Numerical simulation of the 1981 Sichuan flood. Part I: evolution of a mesoscale southwest vortex

Abstract

In this paper we present the results of numerical experiments aimed at 1) testing the capability of a limited-area mesoscale model to predict the evolution of the SW vortex and the accompanying heavy precipitation, 2) examining the structure of the simulated vortex using the model data, and 3) elucidating the role of various physical processes in the evolution of the SW vortex. Principal findings are: 1) the control experiment, which utilized an 80 km grid spacing and simple physical parameterizations, was able to simulate the evolution of the mesoscale SW vortex and the accompanying heavy precipitation. 2) Latent heat release was essential for the development of the SW vortex and the resulting precipitation. 3) Surface sensible and latent heat fluxes were important to the precipitation forecast. 4) Further sensitivity experiments showed that the SW vortex observed in this case was a terrain-induced standing eddy. 5) The differential frictional effect, hypothesized over the past decade to be a plausible mechanism for the formation of the SW vortex, was shown to be unimportant in this case. 6) A trajectory diagnosis and a model experiment with modified topography showed that as the southwesterly monsoon current impinged upon the mesoscale Yun-Gui Plateau, which extends from the southeastern corner of the main Tibetan Plateau, the low-level flow was blocked. The flow aloft then descended into the Sichuan Basin on the lee side of the mesoscale plateau, creating cyclonic relative vorticity over the basin by stretching of earth's background vorticity. -from Authors