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

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.