Gerhard Diendorfer

Initial stage in lightning initiated from tall objects and in rocket‐triggered lightning

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

Some Parameters of Negative Upward-Initiated Lightning to the Gaisberg Tower (2000–2007)

We have analyzed parameters of 457 upward-initiated negative lightning flashes to the Gaisberg tower (GBT) in Austria recorded from 2000 to 2007. Lightning to the radio tower of a height of 100 m and located on a mountain 1287 m above sea level occurs almost independent of season, although a pronounced thunderstorm season exists in Austria during the summer months. A total of 30% of the upward-initiated negative flashes exhibited one or more return strokes with an average multiplicity of 4.4 and a geometric mean interstroke interval of 17.3 ms. The remaining 70% of upward-initiated negative flashes showed an initial continuous current (ICC) only, either with (22%) or without (48%) superimposed current pulses of peak currents greater than 2 kA. Median of total transferred charge to ground is about 30% lower in summer (26.8 C) than in other seasons (39.0 C). For the return stroke current pulses, we determined a median peak current of 9.2 kA (sigmalog10 = 0.25) and a median pulse charge of 0.51 C (sigmalog10 = 0.39). For current pulses superimposed on the ICC, we determined a median of 4.2 kA (sigmalog10 = 0.26) with 22 kA peak current measured for the largest ICC pulse.

Induced voltage on an overhead line due to nearby lightning

Voltage waveshapes induced on an overhead line by nearby lightning return strokes are calculated numerically by solving the transmission-line equations using the time-domain approach of A.K. Agrawal et al. (1980) and a finite-difference technique. The traveling current source (TCS) model is used to calculate the return-stroke vertical and horizontal-electric fields (which are the sources for a line voltage) above a perfectly conducting ground plane. The effect of the striking-point distances and the return-stroke current waveshape on the induced voltages are illustrated. The exact location of the maximum voltage along the line depends not only on the closest distance from the striking point to the line but also on lightning-current parameters. In the case of a 2000-m-long line and a striking point a distance of 100 m from the line center, almost 90% of the induced voltage at the line terminations is due to the horizontal electric field coupling. >

Lightning characteristics based on data from the Austrian lightning locating system

We compare various lightning characteristics measured by the Austrian lightning locating system [Austrian Lightning Detection and Information System (ALDIS)] with those found in the literature. The latter are typically based on measurements of lightning electric fields. We show that lightning peak electric fields due to subsequent strokes measured by the ALDIS are similar to those in the literature. However, the ALDIS data do not show the usual ratio of about 2:1 between the median values of the field peaks for first and subsequent strokes. Although the flash detection efficiency of the ALDIS system in the area of investigation is estimated to be higher than 90%, one of the best for such systems all over the world, the observed percentage of single-stroke flashes and the average number of strokes per flash seem to suggest that the stroke detection efficiency is appreciably less than 90%. The ALDIS data indicate that larger strokes are preceded by longer interstroke intervals. The mean flash duration of 175 ms measured by ALDIS is similar to the typical flash duration found in the literature. Strokes with larger field peaks tend to have higher average field rates of rise. Since many lightning parameters show a large scatter for different thunderstorm days, long-term data from lightning locating systems are more representative of average lightning compared to data derived from electric field measurements typically performed during a few thunderstorms.

Characteristics of upward positive lightning flashes initiated from the Gaisberg Tower

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 ...

Extension of the Diendorfer‐Uman lightning return stroke model to the case of a variable upward return stroke speed and a variable downward discharge current speed

A new lightning return stroke model has recently been proposed by Diendorfer and Uman (1990). In this model, if one specifies a current at the channel base (ground), a constant return stroke speed, and a channel discharge time constant, one can derive analytically the current and charge as a function of time and height associated with the channel above ground. Here we present a more general and more straightforward derivation of the Diendorfer-Uman model. In the new formulation we allow a variable return stroke speed that can be any arbitrary function of height. The influence of a decrease in speed with height, as occurs in nature, on the channel current and charge distributions and on the radiated electric and magnetic fields is determined and compared with the constant speed case. For a given channel base current, a decreasing speed with height does not change the initial peak electric and magnetic fields appreciably from the values found for a constant speed having the same value as the variable speed at the channel base, but a decreasing speed can cause considerable changes in field waveshapes for both the distant radiation fields and the near electrostatic fields. In addition to allowing an arbitrary return stroke speed, the new formulation of the Diendorfer-Uman model allows the downward propagating current waves released by the return stroke front also to have an arbitrary variable speed, whereas the original model assumed that speed to be constant at the speed of light.

Characteristics of upward bipolar lightning flashes observed at the Gaisberg Tower

We analyze current records for 21 natural upward-initiated bipolar lightning flashes observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. Thirteen (62%) of 21 bipolar flashes occurred in non-convective season (September- March) and twelve (57%) of them occurred in seasonal transmission periods of March, August, and November in Austria. Thirteen (62%) of them belong to Type 1 associated with a polarity reversal during the initial stage current, based on the classification as suggested by Rakov and Uman [1]. We also find that the initial polarity reversal from negative to positive occurs more often (16 of 21) than that from positive to negative within a bipolar flash, in agreement with observations in other studies. The geometric mean total absolute charge transfer is 99.5 C with a relatively short total duration of 320 ms.

Lightning electromagnetic fields at very close distances associated with lightning strikes to the Gaisberg tower

In this paper we present and discuss measurements of electric (vertical and radial) and magnetic 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 20 m from the tower. Simultaneously, return stroke currents were also measured at the top of the tower. The data include, for the first time at such close distances, simultaneous records of vertical and horizontal electric fields. The vertical electric field waveforms appeared as asymmetrical V-shaped pulses. The initial, relatively slow, negative electric field change is due to the downward leader, and the following, fast, positive electric field change is due to the upward return stroke phase of the lightning discharge. The horizontal (radial) electric field due to the leader phase has a waveshape similar to that of the vertical electric field. However, the horizontal field due to the return stroke is characterized by a short negative pulse of the order of 1 mu s or so, starting with a fast negative excursion followed by a positive one. The return stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower, which results in a significant decrease of the electric field at distances of about the height of the tower or less. The vertical and horizontal E field changes due to the return stroke were also found to be larger on average than the leader electric field changes. In a significant number of cases (33%), the vertical electric field waveforms due to the return stroke were characterized by a first peak exceeding the typical late-time flattening due to the electrostatic term. This is in contrast with similar measurements related to triggered lightning which do not exhibit such a first peak. About one quarter of the measured vertical electric field waveforms (18 pulses out of 76) featured an unusual waveform characterized by a positive leader field change followed by a bipolar return stroke field change with a zero crossing time of about 60 mu s.

On the Location of Lightning Discharges Using Time Reversal of Electromagnetic Fields

In this paper, we discuss the use of the electromagnetic time reversal (EMTR) method to locate lightning strikes. After a brief description of the EMTR and its application to lightning location, we mathematically demonstrate that the time-of-arrival method can be seen as a subset of EMTR. We propose three different models of backpropagation to address the issue of EMTR not being invariant for lossy media. Two sets of simulations are carried out to evaluate the accuracy of the proposed methods. The first set of simulations is performed using numerically generated fields and the proposed algorithm is shown to give very good results even if the soil is not perfectly conducting. In particular, we show that considering a model in which losses are inverted in the back propagation yields theoretically exact results for the source location. We show also that a lack of access to the complete recorded waveforms may lead to higher location errors, even though the computed errors are found to be within the range of performance of current lightning location systems (LLS). A second set of simulations is performed using the sensor data reported by the Austrian LLS. The locations obtained by way of the proposed EMTR method using only the available sensor data (amplitude, arrival time, and time-to-peak), are observed to be within a few kilometers of the locations supplied by the LLS.

Lightning Characteristics as a Function of Altitude Evaluated from Lightning Location Network Data

Many analyses of lightning parameters and lightning densities are found in literature. Most of the data are based on single station measurements and therefore, strictly speaking, they are only valid for a certain altitude. With lightning location systems it is possible to collect and analyze lightning parameters over large areas. For such analyses, a lightning location network of high detection efficiency over a large area is necessary. Because the Austrian lightning location network is a network of small baselines it has a very high detection efficiency and is therefore ideal for such an investigation. In this paper we show preliminary results of lightning density and lightning peak current as a function of altitude above sea level in Austria.