夏季500hPa副热带高压区域一次暴雨过程环流条件的诊断分析

DIAGNOSIS OF THE CIRCULATION FOR A HEAVY RAIN EVENT IN A 500 hPa SUMMER SUBTROPICAL HIGH REGION

利用NCEP/NCAR再分析资料和地面观测资料对2003年7月26～27日500hPa副热带高压区域一次暴雨过程的形成原因进行了分析。结果表明:中低层来自东北方向的冷空气入侵是此次暴雨发生的重要原因,其作用体现在三个方面:一是致使对流层中低层降温,边界层增温,使得降水区积累了大量的不稳定能量;二是造成边界层风的辐合,进而激发了整层的上升运动;三是这股冷空气为暴雨发生提供了重要水汽源。暴雨发生前南亚高压主体突然东进,高空急流轴不断东伸加强,其出口端不断向南伸展,使得对流层中层副高主体西移,同时造成日本附近的冷涡南压,为冷空气从中低层侵入暴雨发生区提供了有利的环流条件。

In the context of NCEP/NCAR reanalysis and observations, study is undertaken of the causes of a heavy rain formation over the subtropical high center from July 26 to 27, 2003. Results show that cold air from the northeast at the mid-lower levels invading the high's central region acts as an important factor, which functions in the following ways. First, the cold air leads to temperature drop in the middle to lower troposphere, with temperature rise in the boundary layer, thus causing unstable energy to be accumulated in large amounts over the fallout area. Second, it makes the boundary-layer wind converge, which thus excites ascending motion throughout the extent. Finally, this cold air provides rich supplies of water vapor for the rainstorm. Besides, the main body of the South Asia high moves east suddenly before the rainstorm begins and the axis of upper westerly jet keeps moving eastward and strengthening, with its exit region expanding towards the south, thereby shifting westward the main part of the central region of the high in the middle troposphere. In the meanwhile, the invading cold air is responsible for southward displacement of a cold vortex around Japan, creating a favorable condition for cold air to enter the rain zone in mid and lower levels.