锋生及其次级环流对北京2012.7.21最大降水增幅和最大降水的影响

THE ROLE OF FRONTOGENESIS AND SECONDARY CIRCULATION IN MAXIMUM PRECIPITATION ENHANCEMENT AND MAXIMUM PRECIPITATION IN A 21 JULY 2012 RAIN IN BEIJING

利用常规观测、加密站逐时的降水、NCEP/NCAR再分析资料和卫星资料,使用天气动力学诊断方法,分别分析了锋生及其次级环流对北京7.21暴雨过程中最大降水增幅和最大降水的影响。结果表明,北京地区的降水增幅和最大降水发生时刻并不一致。21日14时为北京最大降水增幅时刻,而次大降水增幅时刻的19时却为北京降水最大时刻。北京降水不论是增幅最大还是降水最大都与锋生处于北京的具体位置有关。21日14时,伴随着锋生函数正值区伸展到北京地区,其总锋生函数开始增大,此时高空急流导致的次级环流的上升支与冷锋前的上升支重合,使得地面锋前形成一深厚的上升运动,北京地区出现最大降水增幅;但在北京降水最大时刻,锋生函数大值中心移动到北京上空,其总锋生函数达到最大,在急流-锋系所产生的次级环流中,主要表现为在暖区一侧有强烈的上升运动,这和经典的急流-锋系所揭示的次级环流并不相同。同时,来自孟加拉湾从高原西侧经过河套地区到达北京的准"s"型异常水汽通道,则为北京7.21最大降水增幅时刻提供了良好的水汽条件。而北京最大降水时刻,南海水汽通道成为主要水汽来源,来自孟加拉湾的水汽输送则明显减弱。

Based on the data of conventional observation,surface hourly precipitation,the NCEP reanalysis data and satellite cloud images and using weather diagnosis and analysis methods,the role of frontogenesis and its secondary circulation are studied in maximum precipitation enhancement and maximum precipitation in a rain that took place on 21 July 2012.The results show that the precipitation enhancement was not consistent with maximum precipitation in occurrence in the Beijing area.Maximum precipitation enhancement occurred at 14：00 （Beijing Time,BT） 21 July,and the second largest precipitation enhancement was recorded at 19：00 BT 21 July,in the Beijing area.Whether there is biggest precipitation enhancement or maximum precipitation is related to the location of Beijing area relative to the frontogenesis.At 14：00 BT 21 July,accompanied by a positive frontogenetical function stretching to the Beijing area,the total frontogenesis function began to increase and the ascending branch of the secondary circulation caused by an upper-level jet stream overlapped with the ascending branch of a cold front,resulting in strong ascending motion in front of the surface front and the occurrence of maximum precipitation enhancement in the area.At this moment of biggest precipitation,the high-value center of frontogenesis function arrived in Beijing,making the total frontogenesis function there achieve the maximum.The secondary circulation caused by jet-front systems is mainly manifested as a strong upward movement in the warm zone,which differs from a typical secondary circulation.At the same time,an abnormal Quasi-＂s＂ channel of moisture is transported from the Bay of Bengal on the west side of the plateau through the Great Bend of the Yellow River to reach Beijing,providing a good supply of water vapor for enhancement of maximum precipitation on July 21 in the region.At the time of maximum precipitation,the water vapor channels over the South China Sea become main sources of water vapor while the water vapor transported from the Bay of Bengal to the Beijing area decreases significantly.