Abstract
Time-correlated high-speed video and electric field change data for 139 natural, negative cloud-to-ground (CG)-lightning flashes reveal 615 return strokes (RSs) and 29 upward-illumination (UI)-type strokes. Among 121 multi-stroke flashes, 56% visibly connected to more than one ground location for either a RS or UI-type stroke. The number of separate ground-stroke connection locations per CG flash averaged 1.74, with maximum 6. This study examines the 88 subsequent strokes that involved a subsequent stepped leader (SSL), either reaching ground or intercepting a former leader to ground, in 61 flashes. Two basic modes by which these SSLs begin are described and are termed dart-then-stepped leaders herein. One inception mode occurs when a dart leader deflects from the prior main channel and begins propagating as a stepped leader to ground. In these ‘divert’ mode cases, the relevant interstroke time from the prior RS in the channel to the SSL inception from that path is long, ranging from 105 to 204 ms in four visible cases. The alternative mode of SSL inception occurs when a dart leader reaches the end of a prior unsuccessful branch—of an earlier competing dart leader, stepped leader, or initial leader—then begins advancing as a stepped leader toward ground. In this more common ‘branch’ mode (85% of visible cases), there may be no portion of the subsequent RS channel that is shared with a prior RS channel. These two inception modes, and variations among them, can occur in different subsequent strokes of the same flash.
Highlights
Negative cloud-to-ground (CG) lightning often exhibits multiple ground connections for a single flash or for a single stroke
The previously recognized mode of subsequent stepped leader (SSL) inception (Rakov and Uman 2003) occurs when a dart leader diverts from a prior return stroke channel, the leader continues to advance as a stepped leader to ground
In the set of 40 cases in which the SSL inception is either visible or can be deduced from the channel overlap and leader type at various points, we find six in which the SSL diverts from a dart leader, as it travels along a prior return strokes (RSs)
Summary
Negative cloud-to-ground (CG) lightning often exhibits multiple ground connections for a single flash or for a single stroke. In the full set of 488 subsequent RSs used for this study (including these 88), the average ISI is 87.1 ms with standard deviation of 81.58 ms (which is not very different from those for the subset of 88 SSL strokes), the median value is 65 ms, and 30–40 ms is most common (Fig. 1c). In these 488 subsequent strokes, 30% (145) have ISI > 96 ms and 7% (35) have ISI > 192 ms, quite similar to the values in Rakov and Uman (1990)
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