A numerical study of the rainstorm characteristics of the June 2005 flash flood with WRF/GSI data assimilation system over south-east China

The evolution and structure of rainstorms associated with a flash-flood event are simulated by the Advanced Weather Research and Forecasting (WRF-ARW) model of the National Center for Atmospheric Research and the Gridpoint Statistical Interpolation (GSI) data assimilation (DA) system of the National Oceanic and Atmospheric Administration (NOAA) of the United States. The event is based on a flash flood that occurred in the central Guangdong Province of south-east China during 20-21 June 2005. Compared to an hourly mixed rain-gauge and satellite-retrieved precipitation data, the model shows the capability to reproduce the intensity and location of rainfall; however, the simulation depends on three conditions to a large extent: model resolution, physical processes schemes and initial condition. In this case, the Eta Ferrier microphysics scheme and the initialization with satellite radiance DA with a fine 4-km grid spacing nested grid and coarse 12-km grid spacing outer grid are the best options. The model-predicted rain rates, however, are slightly overestimated, and the activities of the storms do not precisely correspond with those observed, although peak values are obtained. Abundant moisture brought by the southwesterly winds with a mesoscale low-level jet from the South China Sea or Bay of Bengal and trapped within the XingfengJiang region encompassed by northern Jiulian, southern Lianhua and eastern small mountains are apparently the primary elements responsible for the flood event. All simulated rainstorms were initiated over the southern slopes of the Jiulian Mountain and moved south or north-eastward within the Xingfengjiang region. Meanwhile, the Skew-T/Log-P diagrams show that there is a fairly high convective available potential energy (CAPE) over the active areas of the rainstorms. The higher CAPE provides a beneficial thermodynamic condition for the development of rainstorms, but the higher convective inhibition near the northern, eastern and southern mountains prohibits the storms from moving out of the region and causes heavy rainfall that is trapped within the area. Copyright (C) 2010 John Wiley & Sons, Ltd.