Assessing The Urbanization Effects By Applying An Urban Canopy Model And Coupling It With Weather Research And Forecasting

The urbanization causes one of the most significant land use and land cover changes（LUCC）. Especially over the eastern China from Beijing to Shanghai, the greaturbanization occurs during the past half century. It modifies the physicalcharacteristics of land surface, including land surface albedo, surface roughnesslength and aerodynamic resistance and thermodynamic conduction over land. All ofthese play very important role in regional climate change.To tests how the LUCC caused by urbanization affects the regional climate in EastAsia, we built a Modified Single Layer Urban Canopy Model （MSLUCM） andcoupled it with the WRF. In MSLUCM, the energy balances on the five surfaceconditions are considered separately: building roof, sun side building wall, shade sidebuilding wall, pervious land surface and impervious road. MSLUCM is built based onCommunity Land surface Model urban parameterization （CLMU） and have madefurther improvements. Over each surface condition, a method to calculate sky viewfactor （SVF） is developed based on the physically process and got the analyticsolution while most other urban models simply provide an empirical value for SVF.Our approach along with other improvement in short and long wave radiation transfer,for example the multiple scattering and reflection, improves the accuracy of long-wave and shortwave radiation processing over urban surface. The force-restoreapproximation is employed to calculate the temperature of each outer surfaces ofbuilding. The inner side temperature is used as the restore term and was assigned as atuning constant. Based on the nature of the force-restore method and our tests, wedecide to employ the air mean temperature of last72hours as a restore term, whichsubstantially improve the surface energy balance. The surface turbulence calculationis based on Monin-Obukhov similarity theory.We evaluate the ability of the newly coupled model by two experiments: Experiment1is set up to integrate the coupling model from March through September in eachyear during2000–2010, which covers the East Asia Summer Monsoon season. Thesimulating domain covers the Great Beijing Area and the Great Shanghai Area, twomainly urbanized regions in China. The Experiment2is a high-resolution simulatingover the Great Beijing Area. We ran the coupled model from2012-07-19₀0:00to2012-07-22₀0:00, during which the strongest storm of the last60years in Beijinghappened. The highest resolution is1.5km.We designed three run cases for both Experiment1and2: in the control run （CASE1）,the land use map from United States Geography Survey （USGS）, which represent theland use and land cover information in1996, was used. WRFV3.4-SSiB3wereapplied without calling the SLUCM in the control run （CASE1）; An urbanized landuse map, which presents the land use information in2005were used in the CASE2,still without the MSLUCM called; both the2005land use map and MSLUCM wereapplied to WRF model in CASE3. The preliminary results show that the LUCCcaused by the urbanization significantly changes the water and energy fluxes over theland surfaces and then the regional climate. The applying of the MSLUCM makes theperformances of the simulating urban heat island （UHI） effect more reasonable. Byapplying both urbanized land use map and urban canopy model, the existence of theUHIs enhances the convection in lower atmosphere and affects the water vaportransportation of the surrounding area, both of which affect the regional climate,especially the local distribution of precipitation. Our results show that the urbanization makes opposite effects on the precipitation in different scales: theprecipitation caused by East Asia Monsoon were significantly decreases over theurban area, as shown in the climatic scale ten-years averaged results in Experiment1.But in the Experiment2, for the existence and the enhancing of the urban heat island,the structure of the meso-scale convection system is affected, and the precipitationsurround the urban region is redistributed, too. The increasing of the instability inlower atmospheric layer over the urban makes the precipitation even stronger overmost parts of the Great Beijing Urban Area and its upstream zone.