2022

Kohlmann H., W. Schulz, F. Rachidi:
Estimation of Charge Transfer During Long Continuing Currents in Natural Downward Flashes Using Single-Station E-Field Measurements

Journal of Geophysical Research: Atmospheres, 127, e2021JD036197, doi.org/10.1029/2021JD036197, 2022

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.

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Mansouri E., A. Mostajabi, W. Schulz, G. Diendorfer, M. Rubinstein, F. Rachidi:
On the Use of Benford’s Law to Assess the Quality of the Data Provided by Lightning Locating Systems

Atmosphere 13, 552, doi.org/10.3390/atmos13040552, 2022

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.

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Kohlmann H., W. Schulz, H. Pichler:
Simulation elektromagnetischer Felder durch Folge eines Blitzeinschlags auf dem Gaisberg und der anschließende Vergleich der Ergebnisse mit Messungen der ALDIS-Blitzortungssensoren

Elektrotechnik & Informationstechnik (e&i), Volume 139, pages 335–343, doi.org/10.1007/s00502-022-01028-x, 2022

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.

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Heidler F.H., C. Paul, W. Schulz:
Optische Untersuchungen von Blitzeinschlägen in Verbindung mit Feldmessungen und Beurteilung von Blitzortungssystemen

Elektrotechnik & Informationstechnik (e&i), Volume 139, pages 372–378, doi.org/10.1007/s00502-022-01021-4, 2022

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).

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Schwalt L., S. Pack, W. Schulz, G. Pistotnik:
Blitzentladungen im österreichischen Alpenraum mit Fokus auf Single-Stroke Flashes

Elektrotechnik & Informationstechnik (e&i), Volume 139, pages 344–351, doi.org/10.1007/s00502-022-01027-y, 2022

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.

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Manzato A., S. Serafin, M. Marcello Miglietta, D. Kirshbaum, W. Schulz:
A pan–Alpine climatology of lightning and convective initiation

Monthly Weather Review, https://doi.org/10.1175/MWR-D-21-0149.1, 2022

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.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, H. K. Rassoul:
Characterization of the initial stage in upward lightning at the Gaisberg; Tower: 1. Current pulses

Electric Power Systems Research 213, 108626, 2022

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.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, H. K. Rassoul:
Characterization of the initial stage in upward lightning at the Gaisberg; Tower: 2. Electric field signatures

Electric Power Systems Research 213, 108627, 2022

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.

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Stucke I., D. Morgernstern, G. Diendorfer, G. J. Mayr, H. Pichler, W. Schulz, T. Simon, A. Zeileis:
Thunderstorm types and meteorological characteristics of upward lightning

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

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.

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Kohlmann H., W. Schulz, F. Rachidi:
FDTD simulations of lightning strikes to the Gaisberg Tower: Comparison with ALDIS sensor measurements and evaluation of the Gaisberg radiation pattern

36th 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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Poelman D., W. Schulz, S. Pedeboy, D. Hill, M. Saba, H. Hunt, L. Schwalt, C. Schumann, T. Warner:
Global ground strike point characteristics in negative downward lightning flashes

36th 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.

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Schwalt L., Pack S., W. Schulz:
Analyses of Ground Truth Data for Positive Flashes

36th 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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Baron I., P. Koktavý, T. Trcka, M. Rowberry, J. Stemberk, J. Balek, L. Plan, R. Melichar, G. Diendorfer, R. Macků, P. Skarvada:
Differentiating between artificial and natural sources of electromagnetic radiation at a seismogenic fault

Engineering Geology 311, 106912, doi.org/10.1016/j.enggeo.2022.106912, 2022

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.

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Novoselov A., M. Dorninger, G. Diendorfer, G. Bokelmann and the AlpArray Working Group:
Seismoacoustic Study of Thunder and Lightning Using the AlpArray

Seismological Research Letters, 93 (6): 3404–3421, doi.org/10.1785/0220220064, 2022

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.

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