Characteristics of the Environmental Conditions for the Occurrence of Recent Extreme Rainfall Events in Northern Kyushu, Japan

Abstract

Using mesoscale gridded analysis data and radar/raingauge-analyzed precipitation products, this study investigated the environmental conditions for the occurrence of extremely heavy rain events that occurred in northern Kyushu during the warm season in recent years. In all cases, the environmental conditions were not particularly unstable but were almost saturated in the deep layer of the troposphere. The existence of moist absolutely unstable layers (MAULs) was confirmed in and around the rainfall areas. A positive correlation was found between the volume of deep MAULs and the area total rainfall. A large convergence of the water vapor flux is observed in the lower troposphere, which appears to maintain very humid conditions and MAULs. The moisture convergence in the lower troposphere and the presence of deep MAULs is considered to enhance the heavy rainfall in the preceding hours. Humid conditions form deep MAULs that appear to precede the heavy rainfalls.