The initial stage processes of rocket‐and‐wire triggered lightning as observed by VHF interferometry

Abstract

We examine VHF interferometric images, channel‐base currents, and broadband electric field waveforms of the initial stage (IS) in two rocket‐and‐wire triggered lightning flashes. Two types of negative leaders, termed “long‐duration” and “short‐duration” leaders, were imaged by the VHF interferometers during the IS of the two flashes. There were three leaders that had relatively long durations of more than a few milliseconds. These three leaders were not accompanied by a significant change of channel‐base current during their early stages of development, indicating that they corresponded to intracloud (IC) discharges that were not connected to the grounded triggered‐lightning channel. Two of these three leaders eventually connected to the triggered‐lightning channel and initiated initial continuous current (ICC) pulses. The third long‐duration leader apparently developed from the vicinity of an isolated negative charge region toward an upper‐level positive charge region and toward a branch of the grounded channel; it served to bridge the positive charge region and the triggered‐lightning channel, resulting in the opposite polarity portion of the bipolar ICC. The short‐duration negative leaders had durations of some hundreds of microseconds. These negative leaders apparently recoiled along the conductive channels created by branches of the upward positive leader (UPL); they initiated ICC pulses when the grounded channel was sufficiently conductive. It follows that ICC pulses can be initiated either by recoil leaders or via interception of separate in‐cloud leaders by a grounded current‐carrying channel.