Lightning initiation processes imaged with very high frequency broadband interferometry

Abstract

Recent measurements of narrow bipolar lightning events (NBEs) by very high frequency (VHF) radio interferometer have resolved the dynamic development of this special lightning process with submicrosecond time resolution, and showed that the fast positive breakdown (FPB) process is responsible for initiating at least some lightning flashes. In this study, with a newly built and deployed VHF interferometer system, we analyzed 31 intracloud lightning flash initiation events during three thunderstorms at short range from the interferometer. These events separate into two distinct classes that can be identified based on the time scale and the occurrence contexts of the first detectable VHF emissions from the flash. One class has features completely consistent with previously reported FPB events and is associated with continuous VHF emissions of 10–20 μs duration. Downward motion of the FPB region centroid merged continuously into the development of the subsequent upward negative leaders. But the majority of the lightning flashes analyzed began with ultrashort, submicrosecond duration, isolated pulses of VHF emission with no identifiable FPB signatures between these pulses and the leader development. These short VHF pulses begin typically a few hundred microseconds before the upward leader developes and are located at the same position where the leader eventually begins. We suggest that the FPB process is responsible for initiating some but not all lightning flashes, and the extremely short pulse-like VHF emissions play a role in initiating those flashes without any FPB process.