entladungen als Wolke-Erde Entladung oder Wolkenentladung.

Um alle diese Performance Parameter zu überprüfen, sind vom Blitzortungssystem völlig unabhängige Messungen notwendig. Aus diesem Grund  wurden von ALDIS in den letzten Jahren sowohl Blitzstrommessungen am ORS Sendemast am Gaisberg als auch  Video- und E-Feldmessungen an mehreren Orten Ostösterreichs durchgeführt.

Im unserem Beitrag möchten wir die Resultate dieser Messungen präsentieren und einen Ausblick auf weitere Verbesserungen in den nächsten Jahren geben.

6th European Conference on Severe Storms (ECSS), Palma de Mallorca, Balearic Islands, Spain, 2011

Based on lightning records provided by the Austrian Lightning Detection and Information System (ALDIS), the cloud-to-ground (CG) lightning structure was analyzed. The cross-border study site covers an area of 40,000 square kilometres encompassing the Austrian Federal province of Salzburg and south-east Bavaria, Germany. From 1998 to 2009 about 1 million CG lightning flashes occurred in the area populated by roughly 1.3 million inhabitants.

In order to analyze CG lightning activity fine-grain concerning the spatiotemporal distribution of lightning occurrence and the coherence between lightning activity and topographic properties, a geographic information system (GIS) was used. Furthermore, a cross-border approach for the derivation of hazard zones as an indicator for the optimization of preventive measures was developed. Consequently, the results of this study can be used as a decision support for lightning prevention, risk management, spatial planning and civil protection.

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Symposium und Fachmesse für Angewandte Geoinformatik (AGIT), Salzburg, Austria, 2011

Karten zur Blitzdichte sind eine international übliche Darstellungsform der Blitzhäufigkeit und dienen u.a. der Abschätzung des Blitzrisikos und in weiterer Folge der Auswahl geeigneter Schutzmaßnahmen. Blitzortungssysteme erfassen die Koordinaten von Blitzentladungen und damit die Grundlage zur Berechnung der Blitzdichte. Die räumliche Genauigkeit jeder einzelnen Blitzortung wird dabei quantifiziert. Herkömmliche Methoden zur Punktdichteschätzung berücksichtigen die Ortungsgenauigkeit meist nur mit pauschalen Annahmen für alle Punkte, obwohl die Genauigkeit der Blitzortung stark variiert. Der gegenständliche Beitrag stellt mit der probabilistischen Blitzdichte eine innovative GIS-basierte Methode zur Schätzung der Blitzdichte unter Berücksichtigung der individuellen Ortungsgenauigkeit vor und vergleicht diese mit etablierten Methoden der Blitzdichteschätzung.

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7^th Asia-Pacific International Conference on Lightning (APL), Chengdu, China, 2011

DOI: https://10.1109/APL.2011.61102484

We compare in this paper direct measurements obtained at the Towers on Mount San Salvatore (Switzerland) and the Gaisberg Tower (Austria). They are situated in similar topographical environments but in different lightning activity zones. Direct measurements of lightning currents on these towers have revealed a major difference in terms of the number of downward flashes. While Berger and co-workers obtained a significant number of downward flashes, more recent observations on the Gaisberg and Peissenberg Towers were essentially composed of upward flashes. We use in this paper a new method to estimate the proportion of upward/downward flashes to a given tower, based on the data from lightning location systems. The analysis using the proposed method explains the discrepancy in terms of the measured number of downward flashes in Gaisberg Tower and in Monte San Salvatore Tower.

Union Radio-Scientifique Internationale (URSI), 2011

DOI:https://10.1109/URSIGASS.2011.6050698

We present measurements of nearby vertical and horizontal electric fields from leaders and return strokes
associated with lightning strikes to the 100-m tall Gaisberg Tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20 m from the tower’s vertical axis. Simultaneously with the fields, return-stroke currents were also measured at the top of the tower. The measured data are used to test engineering models for the return stroke. In general, the agreement between measured waveforms and model-predicted ones are satisfactory.

IEEJ Transactions on Electrical and Electronic Engineering, Vol. 5, Issue 1, Pages 8-13, 2010

DOI: https://doi.org/10.1002/tee.20486

Simultaneous measurements of lightning current and associated radiated electromagnetic field are of fundamental interest for various applications in lightning research. These data can be used for the evaluation of return stroke (RS) models or to investigate the so-called tower effect when lightning hits an elevated object. In this paper, we show the results of simultaneous measurements of current pulses from lightning strikes on the instrumented Gaisberg tower (Austria) and the correlated vertical E-field components at a distance of 78.8 and 108.7 km, respectively. We have analyzed some main lightning current parameters (peak current Ip, 30–90% rise time TI–30−90, and full width at half maximum TI–FWHM) and the time-correlated field waveform parameters (Ep, 30–90% rise time TE–30−90, TE–FWHM, and the peak-to-zero time TE–PTZ). With a geometric mean of TI–FWHM = 19 μs and Ip = 9.6 kA (N = 73) of the RS current pulses used in this study, those strokes are very similar to the strokes in triggered lightning in Florida and Alabama [1]. With a TE–PTZ of about 10 μs, the zero-crossing time of the radiated E-fields from the tower strokes are significantly shorter than the typical values of 30 − 40 μs (e.g. [2]). Correlation between the current and field parameters TI–FWHM versus TE–FWHM and TE–PTZ, respectively, is low (R2 = 0.29 and 0.14). We assume that the relatively short lightning channel in the case of the RSs in object-triggered upward flashes is the main reason for the observed short zero-crossing time. © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

30th International Conference on Lightning Protection (ICLP), Cagliari, Italy 2010

DOI: https://10.1109/ICLP.2010.7845770

In this paper we present an analysis of 45 negative flashes based on video and field measurement data. The analysis based on video and E-field measurement data is an ideal method to check lightning location data in terms of cloud-to-cloud or cloud-to-ground classification. We will show that the flash and stroke detection efficiency determined by this analysis is quite similar to the result from measurements at the Gaisberg Tower. We further present some lightning parameters for negative flashes and compare them to values available in the literature.

Journal of Geophysical Research, Vol. 115, Issue D24, 2010

DOI: https://doi.org/10.1029/2010JD014330

Although positive lightning flashes to ground are not as frequent as negative flashes, their large amplitudes and destructive characteristics make understanding their parameters an important issue. This study summarizes the characteristics of 103 positive cloud-to-ground (+CG) flashes that have been recorded using high-speed video cameras (up to 11,800 frames per second) in three countries together with time-correlated data provided by lightning location systems (LLS). A large fraction of the +CG flashes (81%) produced just a single stroke, and the average multiplicity was 1.2 strokes per flash. All the subsequent strokes in multiple-stroke +CG flashes created a new ground termination except one. The geometric mean of 21 interstroke time intervals was 94 ms, which is about 1.5 times larger than the average interstroke interval in negative CG flashes (∼60 ms); 75% of the +CG flashes contained at least one long continuing current (LCC) ≥ 40 ms, and this percentage is significantly larger than in the negative flashes that produce LCCs (approximately 30%). The median estimated peak current (Ip) for 116 positive strokes that created new ground terminations was 39.4 kA. Positive strokes with a large Ip were usually followed by a LCC, and both of these parameters are threats in lightning protection. The characteristics presented here include the multiplicities of strokes and ground contacts, the percentage of single-stroke flashes, the average interstroke time interval, the durations of the continuing current, and the distributions of Ip, the total flash durations, and the 2-D leader speeds.

International Conference on Grounding and Earthing (GROUND) & 4th International Conference on Lightning Physics and Effects (LPE), Salvador, Brazil, 2010

In this paper we present the analysis of the width values reported by lightning location system (LLS) sensor for three different subsets of lightning events. The three data sub-sets are (1) lightning to the Gaisberg Tower (GBT), (2) lightning located near the GBT and (3) lightning located near Vienna where correlated video images are available. Median width values for these three data sets are 11 μs, 20 μs and 27 μs, respectively. The sensor reported width of the field pulses from lightning to the GBT is significantly smaller then the sensor reported field pulse width from lightning to ground near the GBT and near Vienna, respectively. This is in agreement with the observations of Pichler et al. [1] reporting peak-to-zero values of electric field pulses radiated by GBT strokes being significantly shorter than typically zero crossing times observed in natural cloud to ground lightning.

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International Conference on Grounding and Earthing (GROUND) & 4th International Conference on Lightning Physics and Effects (LPE), Salvador, Brazil, 2010

Tall structures such as towers or wind turbines are able to initiate upward lightning. This type of discharges typically starts with an initial stage (IS) that may or may not be followed by one or more downward leader/upward return stroke sequences. At the instrumented Gaisberg Tower (GBT) we have measured significant amounts of charge up to 546 As per flash. This values clearly exceeds the 300 As specified in the IEC 62305-1 standard for Lightning Protection Level I (LPL I). Up to 3 735 As were transferred to ground by 22 flashes during a single storm within 25 minutes. During a 10 years period we determined at the GBT an annual average charge transfer of 4 100 As.

Highest charge transfer is observed in the months March and November, respectively, at the beginning and end of the winter season in Austria indicating certain similarities with so-called winter lightning typically observed in Japan.

At the GBT the maximum accumulated charge transfer in one year was 8 888 As corresponding to a total melted volume of 35 cm3, 48 cm3 and 100 cm3 for Steel, Copper and Aluminum, respectively at the lightning channel attachment point.

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IEEE Trans. Electromagn. Compat., Vol. 52, Issue 4, 2010

DOI: https://10.1109/TEMC.2010.2067218

Benford’s law applies to a wide variety of natural and man-made datasets and it has been successfully applied to test the integrity of data and to detect possible fraud and anomalous results in economy, politics, and management studies. In this paper, we investigate the applicability of Benford’s law to lightning data. To do this, we use lightning data in Switzerland obtained using the European Cooperation for Lightning Detection (EUCLID) network. The considered dataset consists of the cloud-to-ground flashes in Switzerland for the period from 1999 till 2007. First the total number of negative and positive flashes per day is considered in the analysis. It is shown that the obtained distribution is in very good agreement with Benford’s law. The same analysis is repeated considering in the dataset only the flashes containing return strokes with absolute peak currents lower than 5 kA, for which the detection efficiency of the lightning location network is expected to be lower. The resulting distribution shows less agreement with Benford’s law. The obtained results suggest that Benford’s law may find an interesting application in the evaluation of the detection efficiency of a given lightning location network.

International Journal of Plasma Environmental Science & Technology, Vo. 4, No. 1, 2010

We present measurements of very close vertical and horizontal electric fields from leaders and return strokes associated with lightning strikes to the 100-m tall Gaisberg tower in Austria obtained in 2007 and 2008. Simultaneously with the fields, return-stroke currents were also measured at the top of the tower. The vertical and horizontal electric field waveforms appeared as asymmetrical V-shaped pulses, the leading edge being slower than the trailing edge. The width of the V for the horizontal electric field is much smaller than that of the vertical electric field. Due to the shadowing effect of the tower, the return-stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning.

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30th International Conference on Lightning Protection (ICLP), Cagliari, Italy 2010

DOI: https://10.1109/ICLP.2010.7845879

Although positive lightning flashes to ground are not as frequent as negative flashes, their large amplitudes and destructive characteristics make understanding their parameters an important issue. This study summarizes the characteristics of 103 positive cloud-to-ground (+CG) flashes that have been recorded using high-speed video cameras (up to 8000 frames per second) in three countries together with time-correlated data provided by lightning location systems (LLS). A large fraction of the +CG flashes (81%) produced just a single-stroke, and the average multiplicity was 1.2 strokes per flash. All the subsequent strokes in multiple-stroke +CG flashes created a new ground termination except one. 75% of the +CG flashes contained at least one long continuing current (LCC) ³ 40 ms, and this percentage is significantly larger than in the negative flashes that produce LCCs (approximately 30%). The median estimated peak current, (Ip) for 116 positive strokes that created new ground terminations was 39.4 kA. Positive strokes with a large Ip were usually followed by a LCC, and both of these parameters are threats in lightning protection. The characteristics presented here include the multiplicities of strokes and ground contacts, the percentage of single-stroke flashes, the durations of the continuing current, and the distributions of Ip.

30th International Conference on Lightning Protection (ICLP), Cagliari, Italy 2010

DOI: https://10.1109/ICLP.2010.7845919

We present and discuss measurements of electric fields (vertical and radial) from leaders and return strokes associated with lightning strikes to the Austrian Gaisberg Tower (GBT) obtained in 2008. The fields were measured at a distance of about 20 m from the tower. Simultaneously with the fields, return-stroke currents were also measured at the top of the tower. The vertical electric field waveforms appeared as asymmetrical V-shaped pulses. The initial, relatively slow, negative electric field change is due to the downward leader and the ensuing fast positive field change is due to the upward return stroke phase of the lightning discharge. The horizontal (radial) electric field due to the leader phase has a similar waveshape to the vertical electric field. However, the radial field due to the return stroke is characterized by a short negative pulse of the order of 1 microsecond or so, starting with a fast negative excursion followed by a positive one.

30th International Conference on Lightning Protection (ICLP), Cagliari, Italy, 2010

DOI: https://10.1109/ICLP.2010.7845921

Lightning data in Switzerland obtained using the European Cooperation for Lightning Detection (EUCLID) network are used to investigate the applicability of the Benford’s law to lightning data. The considered data set consists of the peak current estimates and interstroke intervals. It is shown that the obtained distributions are in good agreement with Benford’s law.

Asia-Pacific International Symposium on Electromagnetic Compatibility (APEMC), Beijing, China, 2010

DOI: https://10.1109/APEMC.2010.5475852

In this paper, we present simultaneous current and electric field records of two upward negative leaders initiated from the Gaisberg Tower (GBT). It is shown that bipolar current pulses occur at the beginning of the upward negative leaders followed by regular unipolar leader pulse trains, and the corresponding electric field exhibits asymmetrical V-shaped pulses with a hump superimposed at the end of the pulse. The stepped leader characteristics in electric field waveforms at close range are similar to that of downward stepped leader pulses in altitude-triggered lightning flashes. Distinct electric field changes prior to the upward negative leader inception are indicative of nearby lightning discharges.

30th International Conference on Lightning Protection (ICLP), Cagliari, Italy 2010

DOI: https://10.1109/ICLP.2010.7845864

We analyze current records for 21 natural upward-initiated bipolar lightning flashes observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Thirteen (62%) of 21 bipolar flashes occurred in non-convective season (September- March) and twelve (57%) of them occurred in seasonal transmission periods of March, August, and November in Austria. Thirteen (62%) of them belong to Type 1 associated with a polarity reversal during the initial stage current, based on the classification as suggested by Rakov and Uman [1]. We also find that the initial polarity reversal from negative to positive occurs more often (16 of 21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean total absolute charge transfer is 99.5 C with a relatively short total duration of 320 ms.

30th International Conference on Lightning Protection (ICLP), Cagliari, Italy 2010

DOI: https://10.1109/ICLP.2010.7845865

We investigate 172 upward-initiated negative lightning flashes containing at least one leader-return stroke sequence observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. A geometric mean value of 8.75 kA of the return stroke peak current is determined, followed by a continuing current of 1.48 ms with a charge transfer of 0.786 C. We find that in case of the continuing current duration larger than 40 ms, usually defined as long continuing current, or when the continuing current charge transfer is greater than 5 C, the initiating peak return stroke current did not exceed 20 kA. Moreover, continuing currents with durations of less than 20 ms or with a charge transfer smaller than 5 C, those continuing currents were initiated by return strokes with peaks of any range.

Journal of Electrostatics, Vol. 68, Issue 5, Pages 415-418, 2010

DOI: https://doi.org/10.1016/j.elstat.2010.05.014

Towers on mountaintops have more incidence of lightning than towers on the flat ground. Therefore towers on mountaintops are ascribed an effective height that is often considerably larger than the physical height of the tower.

In this paper, we review and evaluate the definitions and methods that could be used to estimate the effective height of a given tower on mountaintop and propose a new definition based on an engineering model of lightning attachment. The results can be useful in designing lightning protection of communication/transmission lines and masts on mountaintops.

Journal of Geophysical Research, Vol. 115, Issue D17, 2010

DOI: https://doi.org/10.1029/2009JD013754

In this paper we present and discuss measurements of electric (vertical and radial) and magnetic fields from leaders and return strokes associated with lightning strikes to the 100 m tall Gaisberg tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20 m from the tower. Simultaneously, return stroke currents were also measured at the top of the tower. The data include, for the first time at such close distances, simultaneous records of vertical and horizontal electric fields. The vertical electric field waveforms appeared as asymmetrical V-shaped pulses. The initial, relatively slow, negative electric field change is due to the downward leader, and the following, fast, positive electric field change is due to the upward return stroke phase of the lightning discharge. The horizontal (radial) electric field due to the leader phase has a waveshape similar to that of the vertical electric field. However, the horizontal field due to the return stroke is characterized by a short negative pulse of the order of 1 ms or so, starting with a fast negative excursion followed by a positive one. The return stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower, which results in a significant decrease of the electric field at distances of about the height of the tower or less. The vertical and horizontal E field changes due to the return stroke were also found to be larger on average than the leader electric field changes. In a significant number of cases (33%), the vertical electric field waveforms due to the return stroke were characterized by a first peak exceeding the typical late-time flattening due to the electrostatic term. This is in contrast with similar measurements related to triggered lightning which do not exhibit such a first peak. About one quarter of the measured vertical electric field waveforms (18 pulses out of 76) featured an unusual waveform characterized by a positive leader field change followed by a bipolar return stroke field change with a zero crossing time of about 60 ms.

21^st International Lightning Detection Conference (ILDC) & 3rd International Lightning Meteorology Conference (ILMC), Orlando, Florida, 2010

From the analysis of digital high-speed videos and electric field records of two flashes, some relationships are presented between the channel luminosity preceding the occurrence of the luminosity pulse and the characteristics of the related electric field signature. Some M components may produce E-field signatures very similar to those produced by subsequent return strokes if the preceding luminosity of channel flash is very faint. These M components may be detected by LLS. If the channel luminosity is high, that is, if the continuing current flowing through it is high, then the E-field signature is different from the signature of a subsequent return stroke. The luminosity intensity preceding each M component along with its 10-90% risetime, estimated peak current and peak E-field are presented.

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21st International Lightning Detection Conference (ILDC) & 3rd International Lightning Meteorology Conference (ILMC), Orlando, Florida, 2010

Detailed comparison of located lightning strokes in a 10 km radius area and directly measured lightning currents to the instrumented Gaisberg Tower (GBT) located in the centre of that area is presented. In 84% of the days, when lightning was measured at the GBT, no thunderstorm (TS) activity was observed in the surrounding area. On the other hand, no lightning events at the GBT were recorded in 77 % of the days, when TS activity was present in the area. Upward initiated lightning from the GBT occurs more or less independent of the general CG activity in the area and we conclude that elevated objects neither reduce nor increase the number of strokes to ground in the objects vicinity significantly. Any additional lightning initiated by the object hits directly the object.

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21st International Lightning Detection Conference (ILDC) & 3rd International Lightning Meteorology Conference (ILMC), Orlando, Florida, 2010

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Annales Geophysicae, Vol. 28, Issue 1, Pages 37–46, 2010

DOI: https://doi.org/10.5194/angeo-28-37-2010

We report a study of penetration of the VLF electromagnetic waves induced by lightning to the ionosphere. We compare the fractional hop whistlers recorded by the ICE experiment onboard the DEMETER satellite with lightning detected by the EUCLID detection network. To identify the fractional hop whistlers, we have developed software for automatic detection of the fractional-hop whistlers in the VLF spectrograms. This software provides the detection times of the fractional hop whistlers and the average amplitudes of these whistlers. Matching the lightning and whistler data, we find the pairs of causative lightning and corresponding whistler. Processing data from ~200 DEMETER passes over the European region we obtain a map of mean amplitudes of whistler electric field as a function of latitudinal and longitudinal difference between the location of the causative lightning and satellite magnetic footprint. We find that mean whistler amplitude monotonically decreases with horizontal distance up to ~1000 km from the lightning source. At larger distances, the mean whistler amplitude usually merges into the background noise and the whistlers become undetectable. The maximum of whistler intensities is shifted from the satellite magnetic footprint ~1° owing to the oblique propagation. The average amplitude of whistlers increases with the lightning current. At nighttime (late evening), the average amplitude of whistlers is about three times higher than during the daytime (late morning) for the same lightning current.

IEEE Transactions on Electromagnetic Compatibility, Vol. 51, Issue 3, 2009

DOI: https://10.1109/TEMC.2009.2021616

We have analyzed parameters of 457 upwardinitiated negative lightning flashes to the Gaisberg tower (GBT) in Austria recorded from 2000 to 2007. Lightning to the radio tower of a height of 100 m and located on a mountain 1287 m above sea level occurs almost independent of season, although a pronounced thunderstorm season exists in Austria during the summer months. A total of 30% of the upward-initiated negative flashes exhibited one or more return strokes with an average multiplicity of 4.4 and a geometric mean interstroke interval of 17.3 ms. The remaining 70% of upward-initiated negative flashes showed an initial continuous current (ICC) only, either with (22%) or without (48%) superimposed current pulses of peak currents greater than 2 kA. Median of total transferred charge to ground is about 30% lower
in summer (26.8 C) than in other seasons (39.0 C). For the return stroke current pulses, we determined a median peak current of 9.2 kA (σlog10 = 0.25) and a median pulse charge of 0.51 C (σlog10 = 0.39). For current pulses superimposed on the ICC, we determined a median of 4.2 kA (σlog10 = 0.26) with 22 kA peak current measured for the largest ICC pulse.

X International Symposium on Lightning Protection (SIPDA), Curitiba, Brazil, 2009

In this paper, we review and evaluate the definitions and methods that could be used to estimate the effective height of a given tower on mountaintop based on the statistical observations. We derive the effective heights based on Rizk’s lightning attachment model, which are less than those predicted by the earlier methods based on statistical observations. Then we perform sensitivity analysis to evaluate the effect of uncertainties in model parameters that influence the effective height. Variations in the effective height as a function of model parameters, including the final quasi-stationary leader gradient, minimum positive streamer gradient, upward connecting positive leader speed, and mountain base radius, are presented, with Gaisberg tower as the example. It’s found that the effective height depends primarily on the structure height, mountain shape and upward positive leader speed. This new approach presented here can be employed to estimate the effective height for towers for which no lightning incidence data needed for the earlier methods are available. And the information could be also used in designing lightning protection of communication/transmission line towers and masts on mountain tops.

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X International Symposium on Lightning Protection (SIPDA), Curitiba, Brazil, 2009

Benford’s law applies to a wide variety of natural and man-made data sets and it has been successfully applied to test the integrity of data and to detect possible fraud and
anomalous results in economy, politics and management studies. In this paper, we investigate the applicability of Benford’s law to lightning data. To do this, we used lightning data in Switzerland obtained using the European Cooperation of Lightning Detection (EUCLID) network. The considered data set consists of the total number of
negative cloud-to-ground (CG) flashes per day in Switzerland for the period from 1999 till 2007. It is shown that the obtained distribution is in very good agreement with Benford’s law. The same analysis was repeated considering in the data set only the flashes containing return strokes with absolute peak currents lower than 2 kA, for which the detection efficiency of the lightning location network is expected to be lower. The resulting distribution shows less agreement with the Benford’s law, especially for the first, third and eighth digits, for which significant differences are obtained. The obtained results suggest that Benford’s law may find an interesting application in the evaluation of detection efficiency of a given lightning location network.

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X International Symposium on Lightning Protection (SIPDA), Curitiba, Brazil, 2009

Monitoring units for the measurement of high-frequency voltage transients have been in operation at three different busses of an Italian medium voltage (MV) distribution feeder, mainly composed by overhead lines, in March 2007 – August 2008. The feeder is located in a region characterized by a high ground flash density value (4 flashes/km2/yr); many of the recorded voltage transients may be correlated with the lightning events detected for the same region by the Lightning Location System (LLS) CESISIRF. The paper presents some experimental results obtained using the monitoring units and their comparison with computer results obtained using a LIOV-EMTP model of the considered MV feeder. A procedure aimed at achieving the best fit between measurements and calculations, which takes into account the uncertainties associated with LLS data, is also presented.

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X International Symposium on Lightning Protection (SIPDA), Curitiba, Brazil, 2009

In this paper we show the first data of time correlated lightning video and electric field measurements in Austria. During one thunderstorm on the June 1st, 2008 we recorded nine positive single stroke flashes to ground. The Austrian lightning location system (LLS) detected all nine positive flashes, although one was categorized as intracloud discharge. Additionally to the nine cloud-to-ground flashes the LLS detected some intracloud discharges and misclassified them as cloud-to-ground discharges. We further show some indication that positive flashes in Austria can also exhibit high peak currents and long continuing currents as shown for Brazil [Saba et al., 2006].

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CIGRE SC C4, Colloquium, Kushiro, Japan, 2009

CIGRE TF C4.404 has recently submitted a comprehensive report for publication, dealing with the effects of performance characteristics of lightning location systems (LLS) on lightning parameters based on data from such systems. This paper will provide an overview and summary of this extensive report.
Lightning parameters are essential input variables to procedures for estimating the lightning performance of transmission lines. Parameters that are typically derived from LLS observations are the ground flash density (GFD), ground stroke density (GSD), peak current distribution, flash multiplicity, and polarity. LLS upgrades and/or LLS expansions are causing changes in the network performance that result in changes in LLS-inferred lightning parameters.
The CIGRE report discusses the effect of using different location methods in terms of required number of sensors to obtain a location. For example, median peak current (absolute value) increased by 47%, from -9.8 kA to -14.4 kA, when data from combined direction-finding and time-of-arrival sensors were reprocessed using only the time information and requiring 4 sensors to compute a location. This effect is reduced with shorter sensor baseline distances, or (equivalently) with greater sensor sensitivity.
Direct measurements of currents in lightning striking instrumented towers or in triggered lightning allow estimation of all three major performance characteristics of LLS’s - detection efficiency (DE), for strokes and flashes, location accuracy, and peak current estimation errors. By deploying most recent technology of sensors a flash DE of 95% or higher is achievable. In a network with small sensor baselines and low sensor threshold a flash DE close to 100% is possible. Corresponding stroke DE is generally lower, but can reach values in the range of 80-90%.
Peak current estimates given by LLS in the United States (NLDN) and Austria (ALDIS) are on average in reasonable agreement with the directly measured peak currents in triggered lightning and at electrically short towers, respectively, although significant differences (up to 50%) are observed for individual strokes, likely caused by the natural variation in return stroke speed.

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Atmospheric Research, Vol. 93, Issue 1-3, Pages 381-396, 2009

DOI: https://doi.org/10.1016/j.atmosres.2008.11.005

This study describes the environmental atmospheric characteristics in the vicinity of different types of severe convective storms in Europe during the warm seasons in 2006 and 2007. 3406 severe weather events from the European SevereWeather Database ESWD were investigated to get information about different types of severe local storms, such as significant or weak tornadoes, large hail, damaging winds, and heavy precipitation. These data were combined with EUCLID (European Cooperation for Lightning Detection) lightning data to distinguish and classify thunderstorm activity on a European scale into seven categories: none, weak and 5 types of severe thunderstorms. Sounding parameters in close proximity to reported events were derived from daily high-resolution T799 ECMWF (European Centre for Medium-range Weather Forecasts) analyses.
We found from the sounding-derived parameters in Europe: 1) Instability indices and CAPE have considerable skill to predict the occurrence of thunderstorms and the probability of severe events. 2) Low level moisture can be used as a predictor to distinguish between significant tornadoes or non-severe convection. 3) Most of the events associated with wind gusts during strong synoptic flow situations reveal the downward transport of momentum as a very important factor. 4) While deep-layer shear discriminates well between severe and non-severe events, the storm-relative helicity in the 0–1 km and especially in the 0–3 km layer adjacent to the ground has more skill in distinguishing between environments favouring significant tornadoes and wind gusts versus other severe events.
Additionally, composite parameters that combine measurements of buoyancy, vertical shear and low level moisture have been tested to discriminate between severe events.

Elektrotechnik & Informationstechnik (e&i) 125/5, Pages 209–213, 2008

Seit nunmehr 16 Jahren stehen dank ALDIS detaillierte Informationen über die Gewitteraktivität in Österreich zur Verfügung. Weniger bekannt ist so manchem Anwender, dass die wesentlichen operativen Aktivitäten von ALDIS im Österreichischen Verband für Elektrotechnik (OVE) stattfinden. Der vorliegende Beitrag bietet einen kurzen Überblick über die Entwicklung von ALDIS, sowohl hinsichtlich der angewendeten Technologie in der Blitzortung als auch in Bezug auf die internationale Einbindung von ALDIS im europäischen Verbund von Ortungs-systemen (EUCLID). Zusätzlich wurden einige statistische Daten über das Blitzgeschehen in Österreich seit der Inbetriebnahme von ALDIS im Jahr 1992 zusammengestellt. Die Abteilung ALDIS im OVE betreibt nicht nur das Blitzortungssystem, sondern ist auch sehr aktiv in der Blitzforschung tätig.

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Geophysical Research Abstracts, Vol. 10, EGU General Assembly 2008 © Author(s) 2008

This study describes the environmental atmospheric characteristics in the vicinity of different types of severe convective storms in Europe during the warm season in 2006 and 2007. A sample of 3406 severe weather events from the European SevereWeather Database (ESWD) is examined and provides information about different types of severe local storms, like significant or weak tornadoes, large hail, damaging winds and heavy precipitation. These data were combined with EUCLID lightning detection data, which provide well-defined null cases on a European scale to distinguish between severe and ordinary or no thunderstorm activity. ECMWF T799 analyses are used to calculate sounding parameters in close proximity to reported severe event locations, for every day within the investigated time period.

Instability indices and CAPE have considerable skill to predict the occurrence of thunderstorms and the probability of severe events. In addition, low level moisture can be used as a predictor to distinguish between significant tornadoes or non-severe convection. Most events associated with wind gusts occurred during high synoptic flow situation reveal the downward transport of momentum as the most important factor. While deep-layer shear discriminates well between severe and non-severe events, the storm relative helicity in the 0-1 km (and surface to lifting condensation level) layer adjacent to the ground has more skill in distinguishing between environments favouring significant tornadoes and wind gusts versus other severe events.

Additionally, composite parameters that combine measurements of buoyancy, vertical shear and low level moisture have been tested to discriminate between severe events. No parameters have been found, which distinguish well between significant tornados and local severe wind events.

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Union Radio-Scientifique Internationale (URSI), General Assembly, 2008

This paper presents a preliminary analysis of recently obtained experimental data associated with lightning strikes to the Gaisberg Tower in Austria. Electric field changes were measured at distances of 22 m and 170 m from the tower base and the magnetic field was measured at a distance of 20 m. Simultaneously, the lightning return stroke current was measured using a sensor located near the top of the tower. The electric field waveforms feature the typical asymmetrical V-shaped pulses, the bottom of the V being associated with the transition from the leader to the return-stroke. The obtained results confirm the shadowing effect of the tower in reducing the electric fields in the immediate vicinity of the tower.

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Union Radio-Scientifique Internationale (URSI), General Assembly, 2008

During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs’ foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg’s is presented.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, Issue 4, 2008

DOI: https://10.1109/TDEI.2008.4591238

During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs’ foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg’s are presented. The measured currents at different locations on the 70-m long ground strip are compared with the predictions of a frequency dependant lossy transmission line (TL) model and reasonably good agreement was found. From this validation it is claimed that the TL models are appropriate for lightning transient analysis of grounding systems.

International Journal of Meteorology, Vol. 33, Issue 327, Pages 98-95, 2008

A common ad-hoc-hypothesis tries to explain ball lightning (BL) as an electromagnetic (EM) brain effect caused by ordinary lightning, i.e. as a lightning-induced hallucination. A critical assessment of this alleged effect has to link the physical properties of lightning and its EM field with the neurophysiology of EM-induced hallucinations, so-called magnetophosphens. Using the clinical field of EM brain stimulation – Transcranial Magnetic Stimulation (TMS) and repetitive TMS (rTMS) – with its experimental phosphene data, the authors conclude that EM fields of nearby lightning flashes, because of their spatial configuration and magnetic induction, are unlikely to produce magnetophosphenes. Phosphenes do not appear in lightning accident reports. Phenomenologically, EM phosphenes as elementary hallucinations do not correspond to BL.

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20th International Lightning and Detection Conference (ILDC) + 2nd International Lightning Meteorology Conference (ILMC), Tucson, Arizona, 2008

Knowledge of probability of occurrence of large peak current flashes is important for lighting protection applications. The international standards for lightning protection of objects are based on probabilities of occurrence of lightning peak currents exceeding given values, where these values are mainly extrapolated from data from lightning current measurements on instrumented towers.
Lighting Locations Systems (LLS) infer peak current from the measured electromagnetic fields. In this presentation we show detailed analyzes of strokes with reported amplitudes of more than –150 kA and +200 kA, respectively.

Similar to Cummins (2000) we will show results of analyzes regarding the number of used parameters to estimate the peak current and calculate the location as a function of peak current. Detailed analysis of individual strokes with reported extremely high amplitudes revealed, that in many cases the amplitude calculation is based on a low number of used parameters for the localization and especially for the peak current estimation. As a result of our study we recommend a very careful interpretation of lightning peak current probability distributions in the range above 100 kA.

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20th International Lightning and Detection Conference (ILDC) + 2nd International Lightning Meteorology Conference (ILMC), Tucson, Arizona, 2008

The performance of a lightning location system (LLS) can vary with time as a result of changes in the location and performance of the sensors that comprise the network. The most common changes are (1) the addition or removal of sensors and (2) updated sensor technology. Changes in LLS performance can lead to significant changes in estimated lightning parameters (e.g. peak current and flash multiplicity statistics - Cummins and Bardo, 2004). Given these facts, it is important to be able to quantify these network changes in terms of the relative detection efficiency (DE) between various network configurations. A method to determine the overall relative stroke DE using peak current distributions has been presented by Cummins and Bardo [2004], and is described in detail in an upcoming CIGRE report by Task Force C4.404A. This approach has the limitation that any changes in network configuration that alter the individual stroke peak current estimates will introduce errors in the DE calculation. Further, it does not provide relative flash DE without additional calculations involving multiplicity measurements. The method described in this paper is not subject to the limitations noted above, although we show that it is sensitive to other LLS-derived parameters. The basis for this detection efficiency (DE) correction using multiplicity distributions is the work by Rubinstein [1995], where he shows the relation between flash and stroke DE.

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29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

Electric field measurements of first and subsequent strokes by the Austrian Lightning Detection and Information System (ALDIS) and by electric-field antennas in Florida, United States, show no agreement about the relationship between first-stroke peak field and subsequent stroke peak field in negative cloud-to-ground lightning [Diendorfer et al., 1998]. While in Florida the results are in agreement with what is usually accepted in the literature, that is, the median negative first-stroke peak field (or current) is approximately two times larger than subsequent stroke peak field in the same channel, in Austria the ALDIS network has observed no difference between the median values of the electric field (or current) for first and subsequent strokes. To investigate this controversy in more detail several field measurement campaigns were performed during the last years, e.g. in Austria [Schulz and Diendorfer, 2006] and in Brazil [Filho et al., 2007]. During summer 2006 field measurement data was collected in Sweden also with the same field measurement system which was used in Austria and Brazil [Schulz and Diendorfer, 2006]. In this paper we will show a comparison of the peak field ratio between first and subsequent strokes given by the field measurement data and the Swedish lightning location system. We will further compare the result from Sweden with results from Austria and Brazil.

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29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

LLS infer lightning peak currents from remotely measured electric and magnetic peak fields assuming a linear relationship between peak field and peak current. Directly measured currents at either instrumented towers (e.g Gaisberg Tower, CN Tower, Peissenberg Tower) or at the channel base of rocket triggered lightning are the only available ground truth data to verify the accuracy of LLS peak current estimates.

We compare the directly measured peak currents versus LLS inferred peak currents for lightning to towers of different height, ranging from 100 m (Gaisberg) to 553 m (CN Tower) and for triggered lightning, where the lightning channel termination point is typically close to the ground level. In recent publications [1-4], different relations between measured and inferred peak currents were reported. At the CN Tower, the NALDN peak currents were notably larger than the measured peak currents probably because the assumed relationship between field and current does not account for the transient process in the tower.

Differences in the estimated peak current (relative to the measured one) may also result from differences in the configuration of the employed LLS. A propagation model is used in the NALDN to account for field attenuation due to finite ground conductivity, whereas in the ALDIS system a pure 1/r distance dependency of the fields was used until 02/2005. The effect of the propagation model is expected to be more pronounced when sensors at larger distances from the striking point are used for locating the strokes. After applying the attenuation correction to the ALDIS network, the LLS shows a tendency to underestimate the Gaisberg tower lightning peak currents slightly more than the US-NLDN underestimates the triggered lightning. Significant field enhancement due to strokes to tall towers is only seen for the CN Tower.

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29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

In this paper we present lightning statistics in the regions of the Säntis Tower and the St. Chrischona Tower in Switzerland. We analyze lightning data for an eight-year period from 1999 to 2006. This work is part of a recent study on overall lightning activity in Switzerland. Lightning location data from the EUCLID (European Cooperation of Lightning Detection) lightning location system (LLS) were used in the study.

The two telecommunications towers are situated in two distinct parts of the country, characterized by different geographical conditions. The Säntis Tower is 124 m tall and is located on the top of the Säntis Mountain (2505 m ASL) in the eastern Swiss Alps. The tower location exhibits the highest lightning flash density in Switzerland during the period from 1999 to 2006. The St. Chrischona Tower is 250 m tall and is located in a relatively flat region near Basel in the northern part of Switzerland (493 m ASL).  Various regional maps and statistics around the two towers are presented, including number of flashes and strokes, number of strokes per flash (flash multiplicity), and peak current. The effect of each tower was analyzed by comparing lightning statistics within a defined range around the tower with those obtained on an external ring excluding the tower, as done previously for the analysis of the Gaisberg tower in Austria [1]. The results indicate that the lightning incidence to the Säntis tower (about 100 times a year) is much higher than that to the St. Chrischona tower (less than 10). We found also a relatively high value of flash multiplicity for strikes detected in the Säntis tower region, implying that most of strikes to this tower are upward initiated flashes.

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29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

Today, values of local ground flash density (GFD) are estimated from data from lightning location systems. Lightning is a stochastic phenomenon and it’s occurrence at a given location can be described by a so-called Poisson distribution. Assuming pure random nature of the lightning events from the Poisson distribution we can estimate the achievable accuracy of GFD values as a function of observation period and grid cell size. An accuracy of about ± 20% is achievable when on average more than about 80 events occurred in each grid cell. This finding suggests (1) to adjust the grid cell size A_cell according to the expected GFD and available observation period T_obs and (2) to consider an uncertainty range of at least ± 20% for any N_g value that is based on LLS data by counting lightning events in defined grid cells.

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Journal of geophysical research, Vol. 113, D19, 2008

DOI: https://doi.org/10.1029/2007JD009729

We examine relative magnitudes of electric field peaks of first and subsequent return strokes in negative cloud-to-ground lightning flashes recorded in Florida, Austria, Brazil, and Sweden. On average, the electric field peak of the first stroke is appreciably, 1.7 to 2.4 times, larger than the field peak of the subsequent stroke (except for studies in Austria where the ratio varies from 1.0 to 2.3, depending on methodology and instrumentation). Similar results were previously reported from electric field studies in Florida, Sweden, and Sri Lanka. For comparison, directly measured peak currents for first strokes are, on average, a factor of 2.3 to 2.5 larger than those for subsequent strokes. There are some discrepancies between first versus subsequent stroke intensities reported from different studies based on data reported by lightning locating systems (LLS). The ratio of LLS-reported peak currents for first and subsequent strokes confirmed by video records is 1.7 to 2.1 in Brazil, while in the United States (Arizona, Texas, Oklahoma, and the Great Plains) it varies from 1.1 to 1.6, depending on methodology used. The smaller ratios derived from the LLS studies are likely to be due to poor detection of relatively small subsequent strokes. The smaller values in Austria are possibly related (at least in part) to the higher percentage (about 50% versus 24–38% in other studies) of flashes with at least one subsequent stroke greater than the first. The effects of excluding single-stroke flashes or subsequent strokes in newly formed channels appear to be relatively small.

European Electromagnetics (EUROEM), Lausanne, Switzerland 2008

Considering the importance of global warming and climate change, the problem of possible relationships between climate and lightning activity has recently attracted the attention of researchers (e.g. [1]). Positive correlations between global lightning activity and global temperature variations have been already confirmed in several studies (e.g. [1,2]) where it has been shown that a 1-degree Celsius variation in the temperature would result in an increase of lightning activity, ranging from 10% to 100% [2]. In this paper, we present a preliminary regional study of the link between temperature and lightning activity in Switzerland. Lightning statistics were obtained from the lightning database of the EUCLID (European Cooperation of Lightning Detection) network, recently presented in [3]. Any unknown time variations in the performance of the system are necessarily neglected. Data for the temperature are obtained from the terrestrial meteorology stations in the national network of the Swiss Federal Office of Meteorology and Climatology [4]. Fig. 1a presents the monthly negative cloud-to-ground flash count and the monthly mean maximum daily temperature in Switzerland for the months of August, 1999-2006, where the linear correlation factor is 75%. A similar analysis has been done in the region of the Säntis in northeastern Switzerland, characterized by the highest lightning activity in that country. Fig. 1b shows the monthly mean of maximum daily temperature and the monthly number of the negative cloud-to-ground flashes within 100 km of the Säntis Mountain. The results show an 83% linear correlation.

Acknowledgments - This work has been partially supported by the European COST Action P18 ‘The Physics of Lightning Flash and Its Effects’.

References
1. C. Price, Global surface temperatures and the atmospheric electric circuit, Geophys. Res. Lett. 20, 1363, 1993.
2. E.R. Williams, Lightning and climate: A review, Atmospheric Research, 76, pp. 272-287, 2005
3. P. Manoochehrnia, F. Rachidi, M. Rubinstein, W. Schulz, Lightning statistics in Switzerland, 9th International Symposium on Lightning Protection, SIPDA, Foz do Iguaçu, Brazil, November 2007.
4. M. Begert, T. Schlegel, and W. Kirchhofer, Homogeneous Temperature and Precipitation Series of Switzerland from 1864 to 2000, International Journal of Climatology, vol. 25, pp. 65-80, 2005.

20th International Lightning and Detection Conference (ILDC) + 2nd International Lightning Meteorology Conference (ILMC), Tucson, Arizona, 2008

The UK Met Office VLF arrival time difference (ATD) long range lightning location network has been operating successfully for nearly 20 years. The range includes all of Europe, North Africa, the North Atlantic and most of South America. Recent expansions and improvements to the network have increased the range of detectable lightning to now include all of South America, Africa and central Asia. The improved network (now called ATDnet) has been operating offline in parallel to the original system for testing, but has now replaced the current operational system since December 2007.

The increased coverage and new receiver instrumentation for the ATDnet network will be discussed. The network is compared to other European lightning detection systems to assess the accuracy and detection efficiency over Europe. A significant diurnal variation in the detection efficiency of ATDnet is observed over Europe, which is suggested to be due to a nocturnal enhancement of wave guide modal interference.

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IX International Symposium on Lightning Protection (SIPDA), Foz do Iguaçu, Brazil, 2007

In this paper we present the results of a recent study on lightning statistics in Switzerland during an eightyear period from 1999 till 2006, using data from the EUCLID (European Cooperation of Lightning Detection) LLS (Lightning Location System). After a brief presentation of the history of lightning detection in Switzerland, statistics of some salient lightning parameters in Switzerland are presented. It is shown that there is a relatively high lightning activity in Switzerland especially in the Canton of Tessin, located south of the Alps. Additionally, it is found that the lightning flash density in some regions of Switzerland (Tessin) is higher than the maximum lightning flash density in Austria and Germany while the flash median peak current and the number of strokes per flash (flash multiplicity) are similar in the three countries. We observed a significant improvement of network performance from 1999 to 2006.

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IX International Symposium on Lightning Protection (SIPDA), Foz do Iguaçu, Brazil, 2007

Simultaneous measurements of lightning current and associated radiated fields from tower lightning are of fundamental interest for various reasons in lightning research. These data can be used for the evaluation of return stroke models or to investigate the so called tower effect when lightning hits an elevated object [1].

In this paper we show first results of simultaneously measured far-field waveforms at a distance of 78.8 km together with the corresponding current pulses measured at the top of the instrumented Gaisberg tower in Austria. We have analyzed the Ep/Ip ratios separately for two distinct groups of current pulses observed at the tower, the so called á-pulses, which are superimposed on the initial continuing current and the â-pulses, which occur after the initial continuing current. It is generally accepted, that â-pulses are assumed to be most comparable to subsequent strokes in flashes to ground.

Based on the available experimental data we determined a field enhancement factor of 1.6 compared to the predicted transmission line model Ep/Ip ratio. This observation is comparable to results with triggered lightning data [2] and agrees with a calculated enhancement factor for an “electrically short tower” in [1].

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IX International Symposium on Lightning Protection (SIPDA), Foz do Iguaçu, Brazil, 2007

Nowadays various techniques are employed to locate lightning activity over large areas. Electric and/or magnetic field sensors in the VLF, LF and VHF frequency ranges measure the lightning radiated electromagnetic fields. For locating ground strike points magnetic direction finding (MDF), time-of-arrival (TOA), or a combination of both (MDF+TOA) is employed. Major difference between those techniques is the minimum number of sensor required to calculate a stroke location. Network configuration (mean baseline between sensors, network geometry) and the type of sensors employed strongly affect the achievable detection efficiency (DE) and the resulting peak current distribution. Normally it is the strokes with small peak currents that are missed by lower DE networks and this results in a bias of the peak current distribution to higher values. In this paper we present some results of model calculations and performance evaluation of a LLS based on directly measured lightning to an instrumented tower in Austria.

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IX International Symposium on Lightning Protection (SIPDA), Foz do Iguaçu, Brazil, 2007

In this paper we show details about the relationship between first and subsequent stroke electric field peak in negative cloud-to-ground lightning in Brazil.
A field measurement campaign during summer 2006 in Brazil collected electric field data with a GPS synchronized fast electric field flat-plate antenna. The data analysis shows that the ratio between first stroke fields and subsequent stroke fields in Brazil and in Austria is smaller than in Florida and that the ratio which is including single strokes, differs significantly even between Austria and Brazil. We further found that 38.2% of the flashes in Brazil had at least one subsequent stroke peak field greater than the first stroke peak field.

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13^th International Conference on Atmospheric Electricity (ICAE), Beijing, China, 2007

In this paper we show details about the relationship between first and subsequent stroke electric field peak in negative cloud-to-ground lightning in Brazil. A field measurement campaign during summer 2006 in Brazil collected electric field data with a GPS synchronized fast electric field flat-plate antenna. The data analysis shows that the ratio between first stroke fields and subsequent stroke fields in Brazil and in Austria is smaller than in Florida and that the ratio which is including single strokes, differs significantly even between Austria and Brazil. We further found that 38.2% of the flashes in Brazil had at least one subsequent stroke peak field greater than the first stroke peak field.

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13^th International Conference on Atmospheric Electricity (ICAE), Beijing, China, 2007

The features of lightning flashes occurring on tall structures can give some valuable information about their initiation, especially the cases of upward flashes for lightning protection of tall structures such as wind-turbines. More than 300 flashes which struck the Gaisberg tower (Austria) have been analyzed in order to investigate differences in occurrence conditions between winter and summer. More flashes struck the tower in winter unlike the Austrian lightning activity is much lower during this season. These winter flashes deposit to ground more electrical charge than summer ones. The median values of the -10 ºC isotherm altitudes during winter are 2-km lower than during summer. This difference would produce stronger electric fields at ground level which could favor the triggering of upward flashes.

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Journal of Neurology, Neurosurgery & Psychiatry (JNNP), Vol. 78, Issue 4, 2007

DOI: https://dx.doi.org/10.1136/jnnp.2006.097642

The case of a 23-year-old mountaineer who was hit by a lightning strike to the occiput causing a large central visual field defect and bilateral tympanic membrane ruptures is described. Owing to extreme agitation, the patient was set to a druginduced coma for 3 days. After extubation, she experienced simple and complex visual hallucinations for several days, but otherwise recovered largely. Neuropsychological tests revealed deficits in fast visual detection tasks and non-verbal learning, and indicated a right temporal lobe dysfunction, consistent with a right temporal focus on electroencephalography. Four months after the accident, she developed a psychological reaction consisting of nightmares with reappearance of the complex visual hallucinations and a depressive syndrome. Using the European Cooperation for Lightning Detection network, a meteorological system for lightning surveillance, the exact geographical location and nature of the lightning flash were retrospectively retraced.

Invited Talk presented at the 9^th Int. Conference on EMI&EMC (INCEMIC), Bangalore, India, 2006

Cloud-to-ground (CG) lightning parameters and the spatial distribution of lightning flashes are of fundamental interest for the design of lightning protection systems. In the past lightning location systems (LLS) have been installed in many countries around the world and these systems can provide large scale observation of the lightning occurrence together with some additional information about polarity and peak current of the individual strokes.

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Elektrotechnik & Informationstechnik (e&i), Heft 1/2, 123. Jahrgang, 2006

Die jährlichen Schäden an elektronischen Einrichtungen durch die Einwirkungen von direkten bzw. nahen Blitzschlägen sind enorm. Leider gibt es nur sehr vage Schätzungen der Schäden durch Blitzschlag, da diese durch unterschiedlichste Versicherungspakete abgedeckt werden, bzw. daraus resultierende Folgeschäden, wie Betriebsunterbrechungen, oft gar nicht versichert sind. Im Sinne der EMV Grundstruktur Störquelle - Übertragungsweg - Störsenke stellt die Blitzentladung eine natürliche Störquelle dar. Die Eigenschaften dieser Störquelle sind einerseits bestimmt durch die Stromparameter der verschiedenen Blitzstromkomponenten und anderseits durch die Kenngrößen der mit der Entladung verknüpften elektromagnetischen Felder. In den Vorschriften zum Blitzschutz (IEC 62305-3) [3] findet man Kennwerte für die zu erwartenden Blitzströme, IEC 62305-4 [4] behandelt den Schutz von elektrischen und elektronischen Einrichtungen in Gebäuden im Falle eines direkten oder nahen Blitzschlages. In Anlehnung an die zu erwartenden Störgrößen wurden in der Vergangenheit verschiedenste Prüfimpulse für Stossströme und Stoßspannungen entwickelt, die auch zum Teil bei der EMV Prüfung von Geräten zum Einsatz kommen.

Annual costs of damages caused by direct or nearby lightning on electric and electronic infrastructure are enormous. Unfortunately there are only rough estimates of the total costs because they are covered by different insurance packages and often costs for the lightning caused service interruptions are not recorded or insured at all. Lightning is a natural source of current and electromagnetic field in terms of the electromagnetic compatibility concept Source - Coupling Path - Receiver, whose properties are determined by the parameters of the different lightning current components and their corresponding EM fields. In the standard for lightning protection IEC 62305-3 [3] parameters of the lightning current are listed. IEC 62305-4 [4] deals with the protection of electronic equipment in case of a direct or nearby lightning strike. In the past different current and voltage pulses have been specified to simulate the effect of lightning strikes and some of them are also applied in EMC tests.

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28^th International Conference on Lightning Protection (ICLP), Kanazawa, Japan, 2006

In this paper it is shown how the finitely conducting ground modifies the signature of the radiation field of return strokes striking tall towers. Results are presented for different tower heights and for different ground conductivities varying the current risetime in the return stroke model. The results show that the attenuation of the initial peak of the radiation field resulting from the propagation over finitely conducting ground depends strongly on the current risetime, the tower height and the ground conductivity. In general, the attenuation of the radiation field of lightning flashes striking tall towers is larger than that striking flat ground. In the case where the ground conductivity is extremely poor, namely 0.0001 S/m, the attenuation of the peak radiation field may reach as much as 70% in the case of lightning flashes striking a 300-m tall tower.

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28^th International Conference on Lightning Protection (ICLP), Kanazawa, Japan, 2006

In 2003 we have developed a PC based and GPS synchronized field measurement system which is able to measure and store electric field data continuously. This field measurement system is based on a 12 bit digitizing board operated with a sampling rate of 5 MS/s. The board allows to record a maximum of two channels at the same time. Once every second the field data is stored on the hard disc of the PC. Depending on the number of recorded channels (one or two) the size of the file is 10MB or 20MB, respectively [Schulz et al., 2005a]. The main advantage of such a continuous and GPS synchronized field measurement system compared to a triggered system is that it is not suffering from any trigger threshold and dead time and therefore we basically do not miss any events. Consequently this system is ideal
for critical analyzes of data from lightning location systems (LLS). In this paper we will show a comparison of the flash multiplicity and the interstroke interval derived from data from the Austrian LLS ALDIS (Austrian Lightning Detection and Information System) and the data from the new field measurement system. We will further compare the result with parameters previously published in the literature.

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28^th International Conference on Lightning Protection (ICLP), Kanazawa, Japan, 2006

We compare the characteristics of the M components and the initial continuous current pulses (ICC pulses) in natural upward lightning as observed on the Gaisberg tower (100 m), the Peissenberg tower (160 m), and the Fukui chimney (200 m) with their counterparts in rocket-triggered lightning in Florida for the clarification of the mechanism of the ICC pulses. All lightning events analyzed here effectively transported negative charge to ground. The peak, duration, risetime, half peak width and charge transfer of ICC pulses are similar to those of the M components. However, the parameters of the ICC pulses and the M components in Florida rocket-triggered lightning, except for the charge transfer, are different from those in natural upward lightning flashes. Furthermore, optical images of the ICC pulse have been obtained using video cameras. From these results, we discuss the mechanism of the ICC pulses.

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28^th International Conference on Lightning Protection (ICLP), Kanazawa, Japan, 2006

Lightning to elevated towers is mainly upward initiated. We have recorded 9 flashes to an instrumented tower that lowered positive charge to ground and have therefore been initiated by upward negative leaders from the tower top. All these 9 positive flashes occurred during the cold season or winter time (September - March). The observed overall current waveforms typically exhibit a front section with significant pulsing structure lasting for about 2.2 ms (mean). The total flash duration is in the range from 5 to 200 ms (mean 62 ms) and total charge transfer is in the range from 20 to 356 C (mean 128 C). Mean values of the peak current of these leader pulses are in the range of 1.6 to 13.7 kA. For each of the 9 flashes we have determined for the distinct pulses a mean charge transfer of 0.013 C to 0.321 C per pulse. The similarity of negative downward and upward stepped leaders is discussed.

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European Geosciences Union, Annales Geophysicae, 24, Pages 2921-2929, 2006

We study the penetration of lightning induced whistler waves through the ionosphere by investigating the correspondence between the whistlers observed on the DEMETER and MAGION-5 satellites and the lightning discharges detected by the European lightning detection network EUCLID. We compute all the possible differences between the times when the whistlers were observed on the satellite and times when the lightning discharges were detected. We show that the occurrence histogram for these time differences exhibits a distinct peak for a particular characteristic time, corresponding to the sum of the propagation time and a possible small time shift between the absolute time assigned to the wave record and the clock of the lightning detection network. Knowing this characteristic time, we can search in the EUCLID database for locations, currents, and polarities of causative lightning discharges corresponding to the individual whistlers. We demonstrate that the area in
the ionosphere through which the electromagnetic energy induced by a lightning discharge enters into the magnetosphere as whistler mode waves is up to several thousands of kilometres wide.

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19^th International Lightning and Detection Conference and  1^st International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2006

In the past a considerable number of papers on winter lightning in the costal area of the Sea of Japan has been published. But very limited information is available about winter lightning characteristics in other regions. Every year a few winter thunderstorms occur over the area of Austria. In this paper we present the analysis of one specific winter thunderstorm with special regard to meteorological aspects and lightning activity. The investigation is limited to the area of Salzburg where several flashes have been recorded at the instrumented tower on Gaisberg, which is located very nearby.

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International Conference on Grounding and Earthing and 2^nd International Conference on Lightning Physics and Effects (GROUND/LPE), Maceió, Brazil, 2006

The aim of this work is to obtain some characteristics of the preliminary breakdown (PB) pulses for negative cloud-to-ground flashes. We present the correlation between the electric field peak of first strokes and subsequent new-channel strokes and the time interval from the beginning of the PB and the stroke event. For this purpose we employed a high-speed observation system (with a time resolution of 1000 frames per second), electric field measurements (flat plate antenna) and data from the Brazilian Lightning Location System - RINDAT. All the measurement systems were synchronized with GPS. All measurements and observations were made in S. José dos Campos (45.864°W; 23.215°S), Brazil. The beginning of the PB pulses could be determined both in the electric field data and in the high-speed video recordings (in which it is possible to observe a low intensity light emission from the cloud). The identification of this instant is a very difficult task in the absence of the images. The electric field peak values of the strokes were all normalized to 100 km, based on the location solution given by RINDAT. An amount of 30 first strokes and 16 subsequent new-channel strokes, all from the same day (March 23rd, 2005), were analyzed, and
their data (PB to stroke time interval versus the inverse of the stroke electric field peak) were plotted. The graph obtained for first strokes showed a linear relationship (correlation coefficient: 0.85). No correlation was found for subsequent new-channel strokes.

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19^th International Lightning and Detection Conference and  1^st International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2006

In this paper we present the results of a recent study on lightning statistics in Switzerland during an eightyear period from 1999 till 2006, using data from the EUCLID (European Cooperation of Lightning Detection) LLS (Lightning Location System). After a brief presentation of the history of lightning detection in Switzerland, statistics of some salient lightning parameters in Switzerland are presented. It is shown that there is a relatively high lightning activity in Switzerland especially in the Canton of Tessin, located south of the Alps. Additionally, it is found that the lightning flash density in some regions of Switzerland (Tessin) is higher than the maximum lightning flash density in Austria and Germany while the flash median peak current and the number of strokes per flash (flash multiplicity) are similar in the three countries. We observed a significant improvement of network performance from 1999 to 2006.

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VIII International Symposium on Lightning Protection (SIPDA), Sao Paulo, Brazil, 2005

In 2003 we developed a PC based and GPS synchronized field measurement system which is able to measure and record electric field data continuously. This field measurement system is based on a 12 bit digitizing board operating with a sampling rate of 5 MS/s. The board allows to record a maximum of two channels at the same time. Once every second the field data are stored on the hard disc of the PC. Depending on the number of recorded channels (one or two) the size of the "one-second" file is 10MB or 20MB, respectively. Such a continuous and GPS synchronized field measurement system has some important advantages compared to a triggered system. There is no trigger threshold and no dead time and therefore we basically do not miss any events. In this paper we show some preliminary results of a first measurement campaign in summer 2004. We show results from the amplitude ratios between first and subsequent strokes [Diendorfer et al., 1998] and we give some examples of erroneously classified bipolar flashes [Schulz and Diendorfer, 2003].

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Journal of Geophysical Research, Vol. 110, 2005

In this paper we present lightning statistics for more than three million cloud-to-ground (CG) flashes located during the 10 year operation period 1992-2001 of the Austrian lightning location system (LLS) called ALDIS (Austrian Lightning Detection and Information System). Like a majority of other lightning systems operated worldwide, ALDIS underwent configuration changes and continuous performance improvement. Since these changes can alter the lightning statistics, we also relate the variation of the individual lightning parameters during the period of operation to changes in ALDIS configuration and performance. This analysis should be useful to other network operators and data users. Flash densities in Austria are normally between 0.5 and 4 flashes km-2 yr-1 depending on terrain. Flash densities higher than 4 flashes km-2 yr-1 are typically related to mountain tops or high towers on elevated sites. Flashes are classified as negative, positive or bipolar (both negative and positive strokes comprising the flash). 17% of the flashes were classified as positive (90% single strokes and 10% multistrokes), and 2.3% of the total number of flashes were bipolar. 50% of the positive multiple-stroke flashes were bipolar flashes with positive first stroke -- this influences the positive flash multiplicity and interstroke interval statistics. Compared to many other networks, the ALDIS network reports much lower median negative peak currents. For 2001, the median first-stroke peak current for negative flashes was 10kA. We show that even when using the same configuration parameter as used in the U.S. National Lightning Detection Network (NLDN), the median first-stroke peak current in an NLDN region with similar climate is about 30% higher than in Austria. Some of this difference is likely due to better detection efficiency (DE) in the ALDIS network. Estimated multiplicity of negative flashes for the 10-year period is affected by the algorithm that groups strokes into flashes, as well as the improved DE of the network as a result of the integration of ALDIS into the European LLS (EUCLID). This performance improvement also resulted in a higher number of single stroke flashes. Interstroke interval and median first-stroke peak current show a clear correlation with multiplicity for negative flashes, irrespective of DE. Negative flashes with higher multiplicity show smaller average interstroke intervals and larger first stroke median peak currents. No correlation between interstroke interval and stroke order was found. On average, regions with higher flash density show slightly higher flash multiplicity.

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Journal of Geophysical Research, Vol. 110, Issue D2, 2005

DOI: https://doi.org/10.1029/2003JD004474

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their
counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

International Conference on Lightning and Static Electricity (ICOLSE), Seattle, Washington, 2005

When lightning strikes a tree, the lightning current commonly flows along the trunk to ground or it is diverted to the branches, from where it jumps over to ground. The current flow along the trunk normally damages the bark, which is a strong indication that most of the lightning current flows at the surface of the trunk. In the paper we present two examples where obviously the lightning currents entered the trunk. The currents flowing through the trunks had so high amplitudes and specific energies (? i2 dt), that the trunk was completely destroyed. In April 2000, a very strong destruction of a fir occurred in a forest in the South of Germany about 100 km away from Munich. The struck fir was about 32 m high and the diameter at the bottom was greater than 60 cm. The fir splintered into three major fragments, where each of them was estimated to have a weight in the order of half a ton. The explosion was so severe that the major parts of the tree were blasted away more than 10 m from the remnant stub. Further, lots of smaller fragments with weights up to more than 100 kg were found in the surrounding area up to about 80 m distance from the tree. Due to the extent of the destruction it is concluded that parts of the trunk really exploded. At more than 10 surrounding trees large areas of the bark were damaged obviously by fragments hitting them with high speed. The data from the German lightning detection system (Siemens, BLIDS) revealed, that the fir was probably struck by a positive cloud-toground lightning having a current peak value of roughly 50 kA. Similar destructions occurred in Austria, where also a tree was found which also obviously exploded. With the Austrian lightning detection system (ALDIS) the lightning strike could successfully be detected. Also in this case the tree was obviously struck by a positive cloud-to-ground lightning having a current peak value of about 100 kA.

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Elektrotechnik und Automation (ETZ) 2, 2005

Jahres 2000 erkennbar. 1999 wurden zusätzlich sechs Sensoren in den Benelux-Ländern und drei Sensoren in Polen in das Netzwerk integriert. Wegen Problemen mit dem Ortungsalgorithmus, wenn große Bereiche ausschließlich mit LPATS-Sensoren abgedeckt werden, Das deutsche Blitzortungssystem BLIDS (Blitzinformationsdienst von Siemens) ist seit mehr als elf Jahren in Betrieb. Während dieser Zeit haben mehrere Änderungen im Bereich der Hard- und Software stattgefunden, die zu einer Qualitätsverbesserung des Ortungssystems geführt haben. In diesem Beitrag zeigen wir, welchen Effekt diese Leistungssteigerungen auf verschiedene Blitzparameter während der einzelnen Jahre haben.

siehe auch Jahrbuch der Elektrotechnik 2006, VDE Verlag, Band 25.

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Geophysical Research Abstracts, Vol. 7, 10175, 2005

Lightning to an instrumented tower (height 100 m) on Gaisberg, near to the city of Salzburg, is measured since 1998. From 2000-2003 we have recorded a total of 242 flashes to the instrumented tower. Lightning current is measured by digitizing 20MS/s) the output signal of a 0.25mOhm current viewing resistor (shunt) over a sampling time of 800 ms. The fast majority (236 out of 242) of the recorded flashes were upward initiated, as typical for elevated objects. 9 flashes lowered positive charge to ground and have therefore been initiated by upward negative leaders from the tower top. All these 9 positive flashes occurred during cold season or winter time (September - March). Total flash duration is in the range from 6 to 190 ms (mean 70.8 ms) and total charge transfer is in the range from 20 to 376 As (mean 130 As). Typically the observed overall current waveform exhibits a front section with significant pulsing structure lasting for about 2.1 ms (median).Recorded waveforms are very similar to measured currents of triggered winter lightning in Japan (Yoda et al., 1997, Nakamura et al., 1997). We have analyzed in detail the pulsing front section of the current waveforms. They show 25 to 89 (mean 60) current pulses with pulse duration of 27 to 44µs (mean 34 µs). D The pulses are superimposed on a steadily increasing continuous current. The pulse duration is comparable to the inter-step intervals of 30 to 50 µs determined optically for negative upward leaders observed at Mont San Salvatore (see Rakov and Uman, 2003). Mean values of the peak current of these leader pulses are in the range of 1.6 to 13.7 kA. For each of the 9 flashes we have determined for the distinct pulses a mean charge transfer in the range from 0.013 C to 0.321 C. Wada et al. (2002) measured for the same type of lighting to a 200 m high chimney in Japan a leader propagation speed of 6.106 m/s. Assuming a speed of 6.106 m/s a pulse-duration of 34 µs corresponds to a step length of 204 m, which is in the same range as the optically observed step lengths reported by Wada et al (2002). For three distinct steps Wada et al. (2002) determined a two dimensional length of 45m, 118m and 311m.

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International Workshop on Physics of Lightning (IWPL), Guadeloupe, 2004

Since 1998 direct lighting strikes to a radio tower are measured at Gaisberg, a mountain next to the City of Salzburg in Austria. During the period 2000 - 2003 we recorded 129 negative flashes containing 723 strokes with amplitudes greater than 2 kA. From these 129 negative flashes 114 were detected by the LLS (88% FDE). Overall stroke detection efficiency (SDE) - typically lower than FDE - was determined with 68%. For the subset of ß-strokes, which are assumed to be most similar to subsequent strokes in downward flashes we observed a SDE of 81%.

Locations given by the LLS have a mean location error of 450 m. For the correlation of peak currents provided by the LLS and measured directly at the tower we have determined a regression line I_ALDIS = 1.07*I_GB for a-strokes and I_ALDIS = 1.02 * I_GB for ß-strokes.

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18th International Lightning Detection Conference (ILDC), Helsinki, Finland, 2004

201 lightning flashes with a total of 1015 strokes were recorded at the Gaisberg tower from January 2000 to September 2002. From these 201 recorded flashes 62 (31%) did not show any impulse current superimposed on or following the initial continuing current (ICC) being typical for upward initiated lighting.

From the remaining 139 events showing pronounced current pulses, 129 flashes were classified as upward discharges, 10 as downward flashes. This classification is based on the measured lightning channel current prior to the first stroke current pulse. Unfortunately video images showing the direction of channel branching are not available for many events due to bad visibility. The classification from the current flow only may be questionable in some cases.

In the presentation we will show cumulative frequency distributions for the different lightning current parameters as total charge (see Fig) and impulse charge (Q), peak current (Ip), specific energy (W/R) and current steepness (di/dt). The statistical analysis is done separately for the so called a -pulses (superimposed on the ICC) and ß-pulses (following a period of no current in the lightning channel).

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18th International Lightning Detection Conference (ILDC), Helsinki, Finland, 2004

Recently Diendorfer et al. [2002] compared lightning peak currents measured at the Gaisberg tower with correlated lightning peak currents reported by the Austrian lightning location system ALDIS. They found a surprisingly good agreement between the measurements at the tower and the amplitudes reported by the lightning location system (LLS).

There are several factors which might influence the relation between the directly measured current at the tower and the current reported by the LLS. The effect of the striking object height and the field attenuation are the key elements for a comparison between the two current values. In this paper we will show the possible range of influence for some of the most important parameters.

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27th International Conference on Lightning Protection (ICLP), Avignon, France, 2004

The paper presents two examples of trees severely damaged by lightning strikes. The trees literally burst, when the lightning current passed through the trunk. Huge fragments were blasted away from the trunk and the roots, over distances of several tens of meters. The bursts even caused considerable damage in the surroundings, e.g. removing large patches of bark from nearby trees, when hit by fragments at high speed. The data of lightning location systems revealed that the trees were most likely struck by high amplitude positive cloud-to-ground lightning.

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27th  International Conference on Lightning Protection (ICLP), Avignon, France, 2004

The German lightning location system (LLS) BLIDS (BLitz Informations Dienst von Siemens) is now in service for more than 11 years. During this time the system was improved several times. In this paper we are showing the effect of the performance improvements of the system by evaluating some lightning parameters given by the system during the individual years.

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International Conference on Grounding and Earthing and 1st International Conference on Lightning Physics and Effects (Ground/LPE), Belo Horizonte, Brazil, 2004

Recently Diendorfer et al. [2002] compared lightning peak currents measured at the Gaisberg tower with correlated lightning peak currents reported by the Austrian lightning location system ALDIS. They found a surprisingly good agreement between the measurements at the tower and the peak currents reported by the lightning location system (LLS).

These lightning strikes to the tower radiate higher field peaks due to the presence of two current wavefronts traveling in opposite directions when an elevated object is struck by lightning [Diendorfer and Uman, 1990]. Therefore also the peak currents reported by the LLS should be enhanced compared to lightning to flat ground.

In this paper we will show the reason why there is no significant enhancement in the LLS data. We will do this with the aid of a return stroke model for field calculation and taking into account finite ground conductivity along the propagation path. In addition the limited sensor bandwidth affects the value of measured peak field and therefore the inferred peak current of the strokes.

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IEEE Bologna PowerTech. Proceedings, Bologna, Italy, 2003

DOI: https://10.1109/PTC.2003.1304476

Lightning is one of the major causes for outages in electric distribution and transmission networks. Data provided by a lightning location system in Austria allows detailed correlation of reported outages and the corresponding lightning discharges. Even within the small region of the Austrian territory the lightning flash exposure of HV transmission lines varies in a range from 0,6 to 2,6 km-2yr-1. A comparison of the actual observed lightning related line outages with calculated estimates based on the IEEE flash program show an underestimation of the outage rate by the flash program by a factor of about 2.

International Conference on Lightning and Static Electricity (ICOLSE), Blackpool, UK, 2003

Direct measurement of lightning currents is only possible at the point of impact using a shunt or other measuring instruments (e.g. Rogowsky-coil). On the other hand, the brightness of the lightning channel can be determined by means of optical measurements. So, if there is any correlation between lightning current and channel brightness, there is also an option to estimate the channel current from the measured brightness of a lightning flash.

We have developed a software tool Flash Analyzer (FA) to verify the correlation between the lightning current measured at the top of an instrumented tower and the simultaneously recorded brightness changes of the lightning channel. The optical data are taken by a high speed video system that is installed at a distance of about 200m to the tower. The camera system is operated with 500 frames per second i.e. an image of the lightning channel is taken every 2 ms. The mean value of the measured current during the frame exposure time of 2 ms is calculated from the simultaneously recorded current values. The 2 ms exposure time of the high speed video frames only allows analyses of the initial continuing current (ICC) phase of the upward initiated flashes to the tower.

The FA software has the option to specify up to four arbitrary lines per image, defining cross sections of the lightning channel. The basic idea of the analysis is to place the lines across the lightning channel and to determine the brightness along these cross sections. The result is a plot of the channel brightness as a function of time. This sequence is at first sight (purely visually) very similar to the plot of the measured lightning current. Several analyses under most different conditions have proven the usability of this procedure. Different parameters effecting the correlation between brightness and current (e.g. displacement of the lightning channel due to strong wind, the use of different camera lenses or background light on images taken during daylight time have been analyzed and corrected. For most of the examined lightning flashes coefficients of determination r2 > 0.5 were determined for a linear regression between channel brightness and lightning current. By placing the cross section lines across the individual branches of the lightning channel, we were able with this software tool to assess the corresponding lightning current in the individual branches of the channel. In this paper we show some results of the analysis of lightning flashes to the Gaisberg tower in the years 2000 to 2002.

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VII International Symposium on Lightning Protection (SIPDA), Curitiba, Brazil, 2003

Flashes with subsequent strokes of polarity opposite to the polarity of the first stroke are called bipolar flashes. In 1998 a new central processor (LP2000) for the Austrian lightning location system ALDIS (Austrian Lightning Detection & Information System) was installed. This LP2000 also includes a new algorithm for the grouping of strokes to flashes. This algorithm allows to group strokes to a flash even when the strokes do not have the same polarity, resulting in a bipolar flash. Therefore in our database five years of data is available which contains bipolar flashes.

To provide a ground truth reference for a comparison of natural lightning current parameters and the data of ALDIS, a radio tower on the mountain Gaisberg near the city of Salzburg (Austria) was set up for direct lightning current measurements. Also at this tower some bipolar flashes were measured during the last years.

In this paper details about bipolar flashes measured at the Gaisberg tower and detected with the Austrian lightning location system are presented and analyzed.

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26th International Conference on Lightning Protection (ICLP), Cracow, Poland, 2002

We compare the characteristics of the initial stage (IS) in natural upward lightning as observed on (1) the Gaisberg tower (100 m, Austria), (2) the Peissenberg tower (160 m, Germany), and (3) the Fukui chimney (200 m, Japan) with their counterparts in rocket-triggered lightning in Florida. All current records in Japan and some of the current records in Germany and Austria were obtained in winter, whereas all triggered-lightning data were obtained in summer. All lightning events analyzed here effectively transported negative charge to ground. The geometric mean (GM) values of the overall characteristics of the IS, duration (T), charge transfer (Q), and average current (I), for rocket-triggered lightning (T = 305 ms, Q = 30.4 C, I = 99.6 A) are similar to their counterparts for Gaisberg-tower flashes (T = 235 ms, Q = 29.6 C, I = 126 A) and Peissenberg-tower flashes (T = 290 ms, Q = 38.5 C, I = 133 A), while the Fukui-chimney flashes are characterized by a somewhat shorter GM initial-stage duration (T = 77.1 ms) and a larger average current (I = 505 A). The GM initial stage charge transfer for the Fukui-chimney flashes is 38.9 C. The observed differences in the IS duration is probably related to the difference in the lower current measurement limits: about 200 A for the Fukui data set vs. 15 to 20 A for the other three data sets. The characteristics of the initial continuous current (ICC) pulses in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of two to three, but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter rise times, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

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26th International Conference on Lightning Protection (ICLP), Cracow, Poland, 2002

In 1999 a digital high speed video camera was installed at the lightning experimental site at the Gaisberg tower. The camera is capable of capturing black and white images in speeds of up to 1.000 frames/s. The camera is triggered by a trigger signal derived from the lightning current measured at the tower top. In summer 2000 five flashes were recorded by the digital camera and for those five flashes time correlated current measurements (sampling rate 20 MS/s) are available.

In summer 2000 the frame rate of the camera was set to 500 frames per second with a total re-cording time of 2.1 s and a pretrigger of 50 %. There-fore these data provide sufficient time resolution for investigations on the optical development of the entire flash with a total time duration of up to several hundred milliseconds. On the other hand the 2 ms expo-sure time per frame is insufficient to analyze bright-ness details of individual strokes with a pulse duration of some tens to some hundreds of microseconds only.

From the gray values of the frame pixels of a single frame we calculate a summed brightness value at a given channel cross section. By doing this for all light exposed frames of a flash we can derive a plot of brightness as a function of time. Comparison of the plots of brightness versus the plots of the channel current for four flashes is shown in the paper. For all four flashes we found a very similar wave shape for brightness and current.

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26th International Conference on Lightning Protection (ICLP), Cracow, Poland, 2002

Lightning peak current is one of the most important lightning parameters. Lightning Location Systems infer the peak current from the peak magnetic field. The lightning peak current is calculated from the averaged range normalized signal strengths of the detecting sensors multiplied by a calibration factor [Diendorfer et al, 1998].

The accuracy of the peak current estimate mainly depends on (1) the applied calibration factor and (2) the signal attenuation as a result of finite ground conductivity along the travel path of the signal.

Additionally to these two well known effects on the amplitude measurement we will show in this paper that the so called "site error" of a direction finder does not only have an influence on the angle measurement of a lightning location system but also on the amplitude measurement. This so called "amplitude site error" is a function of the angle of field incidence similar to the angle site error.

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CIGRE Session 2002, Paris, France, 2002

A Lightning Location Systems (LLS) provides large scale information for lightning strikes to ground. In addition to the event time and strike point position, the LLS gives estimates for the lighting peak current. For the end user of LLS data it is important to know the technical limits of the applied network in terms of detection efficiency, accuracy of the stroke location and peak current estimate. To establish a ground truth reference for comparison of natural lightning current parameters and the data of ALDIS (Austrian Lightning Detection & Information System), a telecommunication tower on Gaisberg near the city of Salzburg (Austria) was instrumented for direct lightning current measurement. Since the start of the tower experiment in 1998 we have recorded more than 200 lightning flashes to the tower. Based on the analysis of the current waveforms nearly all of them are so called upward initiated discharges. GPS time synchronization of both data sets allows a precise correlation of individual events measured at the tower and reported by the lightning location network.

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CIGRE Session 2002, Paris, France, 2002

Lightning location systems (LLS) are in operation in Italy and Austria since about 10 years. The two networks are running in collaboration, sharing sensors data, since the beginning. The importance of this collaboration is shown, analyzing the behavior of the two networks as standing alone or as cooperating. In particular a detailed analysis is run on a set of data, evaluating the importance of the other network sensors in the calculation of lightning and in the efficiency and accuracy of the final results. The application of LLS data to improve the lightning performance evaluation of transmission and distribution lines is analyzed. Lightning density at ground is a key factor for performance evaluation of transmission lines. More accurate and detailed maps than those standardized may be derived by LLS, indicating that standardized values (average values for an area) are not sufficiently accurate to describe the situation of a given territory. The use of LLS data to correlate faults and lightning is discussed. In particular, for MV lines, one of the problems discussed is the necessary accuracy in lightning location.

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17th International Lightning Detection Conference (ILDC), Tucson, Arizona, 2002

Lightning strikes to the instrumented Gaisberg tower are often detected by different types of sensors (IMPACT, IMPACT ESP, LPATS III, LPATS IV) installed in Central Europe as part of the EUCLID network. In this paper we present results of an analysis of the response of the different sensor types to lightning discharges to the radio tower. During the two years 2000 and 2001 we have successfully located 310 strokes to the tower with amplitudes in the range from 2 kA to 35 kA. For some of the strokes with higher peak currents reports from more than 30 sensors are available. The field propagation path from the tower site to the different sensors is mainly over flat terrain to the north and over mountains to the south. This arrangement allows evaluating the effects of attenuation to the peak fields measured by the sensors.

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17th International Lightning Detection Conference (ILDC), Tucson, Arizona, 2002

In fall 2001 six European network operators together with Global Atmospherics Inc. (GAI) founded EUCLID (EUropean Cooperation for LIghtning Detection). Currently the EUCLID network consists of 12 LPATSIII, 17 LPATSIV, 33 IMPACT and 25 IMPACT ES/ESP sensors. Due to the fact that the network configuration is very inhomogeneous it is important to monitor and compare the performance in the individual areas of the network. In this paper we will present the current EUCLID network configuration and also a detailed performance analysis.

We will further show some drawbacks of the interconnection which small individual networks do not face. Further first results of lightning parameter variations within the EUCLID network are presented.

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International Conference on Grounding and Earthing & 3rd Brazilian Workshop on Atmospheric Electricity, Rio de Janeiro, Brazil, 2002

EUCLID - the EUropean Cooperation for LIghtning Detection was formally founded in 2001 as a cooperation among operators of lightning location networks in several countries in Europe. Currently the EUCLID network is a composite network utilizing about 100 Vaisala-GAI sensors. Data exchange between the countries is on a cooperation basis. It is also an objective to have a common European forum for discussion, maintenance, technical solutions, and network optimization. The EUCLID network provides lightning data for Europe with homogenous quality in terms of detection efficiency and location accuracy.

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5^th International Workshop on Physics of Lightning (IWPL), Nagoya, Japan, 2001

Lightning is one of the main causes for power line outages. We have correlated HV line outages of either unknown reason or when lightning was already assumed to be the reason with the archived lightning data from ALDIS for about 25 power lines (400 kV and 220 kV). For about 25% of the outages we found very good correlation with a lightning discharge in a corridor of up to +/- 5000 m. Some of the remaining outages are assumed to be either caused by other severe weather related events (strong wind, moisture on insulators, etc) or by lightning flashes that have not been detected by the LLS.

We have also determined values for the actual ground flash densities for the individual lines and we used the software IEEE-FLASH - which is based on the Electro-Geometric-Model (EGM) - to estimate the number of outages per 100 km and year. For some lines the comparison of the estimated values and the actual observed number of outages shows poor correlation.

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International Symposium on Winter Lightning (ISWL), Toyama, Japan, 2001

Lightning is one of the main causes for power line outages. We have correlated HV line outages of either unknown reason or when lightning was already assumed to be the reason, with the archived lightning data from ALDIS for about 25 power lines (400 kV and 220 kV). For about 25% of the outages we found good correlation with a located lightning discharge in a corridor of up to +/- 5000 m. Some of the remaining outages are assumed to be either caused by other severe weather related events (strong wind, moisture on insulators, etc) or by lightning flashes that have not been detected by the lightning location system.

Based on the data archive of the lightning location system ALDIS we have determined values of the actual ground flash densities for the individual lines and we applied the IEEE-FLASH software - which is based on the Electro-Geometric-Model (EGM) - to estimate the number of outages per 100 km and year for each line. For some lines the comparison of the estimated outage rate and the actual observed outage rate are very much different.

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Beitrag zur 4.VDE/ABB Blitzschutztagung, Neu-Ulm, Germany, 2001

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16^th International Lightning Detection Conference (ILDC), Tucson, Arizona, 2000

Lightning is one of the major sources for power line outages. We have correlated outages of unknown reason and when lightning was already assumed to be the reason with the archived lightning data from ALDIS for about 25 power lines (400 kV and 220 kV). For about 25% of the outages we found very good correlation with a lightning discharge in a corridor of up to +/- 5000 m. Some of the remaining outages are assumed to be either caused by other events (strong wind, moisture on insulators, etc) or by lightning flashes that have not been detected by the LLS.

In the past there have been claims of increased lightning activity in the vicinity of high voltage power lines. Detailed analysis of the lightning data near power lines does not support this claim. Power lines in flat area exhibit no effect to the local ground flash density near the power line.

We have also determined values for the actual ground flash densities for the individual lines and we used the software IEEE-FLASH to estimate the number of outages per 100 km and year assuming two distinct peak current distributions (CIGRE distribution with 31 kA and LLS based distribution with 12 kA mean). Comparison of the estimated values and the actual observed number of outages are not well correlated. This leaves some open questions, whether the FLASH program is not applicable for the type of power lines used in Austria or the number of reported outages caused by lightning is not well defined.

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16^th International Lightning Detection Conference (ILDC), Tucson, Arizona, 2000

In 1998 a radio tower on a mountain near Salzburg was instrumented for direct measurements of lightning currents. When lightning strikes the tower, the entire lighting current pulse is recorded using a shunt and a PC based digitizing system. This experimental setup allows to analyze the performance of the Austrian LLS (ALDIS) in terms of location accuracy, detection efficiency and it's ability to provide an estimate for the stroke peak currents.

Since the beginning of the tower measurements we have recorded about 100 events with a total of about 500 strokes. Almost all of the events are assumed to be upward flashes as typical for elevated objects.

In the paper we will present some initial results for the above mentioned performance parameters of ALDIS. Due to the integration of LPATS sensors in Germany installed close to the experimental site, we have both sensor types available for our analysis.

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16^th International Lightning Detection Conference (ILDC), Tucson, Arizona, 2000

Different lightning location systems provided lightning data for the MAP database during the MAP SOP. In this paper we compare lightning data sets recorded during the MAP-SOP from two different lightning location systems, the so called SOP-lightning location system (SOP-LLS) and the lightning location system from the British Met. Office called ATD.

The SOP-LLS was designed [Dorninger, 1999] to provide the best detection efficiency (DE) over the entire alpine region that can be achieved with today's available technology for a large region like the Alps. Thus it was assumed that this system detects more flashes than the ATD system which covers a much larger area than the Alps and had, at the time, a low processing ability. We will show that this assumption was correct and we will determine the overall DE of the ATD system relative to the SOP-LLS. Comparison of the lightning peak current distributions of both systems indicates that the ATD system mainly detects flashes of higher peak currents.

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25^th International Conference on Lightning Protection (ICLP), Rhodos, Greece, 2000

Location accuracy is one of the important performance parameters of a lightning location system. With more and more sophisticated applications of lightning data also an increasing demand on improved location accuracy is observed.

Today lightning location systems are mainly using GPS synchronized time information to locate a lightning discharge. The location is calculated based on the time differences between sensor arrival times and estimated impact time. Assuming speed of light (c0) for the field propagation velocity from this time differences the propagation distances to the sensors are calculated assuming the earth as an ideal ellipsoid. Distances calculated this way do not take into account any elongation of the propagation path due to mountains. This is assumed to be one of the reasons for the existence of a systematic location error in mountainous regions [Schulz, 1997].

In this paper we show how calculated lightning locations are affected by taking into account the wave travelling path elongation due to mountains by applying an elevation model for the earth. Any possible improvement of the resulting locations is evaluated by using data from lightning strikes to high towers.

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25^th International Conference on Lightning Protection (ICLP), Rhodos, Greece, 2000

In a recent investigation on ground flash density around elevated objects in Austria it was found, that the telecommunication tower at mount Gaisberg near Salzburg would be one of the most suitable sites for an experiment to measure direct lightning currents. This about 100 m tall telecommunication tower is located 1287 m above sea level and about 5 km east of the city of Salzburg.

Based on the data from the Austrian Lightning Location System (ALDIS), 40 - 50 flashes with about 200 strokes per year were expected.

The installation of an automatic measuring system to record currents of direct lightning strikes to the tower was started in August 1998. In this paper we present a detailed description of the experimental setup and show first results of recorded current waveforms in 1999.

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