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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