A study on the heavy-rain-producing mesoscale convective system associated with diurnal variation of radiation and topography in the eastern slope of the western Sichuan plateau

Abstract

On 24–25 July 2010, a Plateau Vortex system forming to the north of Tibetan Plateau dramatically changed its moving direction to westward after several days of eastward movement. Observational analysis showed that, during its westward movement, a low-level southeasterly or easterly wind developed over the Sichuan basin. The large-scale forcing became favorable for the convection development. The low-level warm advection was more favorable for convection development than the differential vorticity advection. The daytime scattered convections were organized into a mesoscale convective system (MCS) after sunset, which produced extremely heavy rainfall in the eastern slope of the Western Sichuan Plateau. The observational evidences and numerical simulations have indicated that the topographically induced dynamical lifting over the lower topography and the convergence caused by the topographical blocking provided strong support for the convection initiation. The cold outflows caused by surface evaporative cooling of rain steered the MCS to move away from its original place, while the convergence between the cold outflows and the environmental southeasterly flow apparently helped the maintenance of the MCS. The intensification of the low-level flow, which was associated with the diurnal variation of radiative forcing, contributed to the organization and intensification of the MCS. The results of sensitivity experiments further confirmed the impact of topography in the convection initiation, and the influences of cold outflows caused by surface evaporative cooling of rain on the movement and maintenance of the MCS. The effects of the diurnal variation of radiative forcing on the organization of the MCS are also well represented in the model results.