Characteristics of bolt-from-the-blue lightning discharge produced by a thunderstorm in Japan on August 26, 2018

A thunderstorm that occurred in the Kanto area in Japan, on August 26, 2018, was confirmed to accompany bolt-from-the-blue (BFB) lightning by photographic analysis. To examine the characteristics of BFB lightning flashes caused by this thunderstorm, occurrence frequency, horizontal discharge distance, and direction of discharge progress, quantitative analysis was conducted using lightning mapping array data and extended radar information network (XRAIN) precipitation data for the first time in Japan. This thunderstorm produced 18 BFB flashes. The maximum and average horizontal discharge distance from the edge of the precipitation area were 3,652 m and 1,431 m, respectively. The distance from the edge of the precipitation area and the discharge inception altitude of the BFB flashes seemed to be longer and higher than normal cloud-to-ground (CG) lightning flashes. In particular, the direction of the BFB discharge progress showed a characteristic distribution concentrated on the south side of the thunderstorm and right side of its movement direction. The surface wind around this area flowed from the south side into the thunderstorm. According to XRAIN precipitation data, BFB flashes occur around heavy rain regions (convective region) inside the thunderstorm. The BFB-producing thunderstorm in this study was extremely dangerous because BFB flashes occurred in the opposite direction of the thunderstorm’s movement direction.