Sensitivity of Quasi-Stationary Band-Shaped Precipitation System to Topography: A Case Study for 28 August 2008 Okazaki Heavy Rainfall Event

Abstract

 The Okazaki heavy rainfall event, which occurred at midnight on 28 August 2008 around Okazaki city, Japan, was produced by a quasi-stationary, band-shaped precipitation system. It remained quasi-stationary for approximately five hours over Okazaki city and the surrounding area, producing prolonged, heavy precipitation. This study presents sensitivity numerical experiments to examine the surrounding mountainous topography's effect on the precipitation system using the Weather Research and Forecasting (WRF) model with 500 m horizontal resolution. In an experiment without the mountains to the east of Okazaki city, the quasi-stationary precipitation system was not reproduced. On the other hand, experiments including the eastern mountains produced a low-level convergence south of Okazaki city, resulting in a quasi-stationary precipitation system with prolonged precipitation, as observed near Okazaki city. The convergence was formed by sustained easterly and northerly winds blowing in western Okazaki city. The easterlies were maintained by a westward shift of southeasterly inflow from the Pacific Ocean due to the enhanced pressure gradient on the upstream side of the eastern mountains in the low-level atmosphere with low Froude numbers (Fr < 0.5). The easterlies also steadily supplied warm and moist air to the quasi-stationary system, leading to prolonged heavy precipitation.