The Environment and Precipitation Characteristics of Widespread Extreme Rainfall Events during the Akisame Season

This study investigates the precipitation characteristics and disturbances of widespread heavy rainfall during the Akisame using meteorological data from the Japanese 55-year Reanalysis, precipitation data from the Automated Meteorological Data Acquisition System (AMeDAS), the Global Satellite Mapping of Precipitation (GSMaP) and the Global Precipitation Measurement (GPM) data. First, in the climatological field during the Akisame, northward water vapor transports (WVTs) prevail over eastern Japan (EJPN) due to the eastward shift of the North Pacific subtropical high and the existence of an anticyclone over the continent. This situation differs from the Baiu. Under humid conditions during the Akisame, well-organized precipitation systems with large stratiform precipitation area accompanying strong convective precipitation bring heavy rainfall, as in the Baiu. Second, the analysis of widespread extreme precipitation events (WEPEs) conducted for the Baiu in WJPN (Baiu/WJPN) by Shibuya et al. (2021) is performed for 4 cases: Baiu/EJPN, Baiu/WJPN, Akisame/EJPN and Akisame/WJPN. In the composite of the Akisame/EJPN case, WVTs toward EJPN is enhanced by a northeast-southwest dipole structure of geopotential height anomaly. We newly revealed that WEPEs occur during the Akisame associated with the overlap of the climatological field with anomalous disturbances that are different from those in the Baiu.