Diagnosing Environmental Properties of the July 2018 Heavy Rainfall Event in Japan

The environmental properties of precipitation systems that produced the July 2018 Heavy Rainfall event in Japan were demonstrated. The gridded analysis data of Japan Meteorological Agency's Mesoscale Model were used to diagnose the potential for the development of convective systems in terms of thermodynamic environmental indices. Precipitable water vapor was extremely larger than that seen in the climatology of warm-season quasi-stationary convective clusters (QSCCs, Unuma and Takemi, 2016a). Such an extreme moisture content was realized by very humid conditions at the middle-level troposphere. In contrast, the temperature lapse rate in a convectively unstable layer was not so significant in comparison to the QSCC climatology. Among the environmental indices, K Index was shown to describe the potential for the rainfall development. Based on the analysis, the roles of moisture content and profile on the convection development were discussed. It was suggested that the middle-level high humidity contributes to the occurrence of the present heavy rainfall. In regions where heavy rainfall occurred, an automated algorithm demonstrated the development of QSCCs, which is mostly categorized as a linear shape. This result is consistent with previous findings on Unuma and Takemi (2016b).