Leader observations during the initial breakdown stage of a lightning flash

Abstract

This paper describes luminosity and leader propagation during the initial breakdown (IB) stage of a cloud-to-ground flash, beginning at 6.06 km altitude and 31.86 ms before the return stroke (RS). High-speed video (50,000 frames per s) and time-correlated electric field change (E-change) data show multiple branch ends advance concurrently in the first 6 ms of the flash; each branch begins with IB pulses and propagates first via bursts as an initial leader. Burst luminosity (pixel intensity) is directly related to IB pulse amplitude. Some initial leader branches transition to advancing as stepped leaders after a few milliseconds. Each initial leader branch end makes the transition to a stepped leader branch end at a different time, resulting in a complex E-change waveform including relatively narrow step-type pulses during the IB stage and no apparent Intermediate stage prior to the Leader stage. There is no visible evidence of an upward propagating leader end prior to the RS nor of any light above the earliest visible IB luminosity prior to or during the RS. During the RS, the topmost visible portion of the channel that developed as an initial leader (above 5.1 km) behaves differently from the channel below, indicating it is less conductive. Radar and time-of-arrival lightning source data indicate that the IB luminosity visible to the camera comes from about 6 km inside the thundercloud echo. The 1276 m long initial leader transitions to a stepped leader at 4.9–5.0 km, near the altitude of the radar bright band.