Diendorfer G., W. Schulz:
ALDIS Austrian Lightning Detection and Information System 1992–2008

Elektrotechnik & Informationstechnik (e&i) 125/5: 209–213. DOI 10.1007/s00502-008-0530-3, 2008

Seit nunmehr 16 Jahren stehen dank ALDIS detaillierte Informationen über die Gewitteraktivität in Österreich zur Verfügung. Weniger bekannt ist so manchem Anwender, dass die wesentlichen operativen Aktivitäten von ALDIS im Österreichischen Verband für Elektrotechnik (OVE) stattfinden. Der vorliegende Beitrag bietet einen kurzen Überblick über die Entwicklung von ALDIS, sowohl hinsichtlich der angewendeten Technologie in der Blitzortung als auch in Bezug auf die internationale Einbindung von ALDIS im europäischen Verbund von Ortungs-systemen (EUCLID). Zusätzlich wurden einige statistische Daten über das Blitzgeschehen in Österreich seit der Inbetriebnahme von ALDIS im Jahr 1992 zusammengestellt. Die Abteilung ALDIS im OVE betreibt nicht nur das Blitzortungssystem, sondern ist auch sehr aktiv in der Blitzforschung tätig.

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Kaltenböck R., G. Diendorfer, N. Dotzek:
Proximity sounding parameters obtained from ECMWF Analyses as predictor for local severe storm types in Europe

Geophysical Research Abstracts, Vol. 10, EGU2008-A-00000, EGU General Assembly 2008 © Author(s) 2008

This study describes the environmental atmospheric characteristics in the vicinity of different types of severe convective storms in Europe during the warm season in 2006 and 2007. A sample of 3406 severe weather events from the European SevereWeather Database (ESWD) is examined and provides information about different types of severe local storms, like significant or weak tornadoes, large hail, damaging winds and heavy precipitation. These data were combined with EUCLID lightning detection data, which provide well-defined null cases on a European scale to distinguish between severe and ordinary or no thunderstorm activity. ECMWF T799 analyses are used to calculate sounding parameters in close proximity to reported severe event locations, for every day within the investigated time period.

Instability indices and CAPE have considerable skill to predict the occurrence of thunderstorms and the probability of severe events. In addition, low level moisture can be used as a predictor to distinguish between significant tornadoes or non-severe convection. Most events associated with wind gusts occurred during high synoptic flow situation reveal the downward transport of momentum as the most important factor. While deep-layer shear discriminates well between severe and non-severe events, the storm relative helicity in the 0-1 km (and surface to lifting condensation level) layer adjacent to the ground has more skill in distinguishing between environments favouring significant tornadoes and wind gusts versus other severe events.

Additionally, composite parameters that combine measurements of buoyancy, vertical shear and low level moisture have been tested to discriminate between severe events. No parameters have been found, which distinguish well between significant tornados and local severe wind events.

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Diendorfer G., A. Mosaddeghi, D. Pavanello, H. Pichler, F. Rachidi, M. Rubinstein:
Electric and Magnetic Field Measurements at Very Close Range Associated with Lightning Strikes to the Austrian Gaisberg Tower

Union Radio-Scientifique Internationale (URSI), General Assembly, 2008

This paper presents a preliminary analysis of recently obtained experimental data associated with lightning strikes to the Gaisberg Tower in Austria. Electric field changes were measured at distances of 22 m and 170 m from the tower base and the magnetic field was measured at a distance of 20 m. Simultaneously, the lightning return stroke current was measured using a sensor located near the top of the tower. The electric field waveforms feature the typical asymmetrical V-shaped pulses, the bottom of the V being associated with the transition from the leader to the return-stroke. The obtained results confirm the shadowing effect of the tower in reducing the electric fields in the immediate vicinity of the tower.

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Theethayi N., R. Thottappillil, G. Diendorfer, M. Mair, H. Pichler:
Currents in Buried Grounding Strips Connected to Communication Tower Legs during Lightning Strikes

Union Radio-Scientifique Internationale (URSI), General Assembly, 2008

During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs’ foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg’s is presented.

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Theethayi N., R. Thottappillil, G. Diendorfer, M. Mair, H. Pichler:
Currents in Buried Grounding Strips Connected to Communication Tower Legs during Lightning Strikes

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 4, 2008

During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs’ foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg’s are presented. The measured currents at different locations on the 70-m long ground strip are compared with the predictions of a frequency dependant lossy transmission line (TL) model and reasonably good agreement was found. From this validation it is claimed that the TL models are appropriate for lightning transient analysis of grounding systems.

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Keul A., P. Sauseng, G. Diendorfer:
Ball Lightning – an Electromagnetic Hallucination?

The International Journal of Meteorology, Vol. 33, No.327, 2008

A common ad-hoc-hypothesis tries to explain ball lightning (BL) as an electromagnetic (EM) brain effect caused by ordinary lightning, i.e. as a lightning-induced hallucination. A critical assessment of this alleged effect has to link the physical properties of lightning and its EM field with the neurophysiology of EM-induced hallucinations, so-called magnetophosphens. Using the clinical field of EM brain stimulation – Transcranial Magnetic Stimulation (TMS) and repetitive TMS (rTMS) – with its experimental phosphene data, the authors conclude that EM fields of nearby lightning flashes, because of their spatial configuration and magnetic induction, are unlikely to produce magnetophosphenes. Phosphenes do not appear in lightning accident reports. Phenomenologically, EM phosphenes as elementary hallucinations do not correspond to BL.

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Diendorfer G., W. Schulz:
Critical analyses of LLS detected very large peak current lightning strokes

20th International Lightning and Detection Conference and 2nd International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2008

Knowledge of probability of occurrence of large peak current flashes is important for lighting protection applications. The international standards for lightning protection of objects are based on probabilities of occurrence of lightning peak currents exceeding given values, where these values are mainly extrapolated from data from lightning current measurements on instrumented towers.
Lighting Locations Systems (LLS) infer peak current from the measured electromagnetic fields. In this presentation we show detailed analyzes of strokes with reported amplitudes of more than –150 kA and +200 kA, respectively.

Similar to Cummins (2000) we will show results of analyzes regarding the number of used parameters to estimate the peak current and calculate the location as a function of peak current. Detailed analysis of individual strokes with reported extremely high amplitudes revealed, that in many cases the amplitude calculation is based on a low number of used parameters for the localization and especially for the peak current estimation. As a result of our study we recommend a very careful interpretation of lightning peak current probability distributions in the range above 100 kA.

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Schulz W., K. Cummins:
A method to determine relative stroke detection efficiencies from multiplicity distributions

20th International Lightning and Detection Conference and 2nd International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2008

The performance of a lightning location system (LLS) can vary with time as a result of changes in the location and performance of the sensors that comprise the network. The most common changes are (1) the addition or removal of sensors and (2) updated sensor technology. Changes in LLS performance can lead to significant changes in estimated lightning parameters (e.g. peak current and flash multiplicity statistics - Cummins and Bardo, 2004). Given these facts, it is important to be able to quantify these network changes in terms of the relative detection efficiency (DE) between various network configurations.

A method to determine the overall relative stroke DE using peak current distributions has been presented by Cummins and Bardo [2004], and is described in detail in an upcoming CIGRE report by Task Force C4.404A. This approach has the limitation that any network configuration changes that alter the individual stroke peak current estimates will introduce errors in the DE calculation. It further does not implicitly provide relative flash DE.

The method described in this paper is not subject to the limitations noted above. The basis for the detection efficiency (DE) correction using multiplicity distributions is the work by Rubinstein [1995], where he shows the relation between flash and stroke DE. He shows that the detected flash multiplicity distribution Nf can be calculated from the “actual” flash multiplicity distribution  Nfa where F(n,m) is the probability to detect an n-stroke flash as m-stroke flash. In the case where the individual stroke DE (defined as P) is independent of stroke order, F(n,m) can be calculated according to Eq. (2). In this example F(n,m) is completely specified by the value of P. In our work we also present a somewhat more complicated expression for F(n,m) in cases where there are different stroke DEs for different stroke orders.

Our new method to determine the stroke DEs describes the problem in terms of relative DEs, where the actual flash multiplicity distribution  in Eq.(2) is substituted by the “reference” flash multiplicity distribution. In the case where one assumes two stroke DEs, one for first and one for subsequent strokes (pfirst and psub), then Eq. (2) contains those two stroke DEs as unknowns. With a nonlinear least square algorithm it is possible to determine those two unknowns and therefore determine the relative stroke DEs for the network. Having all the stroke DEs it is an easy task to calculate the corresponding flash DE.

In this paper we provide further details about the method and its’ assumptions, and we present results from applying the method to real data from the NLDN.

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Schulz W., S. Sindelar, A. Kafri, Götschl T., Theethayi N., R. Thottappillil:
The ratio between first and subsequent lightning return stroke electric field peaks in Sweden

29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

Electric field measurements of first and subsequent strokes by the Austrian Lightning Detection and Information System (ALDIS) and by electric-field antennas in Florida, United States, show no agreement about the relationship between first-stroke peak field and subsequent stroke peak field in negative cloud-to-ground lightning [Diendorfer et al., 1998]. While in Florida the results are in agreement with what is usually accepted in the literature, that is, the median negative first-stroke peak field (or current) is approximately two times larger than subsequent stroke peak field in the same channel, in Austria the ALDIS network has observed no difference between the median values of the electric field (or current) for first and subsequent strokes. To investigate this controversy in more detail several field measurement campaigns were performed during the last years, e.g. in Austria [Schulz and Diendorfer, 2006] and in Brazil [Filho et al., 2007]. During summer 2006 field measurement data was collected in Sweden also with the same field measurement system which was used in Austria and Brazil [Schulz and Diendorfer, 2006]. In this paper we will show a comparison of the peak field ratio between first and subsequent strokes given by the field measurement data and the Swedish lightning location system. We will further compare the result from Sweden with results from Austria and Brazil.

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Diendorfer G., K. Cummins, V.A. Rakov, A.M. Hussein, F. Heidler, M. Mair, A. Nag, H. Pichler, W. Schulz, J. Jerauld, W. Janischewskyj:
LLS-Estimated versus directly measured currents based on data from tower-initiated and rocket-triggered lightning

29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

LLS infer lightning peak currents from remotely measured electric and magnetic peak fields assuming a linear relationship between peak field and peak current. Directly measured currents at either instrumented towers (e.g Gaisberg Tower, CN Tower, Peissenberg Tower) or at the channel base of rocket triggered lightning are the only available ground truth data to verify the accuracy of LLS peak current estimates.

We compare the directly measured peak currents versus LLS inferred peak currents for lightning to towers of different height, ranging from 100 m (Gaisberg) to 553 m (CN Tower) and for triggered lightning, where the lightning channel termination point is typically close to the ground level. In recent publications [1-4], different relations between measured and inferred peak currents were reported. At the CN Tower, the NALDN peak currents were notably larger than the measured peak currents probably because the assumed relationship between field and current does not account for the transient process in the tower.

Differences in the estimated peak current (relative to the measured one) may also result from differences in the configuration of the employed LLS. A propagation model is used in the NALDN to account for field attenuation due to finite ground conductivity, whereas in the ALDIS system a pure 1/r distance dependency of the fields was used until 02/2005. The effect of the propagation model is expected to be more pronounced when sensors at larger distances from the striking point are used for locating the strokes. After applying the attenuation correction to the ALDIS network, the LLS shows a tendency to underestimate the Gaisberg tower lightning peak currents slightly more than the US-NLDN underestimates the triggered lightning. Significant field enhancement due to strokes to tall towers is only seen for the CN Tower.

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Manoochehrnia P., W. Schulz, R. Rachidi, M. Rubinstein:
Lightning statistics in the regions of Säntis and St. Chrischona towers in Switzerland

29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

In this paper we present lightning statistics in the regions of the Säntis Tower and the St. Chrischona Tower in Switzerland. We analyze lightning data for an eight-year period from 1999 to 2006. This work is part of a recent study on overall lightning activity in Switzerland. Lightning location data from the EUCLID (European Cooperation of Lightning Detection) lightning location system (LLS) were used in the study.

The two telecommunications towers are situated in two distinct parts of the country, characterized by different geographical conditions. The Säntis Tower is 124 m tall and is located on the top of the Säntis Mountain (2505 m ASL) in the eastern Swiss Alps. The tower location exhibits the highest lightning flash density in Switzerland during the period from 1999 to 2006. The St. Chrischona Tower is 250 m tall and is located in a relatively flat region near Basel in the northern part of Switzerland (493 m ASL).  Various regional maps and statistics around the two towers are presented, including number of flashes and strokes, number of strokes per flash (flash multiplicity), and peak current. The effect of each tower was analyzed by comparing lightning statistics within a defined range around the tower with those obtained on an external ring excluding the tower, as done previously for the analysis of the Gaisberg tower in Austria [1]. The results indicate that the lightning incidence to the Säntis tower (about 100 times a year) is much higher than that to the St. Chrischona tower (less than 10). We found also a relatively high value of flash multiplicity for strikes detected in the Säntis tower region, implying that most of strikes to this tower are upward initiated flashes.

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Diendorfer G.:
Some comments on the achievable accuracy of local ground flash density values

29th International Conference on Lightning Protection (ICLP), Uppsala, Sweden, 2008

Today, values of local ground flash density (GFD) are estimated from data from lightning location systems. Lightning is a stochastic phenomenon and it’s occurrence at a given location can be described by a so-called Poisson distribution. Assuming pure random nature of the lightning events from the Poisson distribution we can estimate the achievable accuracy of GFD values as a function of observation period and grid cell size. An accuracy of about ± 20% is achievable when on average more than about 80 events occurred in each grid cell. This finding suggests (1) to adjust the grid cell size Acell according to the expected GFD and available observation period Tobs and (2) to consider an uncertainty range of at least ± 20% for any Ng value that is based on LLS data by counting lightning events in defined grid cells.

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Nag A., V.A. Rakov, W. Schulz, M. M. F. Saba, R. Thottappillil, C. J. Biagi, A. O. Filho, A. Kafri, N. Theethayi, T. Gotschl:
First versus subsequent return-stroke current and field peaks in negative cloud-to-ground lightning discharges

Journal of Geophysical Research, Vol. 113, D19112, doi:10.1029/2007JD009729, 2008

We examine relative magnitudes of electric field peaks of first and subsequent return strokes in negative cloud-to-ground lightning flashes recorded in Florida, Austria, Brazil, and Sweden. On average, the electric field peak of the first stroke is appreciably, 1.7 to 2.4 times, larger than the field peak of the subsequent stroke (except for studies in Austria where the ratio varies from 1.0 to 2.3, depending on methodology and instrumentation). Similar results were previously reported from electric field studies in Florida, Sweden, and Sri Lanka. For comparison, directly measured peak currents for first strokes are, on average, a factor of 2.3 to 2.5 larger than those for subsequent strokes. There are some discrepancies between first versus subsequent stroke intensities reported from different studies based on data reported by lightning locating systems (LLS). The ratio of LLS-reported peak currents for first and subsequent strokes confirmed by video records is 1.7 to 2.1 in Brazil, while in the United States (Arizona, Texas, Oklahoma, and the Great Plains) it varies from 1.1 to 1.6, depending on methodology used. The smaller ratios derived from the LLS studies are likely to be due to poor detection of relatively small subsequent strokes. The smaller values in Austria are possibly related (at least in part) to the higher percentage (about 50% versus 24–38% in other studies) of flashes with at least one subsequent stroke greater than the first. The effects of excluding single-stroke flashes or subsequent strokes in newly formed channels appear to be relatively small.

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Manoochehrnia P., C. Price, F. Rachidi, M. Rubinstein, W. Schulz:
Link between Lightning Activity and Temperature: A Regional Study in Switzerland

European Electromagnetics (EUROEM), Lausanne, Switzerland 2008

Considering the importance of global warming and climate change, the problem of possible relationships between climate and lightning activity has recently attracted the attention of researchers (e.g. [1]). Positive correlations between global lightning activity and global temperature variations have been already confirmed in several studies (e.g. [1,2]) where it has been shown that a 1-degree Celsius variation in the temperature would result in an increase of lightning activity, ranging from 10% to 100% [2]. In this paper, we present a preliminary regional study of the link between temperature and lightning activity in Switzerland. Lightning statistics were obtained from the lightning database of the EUCLID (European Cooperation of Lightning Detection) network, recently presented in [3]. Any unknown time variations in the performance of the system are necessarily neglected. Data for the temperature are obtained from the terrestrial meteorology stations in the national network of the Swiss Federal Office of Meteorology and Climatology [4]. Fig. 1a presents the monthly negative cloud-to-ground flash count and the monthly mean maximum daily temperature in Switzerland for the months of August, 1999-2006, where the linear correlation factor is 75%. A similar analysis has been done in the region of the Säntis in northeastern Switzerland, characterized by the highest lightning activity in that country. Fig. 1b shows the monthly mean of maximum daily temperature and the monthly number of the negative cloud-to-ground flashes within 100 km of the Säntis Mountain. The results show an 83% linear correlation.

Acknowledgments - This work has been partially supported by the European COST Action P18 ‘The Physics of Lightning Flash and Its Effects’.

1. C. Price, Global surface temperatures and the atmospheric electric circuit, Geophys. Res. Lett. 20, 1363, 1993.
2. E.R. Williams, Lightning and climate: A review, Atmospheric Research, 76, pp. 272-287, 2005
3. P. Manoochehrnia, F. Rachidi, M. Rubinstein, W. Schulz, Lightning statistics in Switzerland, 9th International Symposium on Lightning Protection, SIPDA, Foz do Iguaçu, Brazil, November 2007.
4. M. Begert, T. Schlegel, and W. Kirchhofer, Homogeneous Temperature and Precipitation Series of Switzerland from 1864 to 2000, International Journal of Climatology, vol. 25, pp. 65-80, 2005.

Monthly negative cloud-to-ground flash count and the monthly mean maximum temperature during the months of August, 1999 to 2006.<br>
(a) The whole Switzerland, (b) within 100 km of the Säntis Mountain

Gaffard C., J. Nash, N. Atkinson, A. Bennett, G. Callaghan, E. Hibbett, P. Taylor, M. Turp, W. Schulz:
Observing Lightning Around the Globe from the Surface

20th International Lightning and Detection Conference and 2nd International Lightning Meteorology Conference (ILDC/ILMC), Tucson, Arizona, 2008

The UK Met Office VLF arrival time difference (ATD) long range lightning location network has been operating successfully for nearly 20 years. The range includes all of Europe, North Africa, the North Atlantic and most of South America. Recent expansions and improvements to the network have increased the range of detectable lightning to now include all of South America, Africa and central Asia. The improved network (now called ATDnet) has been operating offline in parallel to the original system for testing, but has now replaced the current operational system since December 2007.

The increased coverage and new receiver instrumentation for the ATDnet network will be discussed. The network is compared to other European lightning detection systems to assess the accuracy and detection efficiency over Europe. A significant diurnal variation in the detection efficiency of ATDnet is observed over Europe, which is suggested to be due to a nocturnal enhancement of wave guide modal interference.

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