Characteristics Analysis of the Senjo-Kousuitai Conditions in the Kyushu Region in Early July: The Case of the July 2020 Heavy Rainfall Event

Abstract

A sequence of heavy rainfall events due to quasi-stationary band-shaped precipitation systems, or “senjo-kousuitai“, was observed in the Kyushu region, Japan, from 3 to 8 July 2020. In this study, we investigate two of six indices that have previously been used to determine conditions favorable for senjo-kousuitai, i.e., water vapor flux at the height of 500 m and storm-relative environmental helicity. We examine the relationship between these indices and the occurrence of senjo-kousuitai over the past 20 years using the Japan Meteorological Agency 55-year reanalysis data. We show that the anomaly in wind speeds rather than humidity contributes more to anomalous water vapor flux. The vertical shear of zonal winds and the meridional flow in the lower layer contribute more to the storm-relative environmental helicity. We conducted 20-member ensemble experiments with a 14 km mesh Nonhydrostatic Icosahedral Atmospheric Model (NICAM) for the senjo-kousuitai event. We found that the initial variabilities of the water vapor over the area stretching from the East China Sea to the South China Sea and the wind fields over the western periphery of the North Pacific High are sensitive to the water vapor flux over the senjo-kousuitai area in Kyushu.