Numerical Simulation of Stepping and Branching Processes in Negative Lightning Leaders

Abstract

A numerical model with physical timing and grid spacing of 3 m is applied to studying the progression (including stepping and branching) of negative lightning stepped leader. The asymmetry between positive and negative streamers is taken into account via using polarity‐dependent initiation and propagation field thresholds. The stepped nature of negative leader is confirmed to be caused by this asymmetry. The step formation process of the negative leader is modeled to begin with the appearance of space stems inside and in the immediate vicinity of its streamer zone (corona streamer burst completing the preceding step). Some of those space stems evolve into space leaders, which can connect to the primary leader channel, thereby facilitating its extension. Model‐predicted morphology and dynamics of negative leaders are in good agreement with the recent recordings of lightning stepped and dart‐stepped leaders obtained using high‐speed video cameras, and their electrical parameters are in line with the current knowledge on negative lightning leaders.

On a Possible Mechanism of Reactivation of Decayed Branches of Negative Stepped Leaders

“…Upward waves associated with individual leader steps were observed to propagate at a mean speed of 6.7 × 10 7 m/s. Similar results were obtained via modeling by Syssoev et al (2020, Figures 7 and 8). (b) Schematic representation of the downward leader stepping process; adapted from Nag and Rakov (2009, Figure 1).…”

“…Everything in the above description is similar to the return stroke, except for the channel being not grounded and the magnitude of positive charge injected at the tip being small compared to the negative charge stored in the channel. The negative leader processes, including upward positive charge waves, are reproduced in the model of Syssoev et al (2020, Figures 7 and 8).…”

“…Everything in the above description is similar to the return stroke, except for the channel being not grounded and the magnitude of positive charge injected at the tip being small compared to the negative charge stored in the channel. The negative leader processes, including upward positive charge waves, are reproduced in the model of Syssoev et al (2020, Figures 7 and 8). Wang et al (1999) observed, for a dart-stepped leader, upward luminosity waves propagating over tens of meters to 260 m at speeds ranging from 1.9 × 10 7 to 1.0 × 10 8 m/s with a mean of 6.7 × 10 7 m/s (see Figure 8).…”

“…They are “still there” only because of residual space charge and elevated air temperature. Mechanism of negative leader branch decay is discussed, based on modeling, by Syssoev et al (2020, p. 16), who have offered the following scenario. Once a negative leader branch starts decaying, it is usually chocked by the space charge of its own corona sheath.…”

Evidence and Inferred Mechanism of Collisions of Downward Stepped‐Leader Branches in Negative Lightning

“…Everything in the above description is similar to the return stroke, except for (a) the channel being not grounded and (b) the magnitude of positive charge injected into the hot channel core being small compared to the negative charge previously stored in the channel corona sheath. The negative leader stepping process, including upward positive-charge waves, is reproduced in the numerical model of Syssoev et al (2020; see their Figures 7 and 8). In the case of return stroke, compensation of leader charge stored in the corona sheath is accomplished via the so-called reverse corona, which is described by Gorin (1985) and Rakov (2006, 2009).…”