Contrasting features of the July 2018 heavy rainfall event and the 2017 Northern Kyushu rainfall event

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, and to the front side of an upper tropospheric trough and to the south of a westerly jet. However, contrasting precipitation properties and environment are observed between the two cases. In the 2018 case, a long-lasting intense precipitation was observed in a wider area with moderately tall precipitation systems. Environment was much moister and relatively more stable compared with the climatology. A deep trough over the Korean Peninsula played a role to provide the environment favorable for organizing precipitation systems through a quasi-geostrophic dynamical forcing. In contrast, in the 2017 case, a short-term extreme precipitation was observed in a narrower area with extremely tall precipitation systems. The environment was moister and relatively more unstable compared with the climatology, but was dryer compared with the 2018 case. A shallow trough over the Korean Peninsula played a role to destabilize atmosphere with upper cold air associated with the trough.

A contrast of characteristics between the 2018 and 2017 cases is similar to that between the composites of extreme rainfall events and extreme convection events shown by the statistical study of Hamada and Takayabu (2018, JC). Amplitudes of temperature anomaly and specific humidity anomaly from climatological values in the 2018 and 2017 cases are several times as large as those in the composites of the extreme events although Hamada and Takayabu (2018) analyzes 99.9 percentile extreme events. This result indicates that the 2018 case is an extreme among the extreme rainfall events and the 2017 case corresponds to an example of the extreme convection events.

Acknowledgments: This study is supported by the University of Tokyo through a project "Research hub for the big data analysis of global water cycle and precipitation in changing climate", the JSPS KAKENHI Grant (15H02132, 18K19951), the JAXA PMM science, and the Environment Research and Technology Development Fund (2-1904) of the Environmental Restoration and Conservation Agency of Japan.