Competing and cutoff leaders before 'upward illumination'-type lightning ground strokes

摘要

High-speed video data provide evidence to examine a hypothesis regarding the physical mechanism resulting in the very weak "upward illumination" (UI) strokes occurring within a few milliseconds after a normal return stroke (RS) of a negative lightning flash. These short-duration (visible < 1 ms) strokes form a new ground connection, without apparent connection to the main RS, over their relatively short (< 3 km) visible upward return path. In a data set of 170 video flashes acquired in 2011 at 50,000 frames per second, there are 20 clear UI examples in 18 flashes at 2.5-32.3 km range from the camera. Average separation values are 1.25 ms and 1.9 km between ground connections of the UI stroke and main RS. In 15 cases, the data show a distinct stepped leader or branch to the UI which develops concurrently with the stepped leader to the main RS. The estimated altitude of the descending UI leader tip just before the main RS occurs ranges from 50 to 610 m, and in seven cases, stepping is visible in the UI leader after the main RS. In most examples, the RS and UI appear as separate channels for their entire visible portion, but in five cases, there is an obvious junction indicating the UI leader is a cutoff branch from the main leader. A generalized schematic of the seven main luminosity stages in a typical UI is described, accompanied by examples of each stage, to support the existence of competing and cutoff leader branches. Key Points High-speed video data show evidence of cut-off gaps in stepped leaders Stepping is visible at the lower end of cut-off leaders after the return stroke Competing, disconnected leader branches result in short UI-type strokes