Environmental Properties of Heavy Rainfall Events under Extremely Humid Conditions during the Rainy Season (Baiu) in Japan

During the rainy season in Japan (i.e., Baiu season), the environmental conditions are generally very humid, with a large amount of precipitable water vapor and a deep tropospheric layer of high relative humidity. Stationary precipitating systems sometimes develop during the rainy season, which will spawn an extreme amount of rainfall, flooding, landslides, and so on. Recent studies have investigated climatological characteristics of stationary precipitating systems over the Japanese islands by using long-term datasets. During the warm season including the rainy season, a humid condition not only at low levels but also at middle levels was commonly seen. Under such a humid condition, moist absolutely unstable layers (MAULs) were sometimes identified in cases of heavy rainfalls. This study investigates the environmental conditions for the generation of extreme-rain-producing, stationary precipitating systems that occurred in northern Kyushu Island, Japan in recent years. The examined cases are those in July 2017, July 2018, July 2020, and August 2021, all of which spawned extreme rainfalls exceeding 500 mm within a few days and caused floodings, landslides, and damages to houses and social infrastructures. It is indicated that precipitable water vapor content is extremely large as compared with the climatology of stationary precipitating systems, which is due to very humid conditions close to be saturated in the deep troposphere. Compared with the moisture content, the temperature lapse rates are around the climatological values for stationary precipitating systems in Japan, leading to low-to-moderate values of CAPE. Under such humid situations, MAULs appear in the lower to middle troposphere, adjacent to or as a part of stationary precipitating systems. It is found that the three-dimensional volume of MAUL regions has a correlation with the amount of heavy rainfalls in later hours, suggesting that MAULs can be regarded as a precursor of the heavy rainfall occurrence. The development of MAULs is supported by a large amount of moisture transport in the layer of 1000-700 hPa along a Baiu stationary front. On the other hand, a moist absolutely unstable condition is a state that indicates a potential for the development of convective motion. Therefore, a certain triggering mechanism is required. In the case of the July 2017 heavy rainfall case in northern Kyushu, small-scale terrain induces a convergence locally, which serves as a triggering in a moist absolutely unstable condition.