Characteristics of hailfall and lightning in a splitting thunderstorm observed on May 4, 2019 in the Tokyo Metropolitan Area, Japan

According to newspapers, hailfall of up to 3–4 cm in diameter was observed in the afternoon of May 4, 2019 in Tokyo, Japan. The thunderstorm cell split into two during the hailfall period, with one propagating northward with hailfall and the other propagating southward with only rainfall. For these two cells, we investigated the number of cloud-to-ground (CG) strokes identified by the lightning detection network, the temporal changes in the ice volume derived from the X-band multi-parameter radar data, and atmospheric electric fields. The results showed that the ice volume of the cell with hailfall (hailfall cell) was greater than that of the cell without hailfall (rainfall cell), whereas the number of CG strokes in the hailfall cell was smaller than that in the rainfall cell. In addition, –CG strokes were more dominant in the rainfall cell than in the hailfall cell, while +CG strokes were more dominant in the hailfall cell than in the rainfall cell. This implies that there was a large loss in negatively charged hail in the hailfall cell due to gravity, which led to an increase in net positive charge in the hailfall cell, causing +CG strokes.