Analysis on the cause of a torrential rain occurring in Beijing on 21 July 2012(Ⅱ):
vertical motion, wind vertical shear and terrain effect

This paper is the second part of“Analysis on the cause of a torrential rain occurring in Beijing on 21 July 2012”. The vertical motion,wind vertical shear and terrain effect of this torrential rain have been investigated by using conventional observation data, intensive automatic weather station observations, Doppler-Radar data and 1°×1° NCEP reanalysis data. The conclusions are as follows. 1) The activities ofthe divergence in high-level jet, the wind shear line in low-level, the upward of the vertical circulation near low-level-jet, the convergenceline of terrain and the terrain forcing jointly acted to trigger the intense upward movement, thus activated and maintained the convective precipitation.The eastward movement of vortex and the frontal system further enhanced the vertical motion and the frontal precipitation. 2) Thestrong wind vertical shear was a unique environmental feature of the“7·21”event. 3) Factors, including the south-west-jet at mid-level, the southwest-northeast direction convergence line at surface level, the cloud clusters moving to the upward motion region, moist flux convergence region and the potential instability region, have resulted in a series of convective cloud clusters emerging, moving along the jet direction,
which acted as a“train effect”, and lead to torrential rain weather. 4) The terrain played an important role in producing torrential rain.The windward slope, flared terrain and mesoscale convergence line make the mountain area in west-south Beijing become a region prone to form and enhance cloud clusters.