Numerical investigation of a localized extremely heavy rainfall event in complex topographic area during midsummer

Numerical simulations with version 3.2 of the Weather Research and Forecasting (WRF) Model are performed to study a localized extremely heavy rainfall event during midsummer in central China. The event occurred in a complex topographical area on 3 July 2008. The extremely heavy rainfall was produced by a quasi-stationary back-building mesoscale convective system (MCS), which was initiated and developed in the exit region of a low-level jet (LLJ). The main mesoscale dynamical process responsible for the MCS formation was the low-level convergence directly generated by the LLJ. The gravity waves excited by the unbalanced dynamics due to the LLJʹs abrupt intensification might be another dynamical factor for the MCS initiation. The LLJ was of obvious diurnal variation, which was nearly in an opposite phase with the variations of the planetary boundary layer (PBL) height and of the surface heat and moisture fluxes. The diurnal variation of the LLJ was mainly dominated by the solar-radiation-driven evolution of the PBL height and the surface heat fluxes below the LLJ. The dynamical uplifts forced by the Wufeng mountainous area and the Wushan Mountain were favorable for the formation and development of MCS. The topographic Froude number was less than one at night due to the increase of atmospheric stability in the lower level. The LLJ was blocked mainly by the Dabashan Mountain, and partly by the Wufeng mountainous area and the Wushan Mountain, leading to the MCS in the exit region of the LLJ to remain quasi-stationary and produce the localized extremely heavy rainfall.