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: water vapor flux at 500 m height and helicity relative to the storm. We examine the relationship between these indices and the occurrence of senjo-kousuitai over the past 20 years. 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 helicity. We conducted 20-member ensemble experiments with a 14 km mesh Nonhydrostatic Icosahedral Atmospheric Model (NICAM) for the senjo-kousuitai event with the initial condition at 0000 UTC on 3 July before the heavy rainfall 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 subtropical high are sensitive to the water vapor flux over the senjo-kousuitai area in Kyushu.