2012年初秋四川盆地两次西南涡暴雨过程的对比分析与预报检验

利用常规观测和雷达、卫星等观测资料,从天气形势配置、对流活动特征以及预报检验等方面对2012年初秋发生在四川盆地的两次西南涡暴雨过程进行了对比分析研究,重点对直接造成暖区对流性暴雨的MCS活动特征和环境条件进行了分析.两次暴雨过程在天气系统配置上,都具有西太平洋副热带高压强盛稳定、高原槽活跃,以及低层有西南涡生成等共同特点；但由于副高脊线的位置差异、西风带短波扰动强度差异等原因,造成系统移动速度以及降水落区的显著区别.在这两次暴雨过程中MCS都较为活跃,第一次降水过程中的MCS较为稳定,系统移动相对缓慢；第二次过程中出现了MCC,局部单点雨强突出,移动速度也相对较快.分析表明第一次过程湿层相对深厚,对形成强降水非常有利,而第二次过程中垂直切变相对较强,高层存在明显干层,从而形成了更强的层结不稳定性,有利于强对流风暴的形成.两次暴雨过程具有明显的暖区对流性质,不稳定性、LLJ急流轴位置在判断初始对流启动位置上占有更加重要的地位.针对这两次过程的预报检验表明,数值模式在反映暖区对流上存在明显缺陷,而预报员在建立精细化的中尺度天气概念模型指导下,主观预报能够在一定程度对数值预报做出订正,提高对暖区对流性降雨的预报能力.

Analysis and Forecast Verification of Two Southwest Vortex Torrential Rain Events in Sichuan Basin in Early Autumn of 2012

Using conventional observations and data from satellite and Doppler weather radar, etc., two extremely heavy rain events that happened in Sichuan Basin in the early autumn of 2012 are compared and analyzed regarding synoptical conditions, convective activities and forecast verification. The emphasis is put on the MCS activities and ambient conditions which induced the convective torrential rains in warm sector directly. The two rain events shared some features in common such as the strong and stable West Pacific subtropical high, the active plateau trough, and the genesis of southwest vortex in the low level in Sichuan Basin, but due to the different positions of the subtropical high ridge line and the different short wave disturbing intensity of westerlies, remarkable differences of the moving speed of southwest vortex and rainfall locations were induced. The MCS in the first rain event was stable and moved relatively slowly, but during the second rain event a MCC emerged with highly intensive precipitation, moving fast. The analysis result shows that there was a deep moist layer in the first rain process which was very favorable for the heavy precipitation, but a notable dry layer in the mid high level during the second rain event which created much stronger instability, good for the forming of severe convective storms. In a word, the two rainstorm processes have clear convections in warm sector and instability, and the position of Low Level Jet is more important for deciding the initiation of the convection. Finally, the forecast verification indicates that, the numerical model has obvious bias in presenting of convection in warm sector, but by the guidance of detailed mesoscale synoptical conceptual model, some correction can be made by subjective forecasting, improving the predictability of convection in the warm sector.