Schulz W., H. Pichler, G. Diendorfer, C. Vergeiner, S. Pack:
Validation of Detection of Positive Flashes by the Austrian Lightning Location System ALDIS

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

In this paper we present a detailed evaluation of the performance of the Austrian Lightning location system (LLS) ALDIS regarding detection of positive flashes. Additional to the performance of the LLS some parameters of positive flashes are determined. We also present detailed information about two positive flashes with a subsequent stroke following the same channel to ground as the first stroke.

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Smorgonskiy A., F. Rachidi, M. Rubinstein, G. Diendorfer, and W. Schulz:
On the proportion of upward flashes to lightning research towers

Atmospheric Research, Vol. 129–130, No. 0, pp. 110–116, Jul. 2013

We compare in this paper direct measurements obtained on the towers on San Salvatore Mountain (Switzerland) and on the Gaisberg Mountain (Austria). They are situated in similar topographical environments but in different lightning activity zones. Direct measurements of lightning currents on these towers have revealed a major difference in terms of the number of downward flashes. While measurements made by Berger and co-workers revealed a significant number of downward flashes on the two towers on San Salvatore Mountain, more recent observations on the Gaisberg and Peissenberg towers were essentially composed of upward flashes. We use in this paper a new method to estimate the proportion of upward/downward flashes to a given tower, based on the data from lightning location systems. The analysis using the proposed method explains the discrepancy in terms of the measured number of downward flashes in the Gaisberg and San Salvatore towers. The analysis presented reveals also that in the evaluation of the percentage of upward flashes initiated from a tall structure, different parameters should be carefully examined, namely (i) the value of the ground flash density, (ii) the topographical conditions, and (iii) the presence of other tall structures in the region from which upward flashes might be initiated.

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Rakov V.A., A. Borghetti, C. Bouquegneau, W. A. Chisholm, V. Cooray, K. Cummins, G. Diendorfer, F. Heidler, M. Ishii, C. A. Nucci, A. Piantini, O. Pinto, X. Qie, F. Rachidi, M. M. F. Saba, T. Shindo, W. Schulz, S. Visacro, and W. Zischank:
CIGRE Technica

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

CIGRE TB 549 (2013) is an update on previous CIGRE documents on the subject, published in Electra more than three decades ago. Lightning parameters needed in different engineering applications are reviewed. New experimental data, as well as the old data, are evaluated. Additional lightning parameters, previously not considered by CIGRE, are included. Possible geographical and seasonal variations in lightning parameters are examined. Specific applications are considered and recommendations are made.

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Smorgonskiy A., F. Rachidi, and G. Diendorfer:
On the Relation between Lightning Flash Density and Terrain Elevation

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

The distribution of lightning flash density over large areas depends on many factors. In this paper, we analyze one of them, namely the terrain elevation. Some previous studies suggest that the lightning flash density decreases above a certain altitude. We show that this conclusion could be affected by the used method of analysis and we suggest two improved methods. The lightning flash density distribution over Switzerland and Austria was used as an example to test the proposed methods. The results of the application of both methods suggest that the lightning flash density grows over the whole altitude range.

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Zhou H., R. Thottappillil, and G. Diendorfer:
Calculation of Electromagnetic Fields in Free Space when Lightning Strikes a Tall Object

12th International Symposium on Lightning Protection (SIPDA), Brazil, 2013

We calculate vertical electric field and azimuthal magnetic field at different elevation angles and distances associated with lightning strikes a tall object. Simple and exact expressions for electromagnetic fields are derived when the current reflection coefficient at tall object top is zero and return stroke propagation speed in the lightning channel is equal to the speed of light. Further, we investigate the effects of current reflection coefficient at tall object top being not zero and the propagation speed is less than the speed of light (e.g., one half) on electromagnetic fields. Interestingly, we find that the vertical electric field has its largest peak value either at the smallest elevation angle or at the largest elevation angle. While for the azimuthal magnetic field, we note that its largest peak value is always at the smallest elevation angle or relatively small elevation angles.

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Chum J., G. Diendorfer, T. Šindelářová, J. Baše, and F. Hruška:
Infrasound pulses from lightning and electrostatic field changes: Observation and discussion

Journal of Geophysical Research: Atmospheres, Vol. 118, No. August, p. n/a–n/a, Oct. 2013

Narrow (~1–2 s) infrasound pulses that followed, with ~11 to ~50 s delays, rapid changes of electrostatic field were observed by a microbarometer array in the Czech Republic during thunderstorm activity. A positive pressure fluctuation (compression phase) always preceded decompression; the compression was usually higher than the decompression. The angles of arrival (azimuth and elevation) were analyzed for selected distinct events. Comparisons of distances and azimuths of infrasound sources from the center of microbarometer array with lightning locations determined by the European Cooperation for Lighting Detection lightning detection network show that most of the selected events can be very likely associated with intracloud (IC) discharges. The preceding rapid changes of electrostatic field, their potential association with IC discharges, and high-elevation angles of arrival for near infrasound sources indicate that an electrostatic mechanism is probably responsible for their generation. It is discussed that distinguishing the relative role of thermal and electrostatic mechanism is difficult and that none of the published models of electrostatic production of infrasound thunder can explain the presented observations precisely. A modification of the current models, based on consideration of at least two charged layers, is suggested. Further theoretical and experimental investigations are however needed to get a better description of the generation mechanism.

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Vergeiner C., W. Schulz, and S. Pack:
On the Performance of the Austrian Lightning Detection and Information System (ALDIS)

11 Höfler´s Days, 2013

In Europe, several Lightning Location Systems (LLS) are operated in order to monitor the lightning activity and to gather information on lightning discharges inside a certain area. For operators of LLS as well as for users of lightning location data, information on the performance of their particular LLS is important. In the past, several studies on the performance of LLS were done with cross comparison of different LLS data sets, but such cross comparisons unfortunately do not provide clear results. Another approach to determine the performance of LLS is a comparison of LLS data with so called “ground truth data”. Such ground truth data are for example “natural lightning to instrumented towers” (e.g. Gaisberg tower), “artificial rocket triggered lightning” or “video and E-field measurements of natural lightning discharges”. Each of these methods has its advantages and disadvantages. In this paper we describe combined video and E-field measurements, the used Video-Field Recording System (VFRS) and our approach to gather ground truth data with such a VFRS. Based on the comparison of VFRS Data with LLS we show the Detection Efficiency (DE) for flashes and for strokes as well as the Location Accuracy (LA) of the Austrian Lightning Detection and Information System (ALDIS) for the southern and eastern part of Austria.

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Poelman D. R. , W. Schulz, and C. Vergeiner:
Performance Characteristics of Distinct Lightning Detection Networks Covering Belgium

Journal of Atmospheric and Oceanic Technology, Vol. 30, No. 5, pp. 942–951, May 2013

This study reports results from electric field measurements coupled to high-speed camera observations of cloud-to-ground lightning to test the performance of lightning location networks in terms of its detection efficiency and location accuracy. The measurements were carried out in August 2011 in Belgium, during which 57 negative cloud-to-ground flashes, with a total of 210 strokes, were recorded. One of these flashes was followed by a continuing current of over 1 s—one of the longest ever observed in natural negative cloud-toground lightning. Lightning data gathered from the lightning detection network operated by the Royal Meteorological Institute of Belgium [consisting of a network employing solely Surveillance et Alerte Foudre par Interfe´rome´ trie Radioe´ lectrique (SAFIR) sensors and a network combining SAFIR and LS sensors], the European Cooperation for Lightning Detection (EUCLID), Vaisala’s Global Lightning Detection network GLD360, and the Met Office’s long-range Arrival Time Difference network (ATDnet) are evaluated against this ground-truth dataset. It is found that all networks are capable of detecting over 90% of the observed flashes, but a larger spread is observed at the level of the individual strokes. The median location accuracy varies between 0.6 and 1 km, except for the SAFIR network, locating the ground contacts with 6.1-km
median accuracy. The same holds for the reported peak currents, where a good correlation is found among the networks that provide peak current estimates, apart from the SAFIR network being off by a factor of 3.

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Farges T., L. J. Gallin, E. Blanc, A. Le Pichon, E. Defer, W. Rison, W. Schulz, M. Fullekrug, and S. Soula:
Overview of acoustic measurements in South-East of France in 2012 during severe weather

European Geosciences Union, General Assembly, Vol. 15, No. p. 6893, 2013

CEA (Commissariat à l’Energie Atomique et aux Energies Alternatives) installed two acoustic networks in South-East of France. The first one was located in Observatoire Haute Provence in the framework of the ARISE European design project (arise-project.eu). It was composed of 4 microbarometers set in an equilateral triangle. The other network was located close to Uzès in the frame of the HyMeX (www.hymex.org) Special Operational Period from August 27th to November 16th. It was composed of 4 microbarometers and 4 microphones. From August to November 2012, several thunderstorms occurred close to these stations. The global thunder spectrum from 0.01 Hz to 250 Hz is studied using the microbarometer and microphone recordings. A very large thunderstorm, in August 30th-31st, produced more than 100 sprites over the south-western part of the Mediterranean Sea. Ten of them were dancing sprites. This kind of sprites can produce infrasound. This experiment offers a unique to triangulate sprites with infrasound measurements. Lastly, a tornado occurred in October 14th. This event will be analyzed with the data of both networks.

Saba M. M. F. , C. Schumann, T. a. Warner, J. H. Helsdon, W. Schulz, and R. E. Orville:
Bipolar cloud-to-ground lightning flash observations

Journal of Geophysical Research: Atmospheres, Vol. 118, No. April, p. n/a–n/a, Oct. 2013

Bipolar lightning is usually defined as a lightning flash where the current waveform exhibits a polarity reversal. There are very few reported cases of cloud-to-ground (CG) bipolar flashes using only one channel in the literature. Reports on this type of bipolar flashes are not common due to the fact that in order to confirm that currents of both polarities follow the same channel to the ground, one necessarily needs video records. This study presents five clear observations of single-channel bipolar CG flashes. High-speed video and electric field measurement observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high-speed cameras, we suggest that positive leader branches which do not participate in the initial return stroke of a positive cloud-to-ground flash later generate recoil leaders whose negative ends, upon reaching the branch point, traverse the return stroke channel path to the ground
resulting in a subsequent return stroke of opposite polarity.

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Schumann C., M. M. F. Saba, R. B. G. Da Silva, and W. Schulz:
Electric fields changes produced by positives cloud-to-ground lightning flashes

Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 92, No. 0, pp. 37–42, Jan. 2013

Positive flashes correspond to approximately only 10% of the total number of flashes produced by a thunderstorm. However, strokes with high peak currents and long continuing currents are usually present in positive flashes. Therefore, positive flashes are responsible for more intense damage than the negative ones. Positive flashes often are preceded by significant and long duration intracloud (IC) discharge activity. We observe in detail the electric field variations produced by 80 cloud-to-ground lightning flashes in 9 different storms in S. Paulo, Brazil during the summers of 2009–2011. Intracloud discharges preceding the positive cloud-to-ground flashes and some characteristics of the electric field changes produced by the return stroke that occurred at ranges of 3–80 km from the site of the electric field measurements were analyzed. All flashes presented breakdown pulses prior to the return stroke. The mean time interval between the preliminary breakdown pulse (PBP) and return stroke was 157 ms. The pulse train duration have a mean value of 3.1 ms. Only 6 out of 80 cases analyzed did not present pulse trains but only one single bipolar breakdown pulse before the return stroke. In 95% of cases the initial breakdown pulse presented the same initial polarity of the succeeding return stroke. Time interval between pulses in a pulse train had a mean value of 280 μs. The mean values of pulse width is 25.2 μs. The mean values of zero-to-peak risetimes and of the 10–90% risetimes for 72 return strokes electric field waveforms are 9.5 and 5.7 μs respectively. The AM value of peak amplitudes of the positive return strokes fields normalized to 100 km is 17.0 V/m.

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