2004年7月冷空气活动及其对西南地区强降水的影响

COLD AIR ACTIVITIES IN JULY 2004 AND ITS IMPACTS ON INTENSE RAINFALLS OVER SOUTHWEST CHINA

利用中国逐日站点降水资料和NCEP/NCAR再分析资料,分析了2004年盛夏(7月)中国西南地区的强降水过程及冷空气活动在降水过程中的作用。结果显示:西南地区强降水与冷空气的活动关系密切,强降水的发生是中高纬环流变化和低纬系统共同作用的结果。首先,中纬度的低槽冷涡与登陆北上台风和东移西南涡合并,以及东亚地区横槽向竖槽的迅速转变,造成了7月上旬和中旬中高纬环流的3次大调整,东亚地区长波迅速增幅,从而导致了盛夏罕见的大范围冷空气向低纬地区的侵袭。与此同时,暖湿的西南夏季风十分活跃,二者在中国南方地区产生强烈相互作用,结果在冷空气南侧形成大范围的对流性降水。其中,冷空气活动对降水的影响主要表现在3个方面:其一,中高层冷空气的迅速南下常伴有偏南高空急流,位于急流中心左前方的次级环流上升支为降水的形成提供了有利的动力抬升条件。其二,冷空气向南推进极大地增强了大气的斜压性,有利于斜压扰动的发展。冷空气迅速南下造成对流有效位能释放,从而形成大范围的对流活动。另外,中高层干冷空气的平流也是暴雨区位势不稳定得以重建并产生持续性强降水的重要原因。

Cold air activities and its impacts on the heavy rainfall events in the mid-summer of July 2004 over Southwest China are investigated by using the daily observed precipitation data from China Meteorological Administration （CMA） and the NCEP/NCAR reanalysis data. The analysis results show that the intense precipitation in Southwest China was closely related to the activities of cold air from middle-high latitudes, and resulted from the cooperative interaction of middle-high and low latitude circulation synoptic systems. The mergence of a mid-latitude cold trough with a northward landfall typhoon and an eastward-moving southwest vortex, and the rapidly veering of an E - W orientation trough over East Asia induced the substantial adjustments of three times of general circulations and the rapid developments of long waves in the first and second dekads of July 2004, which in tern led to the southward intrusion of cold air in a large scale. And meanwhile, the warm, humid southwest summer monsoon flow was aim very active, the both met over the southern China, producing a vast area of the convective precipitation south of the cold air. In the convective precipitation area, the cold air spread southward to as far as 15°N. With regard to the impacts of activities of cold air, it can be summarized in the followings： firstly, the rapid southward incursion of upper level cold air is often accompanied by the southerly upper level jet. The ascending branch of the corresponding secondary circulation on the left front side of the jet core, provides favorite dynamically upward lifting. Secondly, the southward advance of cold air greatly enhances the baroclinicity, thus in favor of the development of baroclinic disturbances. The developed disturbances associated with the intrusion of cold air are able to destroy the potential instability stratification, release the convective available potential energy （CAPE）, and finally induce the convective activities in a large scale area. In addition, the advection processes of dry and cold air at the middle and upper levels aloft the advection of humid and warm air at the lower level is crucial for the reestablishment and maintenance of potential instability, and persistent precipitation in the precipitation area.