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

抄録

 Precipitation characteristics and environment are compared between two rainfall events 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, south of a subtropical jet and to 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. Environment was stable and moist compared with the climatology. A deep trough over the Korean Peninsula played a role to prepare the environment favorable for organizing precipitation systems through moistening of mid-troposphere by quasi-geostrophic dynamically forced ascent. In contrast, in the 2017 case, a 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 the 2018 case. In this 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 is like that found between 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). Temperature anomalies and specific humidity anomalies from climatological values in the 2018 and 2017 cases are several times as large as those in the composites of the extreme events although the previous study analyzed the uppermost 0.1 % of extreme events. This result means that the 2018 case is an extreme among the extreme rainfall events and the 2017 case corresponds to an extreme event of the extremely tall convection events.