Analysis on the cause of a torrential rain occurring in Beijing on 21 July 2012 (Ⅰ)：
Weather characteristics, stratification and water vapor conditions

The weather characteristics and environment conditions of a torrential rain occurred in Beijing on 21 July 2012 have been investigated by using conventional observation data, intensive automatic weather station observations and 1°×1°NCEP reanalysis data. The conclusions are as follows. 1) The rain event consists of convective and frontal precipitation. The convective precipitation, which has longer rainfall time and higher rainfall intensity, was the main rainfall process during the torrential rain. 2) The primary atmospheric circulation characteristics are a east-moving vortex in mid-high level, a north-moving subtropical high and a sharp vorticity wind shear. The rainfall occurred on the low-level sharp wind shear and under the high-level divergence. 3) There were two warm-moist layers in low-level before the heavy rain started. The establishment of the potential instability was mainly related to the wetter low-level and the drier mid-high-level, which was resulted by the change of the wind direction between low and high level. 4) This torrential rain occurred and developed in the whole troposphere layer, and the mid-high level was wetter than low-level. Unlike normal rainfall, the features of deep moist layer, the low condensation level and low free convection level were evident in this case.