Contrasting Features of the July 2018 Heavy Rainfall Event and the 2017 Northern Kyushu Rainfall Event in Japan

Abstract

Precipitation characteristics and their environments are compared between two heavy rainfall events affecting the Northern Kyushu area in Japan: the July 2018 heavy rainfall event (2018 case) and the 2017 Northern Kyushu rainfall event (2017 case). Both events occurred in the later stage of the Baiu season, after the passage of a tropical cyclone, to the south of a subtropical jet, and on the front side of an upper tropospheric trough. However, contrasting precipitation properties and environments are observed between these cases. In the 2018 case, long-lasting, heavy precipitation was observed over a large area with moderately tall precipitation systems. The environment was stable and moist compared to the climatology. A deep trough over the Korean Peninsula prepared a favorable environment for organizing precipitation systems through moistening the mid-troposphere by quasi-geostrophic dynamically forced ascent. In contrast, in the 2017 case, short-term, intense precipitation was observed over a small area with exceptionally tall precipitation systems. The environment was unstable and moist, compared with the climatology, but was dryer than it was in the 2018 case. In the 2017 case, a shallow trough over the Korean Peninsula destabilized the atmosphere via associated high-altitude cold air.

The observed contrast of characteristics between the 2018 and 2017 cases was similar to that found between the composites of extreme rainfall events and extremely tall convection events, included in the previous statistical study by Hamada and Takayabu (2018, doi:10.1175/JCLI-D-17-0632.1). Although the previous study analyzed the uppermost 0.1 % of extreme events, temperature anomalies and specific humidity anomalies from climatological values in the 2018 and 2017 cases are several times larger than those in the composites of the extreme events. This result indicates that the 2018 case was an extreme among the extreme rainfall events, and the 2017 case was an extreme one among the extremely tall convection events.