Mohammad Azadifar

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

In this paper, we present a theoretical analysis of the propagation effects of lightning electromagnetic fields over a mountainous terrain. The analysis is supported by experimental observations consisting of simultaneous records of lightning currents and electric fields associated with upward negative lightning flashes to the instrumented Säntis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Säntis tower is simulated using a full-wave approach based on the finite-difference time-domain method and using the two-dimensional topographic map along the direct path between the tower and the field measurement station located at about 15 km from the tower. We show that, considering the real irregular terrain between the Säntis tower and the field measurement station, both the waveshape and amplitude of the simulated electric fields associated with return strokes and fast initial continuous current pulses are in excellent agreement with the measured waveforms. On the other hand, the assumption of a flat ground results in a significant underestimation of the peak electric field. Finally, we discuss the sensitivity of the obtained results to the assumed values for the return stroke speed and the ground conductivity, the adopted return stroke model, as well as the presence of the building on which the sensors were located.

Evaluation of the performance characteristics of the European Lightning Detection Network EUCLID in the Alps region for upward negative flashes using direct measurements at the instrumented Säntis Tower

In this paper, we present a performance analysis of the European Cooperation for Lightning Detection (EUCLID) lightning detection network using data obtained on lightning currents measured at the Santis Tower (located in northeastern of Switzerland) from June 2010 to December 2013. In the considered period of analysis, a total number of 269 upward negative flashes were recorded at the Santis Tower. The performance of the EUCLID lightning detection network is evaluated in terms of detection efficiency, location accuracy, and peak current estimates for upward flashes. Excluding flashes containing only an initial continuous current with no superimposed pulses exceeding 2kA, the flash detection efficiency for upward flashes is estimated to be 97%. The recorded flashes contained a total of 2795 pulses (including return strokes and International Conference on Communications pulses characterized by risetimes lower than 8 mu s and peaks greater than 2kA). The overall pulse detection efficiency was found to be 73%. For pulses with peak values higher than 5kA, the pulse detection efficiency was found to be about 83%. Peak current estimates provided by the EUCLID network were found to be significantly larger than their directly measured counterparts. This overestimation might be attributed to the enhancement of the radiated electromagnetic fields associated with the presence of the tower and the mountain. The median of the absolute distance error, defined as the median distance between the Santis Tower location and the EUCLIDs stroke locations, was found to be 186m, the majority of large location errors being associated with measured current peaks lower than 10kA. The analysis revealed also that the location accuracy of the EUCLID network improved significantly in 2013 as a result of an upgrade in the location algorithms to take into account propagation effects.

Analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on ToA-based lightning location systems

In this paper, we analyze the propagation effects on lightning-radiated electromagnetic fields over mountainous terrain by using a three-dimensional (3-D) finite difference time domain (FDTD) method. We also discuss the time delay error in the time-of-arrival (ToA) technique currently used to locate lightning in detection networks, specifically. Furthermore, the accuracy of different approximate methods presented in the literature is discussed and tested by using our 3-D FDTD method. It is found that (1) the time delays and amplitudes of the lightning-radiated electromagnetic fields can be significantly affected by the presence of a mountainous terrain and associated diffraction phenomena; (2) for a finitely conducting ground, the time delay shows a slight increase with the increase of the observation distance, but the time delay resulting from the finite ground conductivity appears to be smaller than that caused by the mountainous terrain; and (3) the timing error associated with the ToA technique depends on the threshold times. Threshold times of 10% and 20% of the peak provide very similar results compared to those corresponding to the peak of the first derivative of the magnetic field, and the threshold time exceeds 50% of the initial rising amplitude of the signal. Furthermore, we have assessed the accuracy of two simplified methods (terrain-envelope method and tight-terrain fit method) to account for the time delays resulting from the propagation in a mountainous terrain. It is found that both methods result in time delays that are in reasonable agreement but always overestimating the results obtained using the full-wave 3-D FDTD approach for the perfectly conducting ground. These two methods represent interesting alternatives to account for the time delay over a nonflat terrain using the terrain model.

Fast initial continuous current pulses versus return stroke pulses in tower‐initiated lightning

We present a study focused on pulses superimposed on the initial continuous current of upward negative discharges. The study is based on experimental data consisting of correlated lightning current waveforms recorded at the instrumented Santis Tower in Switzerland and electric fields recorded at a distance of 14.7 km from the tower. Two different types of pulses superimposed on the initial continuous current were identified: (1) M-component-type pulses, for which the microsecond-scale electric field pulse occurs significantly earlier than the onset of the current pulse, and (2) fast pulses, for which the onset of the field matches that of the current pulse. We analyze the currents and fields associated with these fast pulses (return-stroke type (RS-type) initial continuous current (ICC) pulses) and compare their characteristics with those of return strokes. A total of nine flashes containing 44 RS-type ICC pulses and 24 return strokes were analyzed. The median current peaks associated with RS-type ICC pulses and return strokes are, respectively, 3.4 kA and 8 kA. The associated median E-field peaks normalized to 100 km are 1.5 V/m and 4.4 V/m, respectively. On the other hand, the electric field peaks versus current peaks for the two data sets (RS-type ICC pulses and return strokes) are characterized by very similar linear regression slopes, namely, 3.67 V/(mkA) for the ICC pulses and 3.77 V/(mkA) for the return strokes. Assuming the field-current relation based on the transmission line model, we estimated the apparent speed of both the RS-type ICC pulses and return strokes to be about 1.4 x 10(8) m/s. A strong linear correlation is observed between the E-field risetime and the current risetime for the ICC pulses, similar to the relation observed between the E-field risetime and current risetime for return strokes. The similarity of the RS-type ICC pulses with return strokes suggests that these pulses are associated with the mixed mode of charge transfer to ground.

An update on the charaterictics of positive flashes recorded on the Säntis Tower

We present an update on the characteristics of positive flashes recorded on the Säntis Tower in Switzerland from May 2010 to January 2013, during which period 38 positive flashes were recorded. Out of the 38 recorded flashes, 1 was identified as a normal downward flash, 5 were classified as type-1 flashes (characterized by a large unipolar impulse) and the rest of the flashes (32) were classified as type-2 flashes (characterized by a slow waveform with superimposed pulse train). We present a summary of the characteristics of the lightning current parameters for type-1 and type-2 flashes separately and for both types combined. We also present a comparison between the present results corresponding to type-1 flashes and Berger et al.s data for flashes with large impulsive currents. A similar comparison is presented between type-2 flashes in this study and Berger et al.s data for flashes without large impulsive currents. In the latter comparison, the most significant differences between the two datasets are in the peak current and the amount of transferred charge, both of which are substantially larger in our type-2 flash dataset. We finally present a discussion of the oscillatory pulse trains in type-2 flashes. Our data are found to be, in general, consistent with those observed at the Gaisberg tower. However, the overall leader pulse duration and the individual pulse duration observed at Säntis are higher than their counterparts at Gaisberg.

An update on experimental data obtained at the Säntis Tower

The Säntis Tower was instrumented in May 2010 and, to this date, more than 400 flashes were successfully recorded. Lightning current waveforms and their time-derivatives are measured at two different heights along the tower (24-m and 82-m AGL). In this paper, we present a summary of the obtained experimental data.

Characteristics of electric fields of upward negative stepped leaders

We report simultaneous measurements of vertical electric fields and currents associated with positive flashes to the Säntis tower. The fields were measured at a distance of 14.7 km from the tower. All the data correspond to type-2 positive flashes characterized by currents with relatively slow rising portion on which oscillatory pulse trains, inferred to be due to an upward negative stepped leader, are superimposed. Bipolar electric field and current pulses associated with the upward negative stepped leader are examined and analyzed. The widths of the initial and second half cycles are found to be similar. The peak of the initial half cycle is found to be, on average, about twice as high as the peak of the second half cycle. A nearly linear correlation is found between the field peaks and associated current peaks.

An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Säntis Tower

We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Santis Tower in Switzerland. The electric fie ...

A semi-analytical simplified approach to compute lightning radiated electric fields at long distances taking into account ionospheric reflection

In this paper, we present a semi-analytical simplified approach based on the ray tracing method to estimate radiated electric fields associated with lightning return strokes, taking into account ionospheric reflections. The field transfer function at each frequency and each grazing angle is determined by applying the generalized Snells law of refraction, and the antenna radiation pattern concept is used to obtain the source illumination efficiency, from which the radiated field is derived. The proposed method is validated using as reference full-wave FDTD simulations.

Simultaneous records of current and 380-km distant electric field of a bipolar lightning flash

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.