A Survey of Energetic Compact Strokes in Japan

Energetic compact strokes (ECSs) are a special type of cloud‐to‐ground (CG) lightning stroke that were found to be consistently associated with downward terrestrial gamma‐ray flashes (TGFs). In this paper, we made a survey of ECSs in a 500‐km‐radius region in Japan using three years of data from the Fast Antenna Lightning Mapping Array (FALMA), a low‐frequency lightning location system. We found that the vast majority of ECSs occur in winter thunderstorms, especially in November and December, along the west coast of Japan. Compared with normal CG strokes, ECSs in winter are more likely to cluster in mountain regions with elevations of a few hundred meters. The tendency of ECSs to cluster in certain regions provides clear guidance for future campaigns to look for downward TGFs, but it also implies the increased possibility of health hazards posed by downward TGFs in these regions. Some ECSs also occur in spring and mostly at high mountain tops. ECSs have a clear tendency to be temporally isolated from other lightning discharges and rarely occur in thunderstorms with high flash rates. Based on these characteristics, we discuss the favorable conditions for the production of ECSs. A comparison with superbolts observed by the World Wide Lightning Location Network (WWLLN) shows that out of 10 superbolts observed near Japan, nine are negative CG strokes having similar FALMA waveforms as ECSs. It is possible that a significant fraction of WWLLN superbolts are also ECSs, and we suggest that ECSs can also be commonly found outside of Japan.