Taszarek M., J. Allen, T. Púcik, P. Grönemeijer, B. Czernecki, L. Kolendowicz, K. Lagouvardos, V. Kotroni, W. Schulz:
A Climatology of Thunderstorms across Europe from a Synthesis of Multiple Data Sources

Journal of Climate, DOI: 10.1175/JCLI-D-18-0372.1, 2019

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.

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Paul C., F. H. Heidler , W. Schulz:
Performance of the European Lightning Detection Network EUCLID in Case of Various Types of Current Pulses From Upward Lightning Measured at the Peissenberg Tower

IEEE Transactions on Electromagnetic Compatibility, DOI: 10.1109/TEMC.2019.2891898, 2019

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.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, V.A. Rakov, H. K. Rassou:
Characteristics of Currents in Upward Lightning Flashes Initiated From the Gaisberg Tower

IEEE Transactions on Electromagnetic Compatibility, DOI: 10.1109/TEMC.2019.2916047, 2019

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).

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Mostajabi A., D. Li, M. Azadifar, F. Rachidi, M. Rubinstein, G. Diendorfer, W. Schulz, H. Pichler, V.A. Rakov, D. Pavanello:
Analysis of a bipolar upward lightning flash based on simultaneous records of currents and 380-km distant electric fields

Electric Power Systems Research, Vol. 174, 2019

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.

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Azadifar M., M. Rubinstein, F. Rachidi, V.A. Rakov, G. Diendorfer, W. Schulz, D. Pavanello:
A Study of a Large Bipolar Lightning Event Observed at the Säntis Tower

IEEE Transactions on Electromagnetic Compatibility, DOI: 10.1109/TEMC.2019.2913220, 2019

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.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, V. A. Rakov:
Polarity of Upward Lightning based on Currents Measured at the Gaisberg Tower

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

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).

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Poelman D. R., S. Pédeboy, W. Schulz:
Performance validation of a ground strike point algorithm

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.

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Paul C., F. H. Heidler, W. Schulz:
LLS Detection for negative first and subsequent return strokes measured at the Peissenberg Tower, Germany

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

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.

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Schwalt L., S. Pack, W. Schulz:
LiOn Return Stroke Peak Current Analysis for Ground Strike Points in the Austrian Alps

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

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.

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