In Japan, heavy rainfall events often occur in various regions during the Baiu season. Such heavy rainfall events are caused by the large amount of water vapor in the lower atmosphere and unstable atmospheric conditions. Recently, however, the importance of the water vapor content in the middle atmosphere has been highlighted.

In response, the Moist Absolutely Unstable Layer (MAUL) has been studied, and the relationship between the inflow of water vapor from the lower to the middle atmosphere and heavy rainfall has been investigated. The MAUL is a state of conditional instability under saturated atmospheric conditions. Previous studies have shown temporal and spatial correspondences and correlations between MAUL areas and heavy rainfall events during the Baiu season, suggesting that MAUL areas contribute to the humidification of the lower and middle atmosphere prior to the onset of heavy rainfall events. However, previous studies have not quantitatively discussed the influence of volume of MAUL areas on precipitation amount, and the specific mechanism of its contribution to heavy rainfall has not been clarified. Understanding the MAUL that precede heavy rainfall events is very important from the perspective of disaster prevention and mitigation. The purpose of this study is to clarify the mechanism of the contribution of MAUL areas to heavy rainfall and the quantitative relationship between heavy rainfall and volume of MAUL areas for the case of significant heavy rainfall that occurred near the Kuma River in Kyushu region on 3-4 July 2020.

In this study, we use the reanalysis data obtained by assimilating the ground-based observational data using the three-dimensional variational method to the cloud-resolving non-hydrostatic model CReSS. The outputs are obtained at 10-minute intervals over a 6-hour period from 0:00 on 4 July 2020 to 6:00 on 4 July 2020.

The location of the rainfall area around 5:00 on 4 July, when heavy rainfall was observed, indicates the existence of MAUL areas extending from the south to the southwest of the rainfall area. The correspondence between the maul region and the convergence area was examined, and a linear convergence area existed just south of the MAUL areas. It is suggested that the MAUL was formed in the region between the convergence area and the Baiu front as a result of the inflow of water vapor from the southwest by strong winds. The vertical cross section of the MAUL areas shows that cloud water is present in the inner part of the MAUL areas and no rainfall is observed. In addition, the vertically integrated cloud water flux near and southwest of the precipitation area was high. These results suggest that cloud water passing through the interior of the MAUL areas flows into the rainfall area on the leeward side and contributes to heavy rainfall by collision-coalescence process with rain particles in the convective clouds generated in the Baiu frontal zone. Next, the relationship between the MAUL volume ratio and precipitation amounts is examined by extracting a limited amount of MAUL areas that flow into the rainfall area by setting a fan-shaped decision region. The results show that there is a correspondence between the increase in precipitation and the volume of MAUL areas flowing into the precipitation area from 60 and 90 minutes before, and the increase in the volume of MAUL areas flowing into the precipitation area from 120 minutes before, when the central angles of the fan-shaped decision region are 30 and 60 degrees. The time series of MAUL volume ratios and rainfall rates show that the MAUL areas flowing into the rainfall area from 90 minutes before the rainfall precede the rainfall.