2016

Schulz W., G. Diendorfer, S. Pedeboy, D.R. Poelman:
The European lightning location system EUCLID – Part 1:
Performance analysis and validation

Natural Hazards and Earth System Sciences, 16, 595-605, doi:10.5194/nhess-16-595-2016, 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.

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Poelman D.R., W. Schulz, G. Diendorfer, M. Bernardi:
The European lightning location system EUCLID – Part 2:
Observations

Natural Hazards and Earth System Sciences, 16, 607-616, doi:10.5194/nhess-16-607-2016, 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.

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Azadifar M., F. Rachidi, M. Rubinstein, M. Paolone, G. Diendorfer, H. Pichler, W. Schulz, D. Pavanello, C. Romero:
Evaluation of the Performance Characteristics of the European Lightning Detection Network EUCLID in the Alps Region for Upward Negative Fla

Journal of Geophysical Research: Atmospheres, 121, 595–606, doi:10.1002/2015JD024259, 2016

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.

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Li D., M. Azadifar, F. Rachidi, M. Rubinstein, G. Diendorfer, K. Sheshyekani, Q. Zhang, Z. Wang:
Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA-based lightning location systems

Journal of Geophysical Research: Atmospheres, 121, 895–911, doi:10.1002/2015JD024234, 2016

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.

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Gallin L.-J., T. Farges R. Marchiano,F. Coulouvrat, E. Defer, W. Rison, W. Schulz, M. Nuret:
Statistical analysis of storm electrical discharges reconstituted from a lightning mapping system, a lightning location system, and an acoustic array

Journal of Geophysical Research: Atmospheres, 2016

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.

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Poelman D.R., E. Goudenhoofdt, L. Delobbe, W. Schulz:
Determining lightning outliers based on Belgian radar data to evaluate the performance of EUCLID

24th International Lightning Detection Conference and 6th 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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Schulz W., A. Nag, M.J. Murphy:
Relationship between the average number of sensors reporting and the detection efficiency of a network

24th International Lightning Detection Conference and 6th 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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Vergeiner C., S. Pack, W. Schulz, G. Diendorfer:
Negative Cloud-to-Ground Lighting in the Alpine Region
A new Approach

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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Heidler F., W. Schulz:
Lightning current measurements compared to data from the lightning location system BLIDS

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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Schulz W., G. Diendorfer:
Some Field Parameters of Return Strokes in Upward Lightning from Tall Objects

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

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.

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Schumann C., M.M.F. Saba , A.R. de Paiva, H. Kohlmann, W, Schulz, G. Diendorfer, M.A. da Silva Ferro, T.A. Warner, J. Heldson Jr.:
Charge transfer in natural negative and positive downward flashes

33rd 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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Birkl J., G. Diendorfer, S. Thern, J. Kolb, E. Shulzhenko, M. Rock:
Initial Investigation of Influence of Wind Farms to Lightning Events

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

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.

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Fiori E., M. Lagasio, A. Parodi, R. Procopio, A. Smorgonskiy, F. Rachidi, G. Diendorfer:
Implementation and Performance Analysis of the Lightning Potential Index as a Forecasting Tool

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

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).

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Li D., F. Rachidi, M. Rubinstein, G. Diendorfer, Z. Wang:
Location Accuracy Evaluation of ToA-Based Lightning Location Systems over Mountainous Terrain

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

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.

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Rubinstein M., J. Zuber, A. Smorgonskiy, F. Rachidi, g. Diendorfer:
Correlation vs. Causality in Other-Triggered Upward Lightning in Tower Flashes

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

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.

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Saba M.M.F., K.P. Naccarato, A.R. de Paiva, C. Schumann, M.A.S. Ferro, D.M. Custódio, V. Cooray, P. Hettiarachchi, G. Diendorfer, A. Piantini, J.C. de Oliveira e Silva:
The study of lightning strikes to common buildings in Brazil

33rd 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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Azadifar M., D. Li, F. Rachidi, M. Rubinstein, G. Diendorfer, H. Pichler, M. Paolone, D. Pavanello:
Simultaneous Current and Distant Electric Field Waveforms from Upward Lightning: Effect of Ionospheric Reflection

24th International Lightning Detection Conference and 6th 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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Smorgonskiy A., F. Rachidi, M. Rubinstein, G. Diendorfer:
On the Estimation of the Number of Upward Flashes From Wind Turbines in Mountainous Areas

24th International Lightning Detection Conference and 6th 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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Diendorfer G.:
A Review of 25 Years of Lightning Research in Austria from 1991-2015

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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