北京“7.21”特大暴雨环流形势极端性客观分析

Objective analysis of the extreme of circulation patterns during the 21 July 2012 torrential rain event in Beijing

中文关键词:  暴雨  环流分型  低空急流  风垂直切变  可降水量

英文关键词:Torrential rain  Circulation classification  Low level jet  Vertical wind shear  Precipitable water

中文摘要:

      2012年7月21日(简称“7.21”),北京发生了自1951年以来最强的暴雨事件。利用倾斜旋转T模态主成分分析法和美国国家环境预报中心/美国国家大气研究中心再分析资料,探讨了北京“7.21”特大暴雨的大尺度环流形势的极端性。结果表明,北京“7.21”暴雨日所属的大尺度环流型在1951—2012年夏季出现的频率为10.9%,而在“7.21”同类环流型中暴雨出现的概率为4.51%。和同类暴雨日平均场相比,“7.21”暴雨日当天西太平洋副热带高压西伸更强,北京地区对流层低空急流更强,并伴随环境大气中极端充沛的可降水量和较大的风垂直切变。在“7.21”同类环流型下的621 d中,“7.21”暴雨日北京南侧的低空急流排在第54位,北京局地风垂直切变排在第209位,可降水量排在第8位,显示出其在低空急流和可降水量上的极端性。1951—2012年夏季,具有“7.21”暴雨日同类环流形势、且925 hPa低空急流和可降水量均达到或超过“7.21”暴雨日值的个例有3次,相当于每21年发生一次。

英文摘要:

      A torrential rain being the heaviest since 1951 hit Beijing on 21 July 2012 (named "7.21" for short). By using the obliquely rotated T-mode principle component analysis (PCA) and the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Data, the extremes of large-scale circulation patterns during "7.21" are explored. The results suggest that the frequency of the "7.21" type during summers of 1951-2012 is 10.9%, while the frequency of the torrential rain under the "7.21" type is 4.51%. Compared to other rainstorms under the "7.21" type over the study period, the "7.21" case is characterized by more west-stretching subtropical high over the western North Pacific, stronger low level jet in the lower troposphere over Beijing, more ambient precipitable water and stronger vertical wind shear. Among the 621 days under the "7.21" type over the study period, the "7.21" case ranks the 54th place in term of low level jet to the south of Beijing, 209th place in term of the local vertical wind shear over Beijing and 8th place for the local precipitable water over Beijing. The "7.21" case shows its extreme in low level jet and precipitable water. During summers of 1951-2012, cases under "7.21" type bearing equal or greater values with respect to low level jet on 925 hPa level and precipitable water than those of "7.21" case occur three times, namely, such a case occurs once every 21 years.