Classifying Moist Unstable States for the Occurrence of Precipitation During the Summer Season in Japan

Using mesoscale gridded analysis data and radar/raingauge-analysed precipitation products, this study statistically investigated the environmental conditions at the time of precipitation occurrences during the summer season in Japan, from June to August, over the 18 years from 2006 to 2023. We chose five regions with higher precipitation amounts than the surroundings based on the averaged three-month accumulated precipitation and examined various environmental parameters relevant to the stability and moisture conditions, including the characteristics of the moist absolutely unstable layers (MAUL). There was a high correlation between areas with heavy precipitation and deep MAULs, but little agreement between precipitation and other environmental stability indices. A close relationship was found between precipitation and the occurrence of MAULs. Among the environmental parameters examined in this study, the three-dimensional volume of MAUL demonstrated the highest correlation coefficient with the occurrence of precipitation. Although the relationship between the MAUL volume and the precipitation amount indicated a certain magnitude of correlation coefficients, the relationship includes a certain degree of scatter. Through investigating the differences in the relationships between the environmental parameters and the precipitation amount by dividing the cases depending on the volume size of MAUL and the amount of precipitation, the analysis showed that heavy rainfall in Japan during the summertime can be classified into two types based on environmental properties. One type is regarded as “MAUL type” which occurs when MAULs are greatly enhanced in highly humid states throughout the troposphere. The second type is regarded as “CAPE type” which occurs when CAPE and temperature lapse rate become large and hence a convectively unstable state develops. It is considered that mechanisms for the development of heavy rainfalls depend on the tropospheric humidity (which will make the layer moist absolutely unstable) and the convective instability (which will make CAPE larger).