2012

Ballarotti Maurício G., C. Medeiros, M. M. F. Saba, W. Schulz, O. Pinto Jr.:
Frequency distributions of some parameters of negative downward lightning flashes based on accurate-stroke-count studies

Journal of Geophysical Research: Atmospheres, Vol. 117, D06112, doi:10, 2012

This paper presents some parameters of negative cloud-to-ground lightning flashes in terms of frequency distribution. All data are based on so-called “accurate-stroke-count studies” from different climatological regions in the world and were already published in the literature with the exception of our measurements. We used GPS synchronized data from two digital high-speed cameras (at 1–8,000 frames/sec). The parameters considered in this study are: (1) continuing current duration, (2) time intervals between strokes, (3) number of strokes per flash and (4) total flash duration. The analysis includes Berger’s data of Monte San Salvatore (Switzerland), which is the basis for lightning protection standards. The comparison suggests that despite of overall agreement of those parameters that some of them, currently used in protection standards, should be revised in order to be more realistic.

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Zhou H., G. Diendorfer, R. Thottappillil, H. Pichler, M. Mair:
Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower

Journal of Geophysical Research: Atmospheres, Vol. 117, D06110, doi: 10.1029/2011JD016903, 2012

We report the measured current characteristics of positive lightning discharges to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. On the basis of the recorded current waveforms, a total of 26 flashes consisting of initial stage only were identified as upward positive discharges initiated by an upward negative leader from GBT. They accounted for 4% (26/652) of the total flashes recorded at the GBT. Nineteen (73%) out of the 26 positive flashes occurred during nonconvective season (September–March). Median values of flash peak current, flash duration, charge transfer, and action integral were determined as 5.2 kA, 82 ms, 58 C, and 0.16 x 103 A2 s, respectively. Current pulses of high repetition rate superimposed on the initial portion of initial continuous current are inferred to be associated with the upward negative stepped leader process. The weighted arithmetic means of leader pulse peak current, leader pulse duration, leader interpulse interval, and leader pulse charge are 3 kA, 31 µs, 32 µs, and 42 mC, respectively. On the basis of an assumed stepped leader speed in the range of 8 x 104 to 4.5 x 105 m/s an upward negative stepped leader channel charge density of 15–87 mC/m, a leader length of 168–945 m, and an average leader step length of 2.4–13.3 m were estimated. The upward negative stepped leader channel charge density and length are significantly larger and smaller than their counterparts in downward negative stepped leaders, respectively, while the upward leader step length is consistent with previous studies. Possible reasons for this are discussed.

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Lugrin G. L., N. Mora, F. Rachidi, M. Rubinstein, G. Diendorfer:
Localisation de la foudre par retournement temporal

16ème édition du Colloque International sur la Compatibilité ElectroMagnétique (CEM) (french), 2012

Le processus du retournement temporel en électromagnétisme est appliquée à la localization de la foudre. Cette méthode permet de tenir compte des effets du terrain sur la propagation du champ électromagnétique émis par la foudre. Cependant, le problème principal de cette method provient du fait que les pertes ne sont pas invariantes par retournement temporel. C'est pourquoi nous proposons 4 modèles de rétro-propagation pour résoudre ce problème.

Pour évaluer la précision de l'algorithme, différentes simulations ont été effectuées sur des champs électriques générés numériquement. Les résultats sont très bons même si le sol n'est pas un conducteur parfait.

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Zhou H., G. Diendorfer, R. Thottappillil, H. Pichler, M. Mair:
Measured current and close electric field changes associated with the initiation of upward lightning from a tall tower

Journal of Geophysical Research: Atmospheres, Vol.117, D08102, doi: 10.1029/2011JD017269, 2012

We examine in detail the simultaneous lightning current waveforms, close electric field changes, and lightning location system data for upward lightning discharges initiated from the Gaisberg Tower (GBT) from 2005 to 2009. Out of 205 upward flashes, most of them (87% or 179/205) were initiated from the tower top without any nearby preceding lightning activity (called "self-initiated"), whereas 26 upward flashes (13%) were initiated from the tower top with immediately preceding nearby lightning activity (called "nearby-lightning-triggered"), including 15 positive ground flashes, one negative ground flashes, and 10 cloud discharges. The possible reasons for self-initiated upward flashes dominating at the GBT could be the field enhancement due to the Gaisberg Mountain above the surrounding terrain and low altitude of charge region during non-convective season (September to March), since we note that self-initiated lightning at the GBT occurred predominantly (79% or 142/179) during non-convective season. On the other hand the majority (85% or 22/26) of nearby-lightning-triggered upward flashes at the GBT occurring during convective season (April to August) and 80 nearby-lightning-triggered upward flashes out of 81 upward flashes observed at the ten tall towers in Rapid City in South Dakota of USA occurring during summer seasons, could be due to the result of high altitude of charge region. The triggering flashes were detected to be within 1 and 18 km distance and the time intervals between them and upward lightning initiation are in the range of 0.3 to 90.7 ms.

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Müller M.M., H. Vacik, G. Diendorfer, A. Arpaci, H. Formayer, H. Gossow:
Analysis of lightning-induced forest fires in Austria

Teor Appl Climatol, doi: 10.1007/s00704-012-0653-7, 2012

Besides human-caused fires, lightning is the major reason for forest fire ignition in Austria. In order to analyse the causes of ignition and to characterise lightning-induced forest fires, fire records were compared with the real appearance of lightning events by using the Austrian Lightning Detection and Information System for the period from 1993 to 2010. A probability was estimated for each forest fire being caused by lightning by using a decision tree and decision matrices based on flash characteristics (e.g. amplitude, time, location). It could be shown that 15 % of documented forest fires were
lightning-caused. Nearly all lightning-caused fires were found during the summer months, whereas almost 40 % of all fires occurring from June to August were naturally caused. Most lightning-caused fires took place in the south and east of Austria. Lightning fires were more frequent at higher altitudes and primarily affected conifer forests. The median burned area was lower than that for anthropogenic forest fires.

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Mosaddeghi A., A. Shoory, F. Rachidi, M. Rubinstein, G. Diendorfer, H. Pichler, D. Pavanello:
Lightning Return Strokes to Tall Towers: Ability of Engineering and Electromagnetic Models to Reproduce Nearby Electromagnetic Fields

IEEE Transactions on Electromagnetic Compatibility, Vol. 54, No. 4, 2012

We present measurements of nearby vertical and horizontal electric fields from leaders and return strokes associated with lightning strikes to the 100-m-tall Gaisberg Tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20m from the tower's vertical axis. Simultaneously with the fields, return-stroke currents were also measured at the top of the tower. The measured data are used to test engineering and electromagnetic models for the return stroke. In general, the agreement between measured waveforms and model-predicted ones is satisfactory.

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Romero C., M. Paolone, M. Rubinstein, F. Rachidi, A. Rubinstein, G. Diendorfer, W. Schulz, B. Daout, A. Kälin, P. Zweiacker:
A System for the Measurements of Lightning Currents at the Säntis Tower

Electric Power Systems Research (EPSR) Vol. 82, 2012

The paper presents the characteristics of a system suitably developed to measure lightning current waveforms on the Säntis Tower in Switzerland. The paper first describes the configuration of the system then presents the characteristics of the measurement equipment, in terms of bandwidth and accuracy, inferred by means of experimental tests. Finally, we present and discuss examples of waveforms measured using the system. Keywords: lightning current, elevated struck objects, Rogowski coils, lightning current parameters.

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Zhou H., R. Thottappillil, G. Diendorfer:
Distribution of Charge along the Tower and Lightning Channel when Lightning Strikes a Tall Tower

31st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

We derive expressions to calculate the charge density along the tower and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall tower and a series point current source placed at the tower top. These expressions are used to calculate the very close-range electric fields in the monopole (continuity equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

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Lugrin G, N. Mora, F. Rachidi, M. Rubinstein, G. Diendorfer:
On the Use of the Time Reversal of Electromagnetic Fields to Locate Lightning Discharges

31st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

In this paper, we discuss the use of the Electromagnetic Time Reversal (EMTR) method to locate lightning discharges. The main problem of EMTR is that losses are not invariant under time reversal. In this paper, we propose 3 different models of back-propagation to fix this problem. Simulations are made to evaluate the accuracy of the proposed methods. It is shown that by assuming a lossless backpropagation model, the resulting location errors are in the order of some hundreds of meters. It is also shown that a theoretically exact estimate can be obtained considering a back-propagation over a fictitious 'inverted-loss' ground.

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Saba M.F.M., C. Schumann, T. A. Warner, W. Schulz, R. E. Orville:
Bipolar Cloud-to-Ground Flashes Observations

31st International Conference on Lightning Protection (ICLP), Vienna, Austria, 2012

Bipolar lightning is defined as a lightning event where the current waveform exhibits a polarity reversal within the same flash. 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 two clear observations of single-channel, bipolar CG flashes. High-speed video and electric field measurements observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high-speed cameras [1-3] we will suggest that recoil leaders occurring after the positive stroke may generate a subsequent stroke with an opposite polarity.

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Schulz W., C. Vergeiner, H. Pichler, G. Diendorfer, S. Pack:
Validation of the Austrian lightning location system ALDIS for negative flashes

CIGRE C4 Colloquium on Power Quality and Lightning, Sarajevo, Bosnia and Herzegovina, 2012

In this paper we present a detailed performance evaluation of the Austrian Lightning Location System ALDIS in terms of detection efficiency and location accuracy based on ground truth measurements. The ground truth measurements used in this evaluation are video and E-field measurements.

151 out of the 154 negative cloud to ground flashes and 449 out of 540 strokes were detected by the lightning location system (LLS). This results in a flash detection efficiency of 98% and a stroke detection efficiency of 83%. Only two additional cloud to ground strokes (0.4%) were detected but misclassified as cloud discharges. The analysis of 37 flashes which exhibit at least two strokes within one lightning channel results in a median location accuracy of 368 m (STD = 650 m).

The results of those measurements are in good agreement to performance evaluation done with data from current measurements at the Gaisberg Tower in Austria.

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Schumann C., M. M. F. Saba, R. B. Guedes da Silva, W. Schulz:
Electric fields changes produced by positive cloud-to-ground lightning flashes

International Conference on Grounding and Earthing and 5th International Conference on Lightning Physics and Effects (GROUND/LPE), Bonito, Brazil, 2012

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 to 2011. Preliminary breakdown pulses (PBP) preceding positive cloud-to-ground flashes and some characteristics of the electric field changes produced by the return stroke that occurred at ranges of 3 km to 80 km from the site of the electric field sensor were analyzed. All flashes presented PBP prior to the return stroke. The mean time interval between the PBP and return stroke was 157 ms. The pulse train duration have a mean value of 3.1ms. 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 arithmetic mean value of peak amplitudes of the positive return strokes fields normalized to 100 km is 17.0 V/m.

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Schulz W., C. Vergeiner, H. Pichler, G. Diendorfer, K. Cummins:
Location accuracy evaluation of the Austrian lightning locations systems ALDIS

22nd International Lightning Detection Conference and 4th International Lightning Meteorology Conference (ILDC/ILMC), Broomfield, Colorado, 2012

Since 1998 direct measurements of lightning current are performed at the Gaisberg Tower in Austria [1]. One goal of those measurements is to validate the most important performance parameters of the Austrian lightning location system ALDIS, namely the detection efficiency, the location accuracy and accuracy of the peak current estimate. Location accuracy analysis resulted in a median of about 350m [2] which is in agreement with model-based estimates for the location accuracy, but basically this value is only valid for the location of the Gaisberg Tower. In order to evaluate performance in a variety of locations, we developed a portable GPS synchronized video- and field measurement system [3-5]. From the data recorded by this system we can also infer the detection efficiency and the location accuracy of the LLS. Further it is also possible to use these data to evaluate the type-categorization (cloud-to-ground vs. cloud pulse) assigned by the LLS.

In this study we will focus on the parameter location accuracy. We will present a methodology that allows us to determine the location accuracy of the LLS from video records, and we give some theoretical background about comparing location accuracy estimates from tower (triggered lightning) measurements and video records. Finally we will compare results obtained during a combined video and field measurement campaign performed at 13 different locations in the eastern part of Austria in 2009 and 2010 with results from data measured at the Gaisberg Tower.

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Poelman D.R., W. Schulz, C. Vergeiner:
Performance characteristics of three distinct lightning detection networks covering Belgium.

22nd International Lightning Detection Conference and 4th International Lightning Meteorology Conference (ILDC/ILMC), Broomfield, Colorado, 2012

In this paper, we report results from electric field measurements, coupled to high-speed camera observations to test the performance of lightning location networks in terms of its detection efficiency and location accuracy. The measurements were carried out during August 2011 in Belgium, during which 57 negative cloud-to-ground flashes, with a total of 210 strokes, were recorded. Data from the Belgian lightning network, the European Cooperation for Lightning Detection EUCLID and Vaisala's Global Lightning Detection network GLD360 are evaluated against this ground-truth data set.

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