Numerical Simulation Study On The Influence Of Urban Underlying Surface On Precipitation Process In Beijing-tianjin-hebei

In this paper, The Weather Research and Forecasting( WRF) model coupled with a single-layer urban canopy model(UCM) is used to simulate numerically the urbanization effects with high resolution from 18:00 July 14 to 00:00 July 20, 2015 over Beijing-Tianjin-Hebei region with height resolution. Three experiments were designed:(1)expand urban land cover determined by MODIS satellite observations(Big City),urban land cover determined by MODIS satellite observations(CTL) and without urban areas( NO_City) .By comparing the three experiments, the impacts of urbanization on daily precipitation are analysed ,and the possible mechanism is also invertigated.The simulation results show that the WRF model can well reproduce the precipitation from 18:00 July 14 to 00:00 July 20, 2015 over Beijing-Tianjin-Hebei region .The simulated spatial distribution and intensity of precipitation agree well with the observation. The surface temperature and 2 m temperature of each test shows that the surface temperature and the temperature of the 2 m will rise along with the expansion of city underlying surface. Surface energy transfer to the atmosphere with temperature raised by radiation . Radiation heating, easy to cause the upward movement of air is triggered local time of the cause of precipitation. Upward movement will also cause unstable stratification of central city trend abate, this may be caused by the precipitation process is one of the reasons for the city of short duration rainfall. 2 m moisture difference reflects more the higher level of urbanization, the more the urban dry island . Average wind field and divergence field analysis found that both in convergence zone,urban agglomeration region showed that urban expansion for the water vapor convergence is a positive correlation, 850 hpa and 700 hpa water vapor mixing ratio flow convergence, the influence of water vapor mixing ratio is increased.Compared with the diurnal variation characteristics of the meteorological elements in the Beijing-Tianjin-Hebei urban agglomeration, it's found that they show strongest in the afternoon. However, during the period time between 2000UTC to 0800UTC, the change of the underlying surface of the city has the greatest influence on the meteorological elements. It indicates that there exists the strong exchange of material energy between the underlying surface and lower atmosphere during the 1200UTC to 0000UTC. And physical processes are active in the boundary layer of the atmosphere,which may cause the precipitation at night.