一次低涡暴雨过程发生机制及其模式预报分析

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

为了揭示低涡暴雨发生机制,认识高分辨率的区域数值模式对低涡暴雨类天气的预报能力,应用多种观测资料,NCEP再分析资料和区域数值模式WRF和GRAPES预报资料,分析了2012年7月20—23日西南地区一次高原涡和西南涡带来的大暴雨过程。研究结果表明:(1)高原低涡与西南低涡耦合有利于低涡发展维持,中层的正涡度平流、低层的辐合上升运动是低涡发展的重要机制。低涡强烈发展时期,对流系统发展极为强盛,-64℃云盖呈圆形。(2)对流层中低层低涡的维持和发展,使四川盆地处于辐合上升环流控制中,提供了有利于降水的动、热力条件,是盆地强降水发生的重要机制。(3)两个模式都较好地反映了低涡影响下的盆地大暴雨过程。与实况的差异主要表现在降水发生时间提前,降水落区移动偏快或偏慢,有利于降水的动、热力场更强等等。相对而言,WRF模式预报与实况更接近。模式预报的低涡位置及伴随的物理量演变决定了降水预报的差异。

Using data of observations, re-analysis and forecasts from WRF and GRAPES regional models, a vortex-rainstorm occurred during 20 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 mechanism for the vortices development. During the period when the vortices develops violently, severe convective systems have developed and cloud cover with top colder than-64 ℃ has the characteristics of rotundity.(2) The maintenance and development of the vortices at lower-middle troposphere make the Sichuan Basin in the control of convergence and ascending movement, and provide favorable dynamics and thermodynamics conditions to precipitation occurrence, which is the important mechanism of the basin heavy rainfall occurrence.(3) The two models have given reasonable vortex-rainstorm predictions. The main differences between the model's forecasts and the observations are that the time of precipitation occurrence forecasted by the models is ahead, the movements of the precipitation area are either slower or faster, and dynamics 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 have made the differences in their precipitation forecasts.