The analyses of occurrence mechanism and its numerical forecasts of a vortex rainstorm

Using data of observations, re-analysis and forecasts from WRF and GRAPES regional models, a vortex-rainstorm occurred during20 to 23 July 2012 has been examined in seeking to understand the rainstorm’s occurrence mechanism and the capabilities of the models.The results show the following. (1) The coupling of plateau vortex and southwest vortex is in favor of vortices’s development and maintenance.Positive vortex advection at middle troposphere, convergence and ascending movements at lower troposphere are important mechanismfor the vortices development. During the period when the vortices develops violently, severe convective systems have developed and cloudcover with top colder than -64 ℃ has the characteristics of rotundity. (2) The maintenance and development of the vortices at lower-middletroposphere make the Sichuan Basin in the control of convergence and ascending movement, and provide favorable dynamics and thermodynamicsconditions to precipitation occurrence, which is the important mechanism of the basin heavy rainfall occurrence. (3) The two modelshave given reasonable vortex-rainstorm predictions. The main differences between the model’s forecasts and the observations are that thetime of precipitation occurrence forecasted by the models is ahead, the movements of the precipitation area are either slower or faster, anddynamics and thermodynamics fields are more in favor of precipitation occurrence, compared to actual observations. Between the two models,the prediction of WRF is better than that of GRAPES. The place of the vortices and the evolution of their associated physical elements havemade the differences in their precipitation forecasts.