The numerical modeling of electrical discharges occurring in atmospheric air has been in continuous development during the past decades in different fields, such as high-voltage techniques and lightning protection. Different methodologies have been proposed to represent the physical phenomena taking place at a single full discharge event, departing both from experimental and theoretical approaches. The implementation of these methodologies in numerical routines combined with the use of numerical methods to determine the electric potential distribution permits the creation of models whose predictions closely agree with the real case situations, where electrode arrangements might have nonsymmetric geometries. In this paper, we present an improved version of a simulation methodology for representing electrical discharges in long air gaps. This simulation methodology includes new elements like: 1) the 3-D leader channel tortuosity based on laboratory experimental measurements and 2) two new methods for the estimation of the electric charge contained in the so-called leader-corona region based on the electrostatic potential of fictitious potential rings representing the active region in front of the leader tip. Results from the simulation were compared with experimental records and a reasonably good agreement is found between them.

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.

A physical model based on the mechanism observed in experimental investigations is introduced to describe the formation of negative leader steps. Starting with a small length of a space leader located at the periphery of the negative streamer system of the stepped leader, the model simulates the growth and the subsequent formation of the leader step. Based on the model, the step length, the step forming time, and the propagation speed of stepped leaders as a function of the prospective return stroke peak current are estimated. The results show that the step length and the leader speed increase with increasing prospective return stroke current. The results also show that the speed of the stepped leader increases as it approaches the ground. For prospective return stroke currents in the range of 15 kA–60 kA, the step lengths lie within the range 5 m–100 m, the step forming times lie within the range 10 μs–250 μs, and the leader speed lies within the range 10⁵ m/s −1.5 × 10⁶ m/s. The results obtained are in reasonable agreement with the experimental observations.

Narrow Bipolar Pulses are generated by bursts of electrical activity in the cloud and these are referred to as Compact Intracloud Discharges (CID) or Narrow Bipolar Events in the current literature. These discharges usually occur in isolation without much electrical activity before or after the event, but sometimes they are observed to initiate lightning flashes. In this paper, we have studied the features of CIDs assuming that they consist of streamer bursts without any conducting channels. A typical CID may contain about 10⁹ streamer heads during the time of its maximum growth. A CID consists of a current front of several nanosecond duration that travels forward with the speed of the streamers. The amplitude of this current front increases initially during the streamer growth and decays subsequently as the streamer burst continues to propagate. Depending on the conductivity of the streamer channels, there could be a low-level current flow behind this current front which transports negative charge towards the streamer origin. The features of the current associated with the CID are very different from those of the radiation field that it generates. The duration of the radiation field of a CID is about 10–20 μs, whereas the duration of the propagating current pulse associated with the CID is no more than a few nanoseconds in duration. The peak current of a CID is the result of a multitude of small currents associated with a large number of streamers and, if all the forward moving streamer heads are located on a single horizontal plane, the cumulative current that radiates at its peak value could be about 10⁸ A. On the other hand, the current associated with an individual streamer is no more than a few hundreds of mA. However, if the location of the forward moving streamer heads are spread in a vertical direction, the peak current can be reduced considerably. Moreover, this large current is spread over an area of several tens to several hundreds of square meters. The study shows that the streamer model of the CID could explain the fine structure of the radiation fields present both in the electric field and electric field time derivative.