Two Possible Scenarios for the Formation of Category 1 Bipolar Lightning Flashes

Mohammad Azadifar, Qi Li, Davide Pavanello, Marcos Rubinstein
2019
Conference paper

Abstract

We present and discuss the current and electric field waveforms associated with two upward bipolar lightning flashes that occurred at the Säntis Tower. According to the traditional classification of bipolar flashes, both observed flashes could be classified as Category 1 flashes. The first one belongs to the subcategory 1a in which the polarity reversal during the initial stage is associated with an in-cloud process, while the second one involves the sequential initiation of two opposite-polarity leaders from the tower.

Official source

https://infoscience.epfl.ch/record/265996?ln=en

About this result

This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.

Related concepts

Related publications

, , ,

We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Säntis Tower in Switzerland. The electric field was measured 15 km from the tower. Upward flashes always begin with the initial stage composed of the upward‐leader phase and the initial‐continuous‐current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return‐stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no‐current time intervals; (2) mixed‐mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively small junction heights; (3) “classical” M‐component pulses superimposed on the continuing current following some return strokes; and (4) M‐component‐type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively large junction heights. We consider a data set consisting of 9 return‐stroke pulses, 70 mixed‐mode ICC pulses, 11 classical M‐component pulses, and 19 M‐component‐type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed‐mode and M‐component‐type pulses, which is based on the current waveform features. The characteristics of M‐component‐type pulses during the initial stage are found to be similar to those of classical M‐component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed‐mode ICC pulses were preceded by microsecond‐scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond‐scale electric field pulses observed for M‐component‐type ICC pulses and which can be attributed to the junction of an in‐cloud leader channel to the current‐carrying channel to ground. Classical M‐component pulses and M‐component‐type ICC pulses tend to have larger risetimes ranging from 6.3 to 430 μs. On the other hand, return‐stroke pulses and mixed‐mode ICC pulses have current risetimes ranging from 0.5 to 28 μs. Finally, our data suggest that the 8‐μs criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M‐components. However, mixed‐mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28 μs, as observed in this study.

2018

Pulses in Upward Negative Lightning at the Säntis Tower in Northeastern Switzerland

Mohammad Azadifar, Lixia He, Davide Pavanello, Marcos Rubinstein

Upward lightning happens predominantly from tall objects. The channel-base currents associated with upward negative lightning initiated from towers exhibit pulses that can be classified, based on their waveform features and on the concurrence with other processes in the flash, into four categories. Three of them (M-component-type pulses, M-component pulses, and Mixed-mode pulses) are always superimposed either on the continuous or on the continuing current, and the remaining type (Return Stroke pulses) is initiated during a zero-current interval. Simultaneous measurements of the channel-base current and electric fields 15 km from the tower were analyzed. Good agreement was found between measured and calculated electric field waveforms when simulations were carried out using the MTLE model for the return stroke and for the mixed mode pulses, and when the model of Rakov at al. was used for M-components and M- component-type pulses. The analysis validates the similarity between M-component-type pulses superimposed on the Initial Continuous Current during the initial stage of upward negative lightning and M-components pulses superimposed on the continuing current that follows some return strokes. In addition, the results indicate that mixed-mode pulses are similar to return strokes. In the two studied flashes, the initial stage was associated with a larger charge transferred to ground than the ensuing return strokes and their associated continuing currents and M components, the factor being almost four-to-one.

2019

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

Mohammad Azadifar, Dongshuai Li, Stefan Metz, Mario Paolone, Davide Pavanello, Marcos Rubinstein, Zhaoyang Wang, Qilin Zhang

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 Santis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Santis 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 Santis 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.

Institute of Electrical and Electronics Engineers2016

, , ,

2018