Compact Lightning Strokes in Winter Thunderstorms
Ting Wu, Daohong Wang, Nobuyuki Takagi
Abstract
Abstract With the observation of a 14‐site Fast Antenna Lightning Mapping Array in the Hokuriku region of Japan, we identified a special type of lightning stroke called “compact stroke” in winter thunderstorms. A compact stroke starts with a fast downward negative leader followed by a “compact return stroke.” Compact strokes have the following characteristics that are not seen in normal lightning strokes: (a) Durations of preceding discharges (defined as the time difference between the return stroke and the start of the flash) are generally smaller than 200 μ s. (b) Most compact strokes are inferred to be produced in small gaps between mountain tops and negative charge regions in winter thunderstorms. (c) Channel lengths of compact strokes are estimated to be shorter than 600 m with an average of about 300 m. (d) Characteristics of electric field change waveforms of compact return strokes are closely related with durations of preceding discharges. (e) Peak currents of compact return strokes increase with increasing channel lengths. It seems that elevations of mountain surfaces where compact strokes occur are comparable to the altitude of the lower positive charge region, so we speculate that the mountain surface and the near‐surface space carry positive charges when a storm is overhead and are virtually the lower positive charge region of a normal tripole structure. As a result, compact strokes are confined between the main negative and lower positive charge regions, and small pulses with short durations preceding compact return strokes are produced by downward negative leaders corresponding to the preliminary breakdown stage.