Preliminary Study on Mechanism of b Mmesoscale Rainstorm Forming in Beijing City

Chen Yun^a  Li Qiang^b ChenTao^a Li Zechun^a  Bi Baogui^a  He lifu^a

^a National Meteorological Center, CMA , Beijing, China, 100081

^b Xining Institute of Health Inspection and Supervision, Xining, Qinghai Province, China, 810001

Abstract

The mechanism of b mesoscale rainstorm forming in Beijing City was studied based on integrated analysis of conventional and unconventional observation data of Beijing City and numerical experiment with non-static equilibrium mesocale simulation system (ARPS). The results show that: (1) b mesoscale rainstorm development would be strongly dependant on surrounding topographty and easily formed ahead of the hill with high topographic grade. (2)The "warm boot" of mesoscale model was carried out by analytical field of mesoscale model based on ADAS and assimilating many data such as reflectivity and radial velocity of WSR-88D Radar, cloud top temperature on infrared channel of FY-2C and so on in a way. (3) It is closely related to atmospheric circulation pattern. During the active period of Westerly Trough, warm rainstorm may be significantly developed when Beijing city is controlled by the warm and wet currents ahead of the trough. It can be also favored from the easterly currents in low level with plenteous water vapor and instability energy in the troposphere in the north of typhoon in the meantime. (4) The topography boundary was corresponding with the echo belt of topographic convergence. Convergence area of mesoscale wind field was found at the centre of rainstorm. Mesocyclone would have important impacts on rainstorm forming. (5) The dynamic condition of rainstorm forming could be provided by current lifting at the convergence center where the surface southeastern winds from plain area meet ahead of the hill near city. The convergence ascending ahead of hill may be strengthened by the vertical shear between southwestern wind in the high level and northeastern wind in the low level, resulting in maintaining and enhancing of mesoscale convective system. (6)Rainstorm would be led to by mesoscale convective system resulting from the front of mesocale temperature gradient  in boundary layer around city and themodynamic and dynamic instability induced by city greenhouse effect. (7) Meso-scale rainstorm jointly induced by topographty and urban thermal dynamics normally occurs at dusk or dawn.

Keyword: Beijing City; b Mesoscale Rainstorm Around City; Surrounding Topographty , Urban Thermal Dynamics