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This study investigated the environmental factors responsible for the development of heavy rainfall in eastern Japan during the passage of Typhoon Hagibis (2019) by using mesoscale gridded analysis data as well as observed data. Environmental indices for diagnosing stability and moisture conditions were examined. It was found that the whole troposphere is almost saturated and the column total water vapor content is extremely large. In the lower troposphere we identified layers of moist absolutely unstable states with the thickness deeper than 2 km. Such deep moist absolutely unstable layers as well as abundant moisture content and almost saturated troposphere set a high potential for convective development. Under these favorable environmental conditions, the fact that the heights of the absolutely unstable layers' bottom are comparable to the mountain elevations is considered to be favorable for topographic lifting of unstable, moist air, which will trigger and activate strong convection and hence heavy rainfall. In spite of a moderate amount of convective available potential energy and a nearly moist-adiabatic lapse rate, moist absolute instability, abundant moisture, and high humidity jointly play a key role to increase the potential for generating the present heavy rainfalls., 2019年台風19号による豪雨の発生メカニズムを解明 --湿度100%で絶対不安定な大気層の役割--. 京都大学プレスリリース. 2020-02-28.

This study documented the environmental properties of precipitation systems that produced the July 2018 Heavy Rainfall event in Japan. The gridded analysis data 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). Such an extreme moisture content was realized by very humid conditions at the middle-levels. In contrast, 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 by minimizing negative effects of environmental mixing and by decreasing vertical displacements to reach levels of free convection. In regions where heavy rainfall occurred, an automated algorithm detected the development of QSCCs, which were mostly categorized as a linear type., 平成30年7月豪雨時の積乱雲群の発生機構を解明 --上空の高湿度の条件で積乱雲が広域で持続的に発達--. 京都大学プレスリリース. 2019-09-24.

The characteristics and environmental properties of warm-season quasi-stationary convective clusters (QSCCs) in Japan were statistically investigated using operational weather radar and upper-air sounding data from May to October during 2005-2012. The characteristics of the environmental conditions for the development of QSCCs were described through a comparison with those for no-rain cases. We identified 4133 QSCCs over the Japanese major islands. By compiling numerous QSCC samples, the horizontal scales of QSCCs with a circular shape are about 20 km on average with a maximum of 72 km, indicating that warm-season QSCCs in Japan are meso-β-scale phenomena. The analyses of the environmental conditions for the QSCC and no-rain cases showed that the amount of moisture in the lower layer controls the stability condition for the development of the QSCCs, and that the magnitudes of the wind shear and the helicity in the lower troposphere distinguish the kinematic environments for the development of the QSCCs. An increase in the middle-level moisture leads to a larger amount of precipitable water vapour in the QSCC environments, suggesting that atmospheric moistening before the development stage of convection plays an important role in the development of the QSCCs. Additionally, the precipitation intensity has a higher correlation with the convective instability, whereas the precipitation area correlates with the shear intensity. A comparison between slower- and faster-moving CCs indicated that the precipitation intensity of the slower-moving CCs is stronger. This feature is related to a higher convective instability for the slower-moving ones.

A role of environmental shear on the organization mode of warm-season quasi-stationary convective clusters (QSCCs) was statistically investigated by using operational weather radar and radiosonde data from May to October during 2005-2012. With the use of an ellipse-fitting method, the total number of QSCCs whose shape was determined was 2549. It was found that 87% of the QSCCs have an aspect ratio of larger than 1.4, suggesting that the elongated mode is dominant during the warm season in Japan. The elongated QSCCs were mostly oriented southwest–northeast. The analyses of the environmental shear direction with respect to the orientation of the elongated QSCCs showed that the wind shear direction at the lower troposphere is mainly parallel to the orientation of the elongated QSCCs. A comparison between the elongated and the circular QSCCs with the environmental parameters showed that the lower convective instability and stronger intensity of the low-level shear clearly characterize the elongated QSCC environments. A parameter combining convective instability and shear, bulk Richardson number, characterizes the environmental conditions for determining the organization mode of the QSCCs, suggesting that a back-building mechanism should play a role in generating the elongated QSCCs.