Analysis on Very High Frequency Radiation and Development Characteristics of a Negative Cloud-to-Ground Lightning Discharge with Multiple Return Strokes

A negative cloud-to-ground lightning flash with multiple return strokes was observed using a short-baseline time-of-arrival lighting VHF (Very High Frequency) radiation location system. Combining the synchronous data of electric field change, the discharge processes of the preliminary breakdown, stepped leaders, dart leaders, and discharges after the last return stroke inside the cloud are analyzed. The results suggest that the preliminary breakdown depicts bi-directional propagations, with the magnitude of the average speeds of two concurrent channels of 10⁴ m s^{-1 in opposite directions. The stepped leaders developed from the downwards channels of preliminary breakdown and split into two downward channels, only one off which contacted the ground. The propagation speeds of the two leader channels were about 105 m s-1}. Dart leaders appeared to initiate the return stroke along the first return stroke discharge channel, at a speed of about 10⁵-10⁶ m s^-1. They were accompanied by several branches developing to the ground simultaneously at a speed of about 10⁵ m s^-1. The discharge processes were notably complex and intensive. VHF radiation was produced during the intermission between return strokes and after the last return stroke. During the discharge between the return strokes and the last return stroke, the radiation started from the source region of preliminary breakdown and extended the discharge channels in cloud. After the last return stroke, several K processes (Kitagawa and Kobayashi, 1958) were found to be caused by the negative streamers propagating backwards along the positive electrified channel. The propagation speeds of the K process were about 10⁶ m s^-1.