Schwalt L., S. Pack, W. Schulz, G. Pistotnik:
Percentage of single-stroke flashes related to different thunderstorm types

Electric Power Systems Research, Vol. 194, 2021

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.

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Kolmašová I. et al.:
First Observations of Elves and Their Causative Very Strong Light-ning Discharges in an Unusual Small-Scale Continental Spring-Time Thunderstorm

Journal of Geophysical Research: Atmospheres, DOI: 10.1029/2020JD032825, 2021

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.

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Poelman D. R. , W. Schulz, S. Pedeboy, D. Hill, M. Saba, H. Hunt, L. Schwalt, C. Vergeiner, C. T. Mata, C. Schumann, T. Warner:
Global ground strike point characteristics in negative downward lightning flashes – Part 1: Observations

Natural Hazards in Earth System Sciences, DOI: 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.

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Poelman D.R., W. Schulz, S. Pedeboy, L. Z. S. Campos, M. Matsui, D. Hill, M. Saba, H. Hunt:
Global ground strike point characteristics in negative downward lightning flashes – Part 2: Algorithm validation

Natural Hazards and Earth System Sciences, DOI: 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.

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Kohlmann H., W. Schulz:
Comparison of 3-D and 2-D Cylindrical Symmetry FDTD Simulation Results of a Lightning Strike to Gaisberg With ALDIS Sensor Measu-rements

IEEE Transactions on Electromagnetic Compatibility, 2021

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.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, H. K. Rassoul:
Characterization of initial stage current pulses in upward lightning initiated from the Gaisberg Tower

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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Kohlmann, H., W. Schulz, T. Auzinger:
FDTD simulations of lightning electromagnetic fields over irregular terrain using MEEP

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

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).

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Schwalt L., S. Pack, W. Schulz:
Specific Ground Truth Data Analysis of Lightning Discharges in Austria

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

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.

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Becerra M., D. Wang, G. Diendorfer:
Self-initiated Upward Lightning: Terrain and Seasonal Effects

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

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.

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Kohlmann H., W. Schulz, F. Rachidi:
Evaluation of Site Errors in LLS Magnetic Direction Finding Caused By Large Hills Using the 3D-FDTD Technique

Earth and Space Science, 2021

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.

Accepted for publication, Earth and Space Science

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