2017

Simon T., N. Umlauf, A. Zeileis, G.J. Mayr, W. Schulz, G. Diendorfer:
Spatio-temporal modelling of lightning climatologies for complex terrain

Natural Hazards and Earth System Sciences, 17, 305–314, doi:10.5194/nhess-17-305-2017, 2017

This study develops methods for estimating lightning climatologies on the day-1 km-2 scale for regions with complex terrain and applies them to summertime observations (2010–2015) of the lightning location system ALDIS in the Austrian state of Carinthia in the Eastern Alps.

Generalized additive models (GAMs) are used to model both the probability of occurrence and the intensity of lightning. Additive effects are set up for altitude, day of the year (season) and geographical location (longitude/latitude). The performance of the models is verified by 6-fold cross-validation.
The altitude effect of the occurrence model suggests higher probabilities of lightning for locations on higher elevations. The seasonal effect peaks in mid-July. The spatial effect models several local features, but there is a pronounced minimum in the north-west and a clear maximum in the eastern
part of Carinthia. The estimated effects of the intensity model reveal similar features, though they are not equal. The main difference is that the spatial effect varies more strongly than the analogous effect of the occurrence model.
A major asset of the introduced method is that the resulting climatological information varies smoothly over space, time and altitude. Thus, the climatology is capable of serving as a useful tool in quantitative applications, i.e. risk assessment and weather prediction.

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Schulz W., D.R. Poelman, R. Kaltenböck, E. Goudenhoofdt, L. Delobbe:
Analysis of Outliers in the EUCLID Network

10th Asia-Pacific International Conference on Lightning (APL), Thailand, Krabi, 2017

In this paper we provide details about a performance parameter of the EUCLID lightning location system (LLS) called the percentage of outliers. The term outlier means an event (CG stroke or IC pulse) located by the LLS on a wrong place. In this study we use data from weather radar networks in two regions of the EUCLID network (Belgium and Austria) to distinguish between outlier and non-outlier. It is shown that the percentage of outliers is sensitive to changes in the network and also changes related to the location algorithm itself. The overall percentage of outliers for both regions is between 0.8% and 1.9% for a distance to the nearest precipitation of 2km.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz:
Close and Distant Electric Fields due to Lightning Attaching to the Gaisberg Tower

4th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

We examine current and electric field waveforms produced by lightning strikes initiated from the Gaisberg Tower located in Salzburg, Austria. Current was measured at the top of the tower and electric field measured simultaneously at close (170 m from the tower), and far (79 or 109 km from the tower) distances. In this preliminary study, we establish the criteria for characterizing current and electric field pulses that occur during the initial stage of upward lightning flashes (including those at the initiation of the initial stage) based on the characteristics of the measurement system used to record the current and electric field waveforms and the occurrence context of the pulses in the flash. Of the seven negative upward flashes analyzed in this study, two flashes had bipolar IS background current, which were first negative, followed by brief (duration < 2 ms) positive current, and then negative again. The initial stage background current was negative in the other five flashes. Overall, 71% of the pulses occurring during the initial stage were positive bipolar, 2% were positive unipolar, and 27% were negative unipolar. No negative bipolar pulses were found. The total duration of unipolar pulses ranged from 4.9 to 702 μs and that of bipolar pulses ranged from 4.1 to 197 μs.

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Azadifar M, G. Diendorfer, M. Paolone, D. Pavanello, F. Rachidi, V. A. Rakov, M. Rubinstein, W. Schulz:
A Large Biploar Event Recorded at the Säntis Tower

4th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

We present a 4-stroke negative lightning flash recorded at the Säntis Tower for which the current waveform associated with the first return stroke was unusual and resembled a Gaussian pulse. Such current pulses could be the sources of similarly-shaped electric field waveforms that are attributed to LBEs. Correlated data form the EUCLID lightning detection network show that this flash was preceded by a positive flash located 0.8 km from the tower and 1 ms prior to its first stroke. We also present simulation results of the radiated electric fields considering two different models for the LBE and we show that the simulated waveforms agree well with the experimentally observed characteristics of the radiated fields associate with LBEs.

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Pedeboy S., M. Bernardi, W. Schulz, A. Rousseau:
Characteristics and distribution of intense cloud-to-ground flashes in Western Europe

CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

This work aimed at analysing the occurrence of Intense Cloud-to-Ground (ICG) in Western Europe, including a large part of maritime areas, defined as lightning flashes exhibiting at least one return stroke peak current larger than 200 kA based on lightning data collected by EUCLID between 2007 and 2016. As expected, the rate of ICG is low in average, about 0.18 % of the total Cloud-to-Ground (CG), but because of a pronounced seasonal trend it can increase up to 1.5% in winter. Around 70% of ICG occurring over the Atlantic Ocean and the Mediterranean Sea are of negative polarity whereas, in around the same proportion, they are positive over the continental regions. The geographical distribution of ICG shows a clear enhancement of ICG occurrences during winter time along coastal areas exhibiting elevated terrain, in northern Spain and western Italy and in Balkans. In these regions ICG are mainly located in land and surprisingly their polarity is negative on the contrary to the general trend stating most ICG are positive on the Continent. The discrepancies observed in the geographical, seasonal and polarity distributions are thought to be related to the different type of thunderstorms occurring across Europe and particularly oceanic and Mediterranean winter and continental deep-convective clouds. Finally, some high-density areas along Italian or Balkan coastlines can reach up to 0.45 ICG/km²/year, both polarities combined.

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Schulz W., D.R. Poelman, R. Kaltenböck, E. Goudenhoofdt, L. Delobbe:
Outliers in the EUCLID Network

CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

In this paper we provide details about a performance parameter of the EUCLID lightning location system (LLS) called the percentage of outliers. The term outlier means an event (CG stroke or IC pulse) located by the LLS on a wrong place. In this study we use data from weather radar networks in two regions of the EUCLID network (Belgium and Austria) to distinguish between outlier and non-outlier. It is shown that the percentage of outliers is sensitive to changes in the network and also changes related to the location algorithm itself. The overall percentage of outliers for both regions is between 0.7% and 1.9% for a distance to the nearest precipitation of 2km.

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Schwalt L., S. Pack, W. Schulz, G. Diendorfer, G. Pistotnik:
Number of single-stroke flashes in the alpine region determined with a video and field recording system

CIGRE International Colloquium on Lightning and Power Systems (ICLPS), Ljubljana, Slowenien, 2017

Since 2008, measurements of natural cloud-to-ground (CG) lightning were performed during warm season thunderstorms in the Alpine region of Austria to generate a ground truth data set of lightning strikes. Those measurements were performed with a mobile high speed video and an electric field recording system (VFRS) to observe the optical properties of lightning discharges and to record the ambient electric field. In 2015, the VFRS`s high speed camera was upgraded in order to significantly increase the optical and temporal resolution of the video data. Due to the upgrade it was possible to record a high quality data set during 20 thunderstorm days at 15 different sites between May and August 2015.
For this paper these data sets of the Alpine region are used to analyze possible reasons for the detected variation of single-stroke flashes. The ground truth data sets are also compared to formerly published values from different countries. To provide additional information, data of the Austrian Lightning Location System (LLS), ALDIS (Austrian Lightning Detection and Information System), is compared to the VFRS ground truth data, operated by Graz University of Technology, to analyze the reasons for the varying amount of single-stroke flashes in the considered region. Thunderstorm types are classified with radar data and with wind measurements in order to investigate the effect of thunderstorm organization on their lightning characteristics.
Compared to values from the literature the percentage of single-stroke flashes in this study present a higher value for negative flashes (26 %) and values in the same range for positive flashes (89 %).
Results of this report shall contribute to a better understanding of the lightning process in general and the behavior of thunderstorms in the Alpine region in particular.

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Kohlmann H., W. Schulz, H. Pichler:
Compensation of integrator time constants for electric fieldmeasurements

Electric Power Systems Research (EPSR) Vol. 153, 2017

Rubinstein et al. (2012) [1] examined a method to compare electric fields from lightning discharges measured with analogue integrators of different time constants. The time constant distorts the waveforms and has to be corrected numerically. We have extended the work of Rubinstein et al. (2012) [1] by considering the antenna characteristics in the system equations. In this paper we focus on the compensation of the integrators time constant and present some cases and results after applying the method. Furthermore we discuss the importance of any existing offset errors. A simple approach for handling the offset will be presented. Examples and determination of continuing currents are given in section IV. Advantages of the compensation method are mentioned in the conclusion. Together with Appendix A (system equations) this paper can be seen as a reference to a profound understanding of the measurement methods of E-fields with flat plate antennas for lightning researchers.

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Birkl J., E. Shulzhenko, F. Heidler, G. Diendorfer:
Measuring Lightning Currents on Wind Turbines

4th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

This paper is dedicated to the question of measuring lightning current events on tall objects such as wind turbines. Because of their height, location at the open or uplands area, the probability of lightning strike increases significantly. Modern wind power plants with total height up to 200 m are able to trigger upward or ground-to-cloud flashes, especially during winter season, which are different from downward flashes. In general, upward lightning is critical for the air-termination system of wind turbine with regard to transferred charge, which can easily exceed the value of 300 As specified for lightning protection level I (LPL I) in the international standard IEC 62305 [1]. For proper operation and efficient maintenance regimes measurement of the lightning events on wind turbines is needed. The measuring principle, based on Rogowski coil sensors is presented in this paper. The data obtained from the measuring system allow to evaluate the effects of lightning strikes on wind turbines. Some specifics during the measuring of lightning events on tall objects are discussed as well. In particular, the peak value of lightning impulse currents, to be able to be measured, should be greater than 200 kA specified again for LPL I. Also upward lightning may have only a long duration initial continuing current (ICC-only), which is difficult to be detected by LLS and to be measured. Both of these lightning parameters have different effects on components of wind turbines, which are discussed in detail in the paper.

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Diendorfer G.:
Review of seasonal variations in occurrence and some current parameters of lightning measured at the Gaisberg Tower

4th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

Upward lightning initiated from tall objects has gained considerable interest in the past years. Modern wind turbines are reaching total heights of 200 m and more and these structures are often exposed to this type of discharge. The data set collected by directly measured lightning current waveforms at the Gaisberg Tower (GBT) in Austria is used to evaluate the seasonal variations in the occurrence of upward lighting and variations in their current parameters. Initial continuing currents with superimposed pulses (ICCP) transfer the highest amounts of charge and occur mostly during non-convective season. All flashes with transferred charge exceeding 300 As occurred during non-convective season and were mostly of negative polarity. All days, where more than 10 flashes were recorded at the GBT within 24 hours, belong to the non-convective season.

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Smorgonskiy A., M. Rubinstein, F. Rachidi, G. Diendorfer:
Prediction of Lightning Incidence to Tall Structures Before Construction

4th International Symposium on Winter Lightning (ISWL), Joetsu, Niigata-ken, Japan, 2017

A new approach is presented to evaluate lightning incidence for the cases when wind turbines are located close to each other and in complex terrain. Lightning incidence to a wind turbine park consisting of 16 wind turbines and located on the Mont-Crosin Mountain in Switzerland is analysed. A significant increase in the number of upward flashes is observed following the construction of the new wind turbines and replacement of the old wind turbines in the area of Mont Crosin.

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Saba M. M. F., A. R. Paiva, K. P. Naccarato, F. V. C. Siqueira, F. S. Sabbas, J. C. O. Silva , M. A. S. Ferro, D. M. Custodio, C. Schumann, G. Diendorfer, A. Piantini:
Current measurements of upward leaders from buildings

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

Although there are some data on lightning attachment to tall towers (height over 60 m), there are no observational data of lightning attachment to common structures or buildings (under 60 m) that are present in almost every city. In this paper we analyze current measurements of upward leaders induced by a downward negative lightning flash that struck a building located in São Paulo, Brazil. The attachment process was recorded by two high-speed cameras running at 37,800 and 70,000 images per second, two current sensors and an electric field sensor.

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Mostajabi A., M. Azadifar, F. Rachidi, M. Rubinstein, G. Diendorfer, W. Schulz, H. Pichler, V. A. Rakov, M. Paolone:
Simultaneous Records of Current and 380-km Distant Electric Field of a Bipolar Lightning Flash

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

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 height of the ionospheric reflection for the positive pulse was inferred to be about 94.9 km, a value which is significantly higher than for negative pulses of this same flash, which range from 73 to 81 km. 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 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, than for the positive pulse.

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Cooray V., G. Diendorfer:
Merging of current generation and current dissipation lightning return stroke models

Electric Power Systems Research (EPSR), 5153, 2017

Current generation and current dissipation return stroke models are engineering models based on the theory associated with the propagation of current pulses along transmission lines undergoing corona. However, neither of these models incorporates the complete theory associated with the phenomenon. One can make the physical scenario complete by combining the current generation concept with the current dissipation concept. In this paper how this can be done is demonstrated by creating a return stroke model which is a combination of these two model types. The new model encompasses the full theory associated with the pulse propagation along transmission lines under corona. The paper provides a full description of the model together with a description of the spatial and temporal variation of the return stroke current and the electric and magnetic fields generated at different distances as predicted by the model.

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Li D. , M. Rubinstein , F. Rachidi , G. Diendorfer, W. Schulz, G. Lu:
Location Accuracy Evaluation of ToA-Based Lightning Location Systems Over Mountainous Terrain

Journal of Geophysical Research: Atmospheres, Vol. 122, doi.org/10.1002/2017JD027520, 2017

In this paper, we analyze the location error of time of arrival (ToA)-based lightning location
systems (LLSs) caused by propagation effects over mountainous terrain around the Säntis Tower located in the Swiss Alps. The study is based on a full-wave three-dimensional (3-D) finite difference time domain approach using the topographic map including the Säntis Tower and the nearby sensors belonging to LLSs. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity and that the location error associated with the ToA technique depends strongly on the used onset time estimation technique. The evaluated location errors associated with amplitude thresholds of 10% and 20% and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest (about 300mor less). Finally, we assess the accuracy of two simplified methods (terrain envelope method and tight-terrain-fit method) to account for the location error due to propagation over mountainous terrain. These two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a nonflat terrain by using available topographic data. In addition, a possible real-time location error compensation algorithm using the elongated propagation path method to improve the location error of the LLSs in mountainous regions is presented and discussed.

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Poelman D. R., W. Schulz, R. Kaltenboeck, L. Delobbe:
Analysis of lightning outliers in the EUCLID network

Atmospheric Measurement Techniques, 10, 4561-4572, 2017

Lightning data as observed by the European Cooperation for Lightning Detection (EUCLID) network are used in combination with radar data to retrieve the temporal and spatial behavior of lightning outliers, i.e., discharges located in a wrong place, over a 5-year period from 2011 to 2016. Cloud-to-ground (CG) stroke and intracloud (IC) pulse data are superimposed on corresponding 5 min radar precipitation fields in two topographically different areas, Belgium and Austria, in order to extract lightning outliers based on the distance between each lightning event and the nearest precipitation. It is shown that the percentage of outliers is sensitive to changes in the network and to the location algorithm itself. The total percentage of outliers for both regions varies over the years between 0.8 and 1.7 % for a distance to the nearest precipitation of 2 km, with an average of approximately 1.2 % in Belgium and Austria. Outside the European summer thunderstorm season, the percentage of outliers tends to increase somewhat. The majority of all the outliers are low peak current events with absolute values falling between 0 and 10 kA. More specifically, positive cloud-to-ground strokes are more likely to be classified as outliers compared to all other types of discharges. Furthermore, it turns out that the number of sensors participating in locating a lightning discharge is different for outliers versus correctly located events, with outliers having the lowest amount of sensors participating. In addition, it is shown that in most cases the semi-major axis (SMA) assigned to a lightning discharge as a confidence indicator in the location accuracy (LA) is smaller for correctly located events compared to the semi-major axis of outliers.

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Sari A. P., R. Zoro, C. Vergeiner, S. Pack, W. Schulz:
Evaluation of the LLS ALDIS Performance Characteristics by using a Video and Field Recording System

10th Asia-Pacific International Conference on Lightning (APL), Thailand, Krabi, 2017

Lightning Location Systems (LLS) are the most common way to geolocate lightning. The performance characteristics of the Austrian Lightning Detection and Information System (ALDIS) as an Austrian ground-based LLS are determined by their ability to detect lightning flashes and strokes as well as their ability to geolocate lightning events accurately and provide relevant information such as time, peak current, number of strokes, etc. The performance of LLS is very important for LLS operators as well as for users to get the right information about lightning. During the years the configuration of ALDIS in terms of hardware and software changed several times resulting in continuous performance improvements. Comparing ALDIS data with data of a Video and Field Recording System (VFRS) is a ground-truth data method that can be used to validate the performance characteristics of LLS for a large region. The VFRS consists of a high speed camera, an electric field sensor and a GPS clock for time synchronization. Performance characteristics of ALDIS for cloud-to-ground lightning were obtained by comparing ALDIS data with VFRS data recorded during a Lightning Observation Project in 2015. VFRS data were recorded between May and August 2015 during 20 days. In the recorded data, 153 negative, 28 positive and 6 bipolar flashes could be identified. The most important performance parameters of LLS are the Detection Efficiency (DE) of cloud-to-ground (CG) flashes and strokes and the Location Accuracy (LA) of the located strokes. Our analyses showed that the DE for negative flashes and strokes are 96,1% and 87,9% respectively and 100% and 91,2% for positive flashes and strokes, respectively. The median LA could be identified to be 111 m.

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Plesch J., L. Schwalt, S. Pack, W. Schulz, G. Achleitner:
Transient Measurements in the Austrian High Voltage Transmission System

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

Transient measurements in several substations in the Austrian transmission system were performed in the last years. Based on these measurements it was possible to detect signal sequences which show direct and nearby lightning discharges as well as the coupling effect to phases after lightning strikes to the ground wire. These transient voltages were measured by using resistive-capacitive voltage dividers to generate a high quality dataset. The high bandwidth of the RC-divider and its resulting frequency stability is needed to analyzed such transient events. A correlation between the measurements and the data of the Austrian Lightning Location System was performed. In this paper, three different representative measurements of atmospheric discharges to or close to transmission lines will be presented.

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Kohlmann H., W. Schulz, S. Pedeboy
Evaluation of EUCLID IC/CG Classification Performance Based on Ground-Truth Data

International Symposium on Lightning Protection (XII SIPDA), Natal, Brazil, 2017

This work compares the classification accuracy (CA) of two algorithms applied to data from the EUCLID lightning location system (LLS). As CA we call the accuracy of a LLS to correctly distinguish between cloud-to-ground (CG) and intra cloud (IC) discharges. The ground-truth data, used for this evaluation, was taken from optical and electric field data measured in various regions in Austria (2012 and 2015) and France (2014). The data set contains CG and IC discharges of positive and negative polarity. The data set was split up into further sub-categories as long as the number of data was still sufficient to give reasonable results. For a coarse overview of the algorithm performances, the total CA was first calculated for each year and country for both polarities. Furthermore, for the class of CG discharges, the CA of first return strokes, the CA of subsequent return strokes (with and without respect to the polarity) as well as the CA of IC events with respect to their polarity was evaluated. Specifically the subdivision into classes of amplitudes of the peak currents for different events can give further insight to the performance of the algorithms. For that reason the total CA, the CA of negative and positive events and CG and IC was analyzed.
The evaluation shows that in combination with the new sensor data format LS the new algorithm exhibits an improvement of 2% at the CA. In combination with the old data format IMPACT, the new algorithm performs worse than the old one. In total, IC events have been classified much better by the new algorithm, irrespective of the sensor data format. CG discharges on the other hand show a worse CA throughout all years for the new algorithm.

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