Characteristics of Atmospheric Environments of Quasi-Stationary Convective Bands in Kyushu, Japan during the July 2020 Heavy Rainfall Event

This study investigated characteristics of atmospheric environmental fields in the occurrence of quasi-stationary convective bands (QSCBs) in Kyushu, western Japan during the July 2020 heavy rainfall event. We performed case studies of extreme rainfall subevents in the Kumamoto and Kagoshima prefectures on 3-4 July (2020KK) and northern Kyushu on 6-7 July 2020 (2020NK), compared with two heavy rainfall events in northern Kyushu in 2017 and 2018. Nine QSCBs were objectively extracted during the July 2020 heavy rainfall event, causing hourly precipitation amounts exceeding 100 mm twenty times. In 2020KK, the environmental field with extremely large precipitable water due to low-level and middle-level humidity was affected by the upper-level cold airflow, which resulted in favorable condition for the deep convection development. Consequently, the lightning activity became high, and cloud tops were the highest in comparison to previous events. QSCBs in 2020KK and 2020NK were located along a low-level convergence line/zone associated with an inflow that had extremely large water vapor flux on the south side of the mesoscale Baiu frontal depressions. In most of the QSCB cases in 2020, mesoscale depressions were observed and enhanced horizontal winds, which led to extremely large low-level water vapor flux to produce short-term heavy rainfall.

Automated summary

This study investigated the characteristics of atmospheric environmental fields during the occurrence of quasi-stationary convective bands (QSCBs) in Kyushu, Western Japan in July 2020 heavy rainfall event. Nine QSCBs were identified, causing hourly precipitation exceeding 100mm twenty times. QSCBs in 2020KK had extremely large precipitable water due to low-level and middle-level humidity, affected by upper-level cold airflow, leading to high lightning activity and the highest cloud tops compared to previous events.