Preliminary analysis on synoptic configuration evolvement and mechanism of a torrential rain occurring in Beijing on 21 July 2012

The synoptic configuration features and contributing factors of a torrential rain occurring in Beijing region on 21 July 2012 have been preliminarily investigated by using conventional observation data, intensive automatic weather station observations, Doppler-Radar data etc. and low-level thermal and dynamical retrieval system of 4Dvar based on Variational Doppler Radar Analysis System (VDRAS). The conclusions are drawn as follows. (1) There are two stages during the torrential rain. The first one is from 10 a.m. to 20 p.m. (LTC) on 21 July 2012 and presents convective precipitation with short-time, high rainfall intensity and obvious fluctuation. The second one is from 20 p.m. on 21 July to 04 a.m. on 22 July 2012 and presents smooth frontal precipitation. (2) The synoptic scale circulation situations on 21 July are typical circulation pattern of North China heavy rain. The divergence caused by high-level jet, low-vortex and shear in the lower and middle troposphere, and deep convergence zone triggered by surface inverted trough cause the formation and continuance of strong vertical upward motion,which lead to 16 h severe precipitation process. The southeast moisture channel at the periphery of subtropical high and the southwesterly low-level jet supply sufficient moisture condition for the occurrence of torrential rain. (3) The precipitation echoes in the first stage of the process have an obvious train effect whose initiate convection originates from both mesoscale convergences in warm regions caused by topographic forcing effect and wind velocity fluctuation in the enhancing process of low-level jet. The mesoscale convective cells moving along the left side of low-level jet axle have a significant gravity wave transmitting feature. The stable continuance of southwesterly jet made inertia-gravi-tational wave obtain energy from ambient field and develop continuously. (4) Topography promotes precipitation increment. The southeast
wind in boundary layer joins southwest wind or northeast wind in front of the Taihang Mountain and forms a continuing convergence zone in front of the mountains, which is in favor of the accumulation of vapor, meanwhile, the southeast airflow is uplifted at the windward slope of themountains, which leads to the increase of precipitation in the regions in front of the mountains.