Mechanism For The Impact Of Urbanization In City Cluster Regions Of Eastern China On Summertime Weather And Climate

Ease China has been experiencing rapid economic growth and intensive urbanization since the 1980s.It has become one of regions with the largest adjacent metropolitan areas in the world.The urban clusters in China have resulted in significant modifications in the underlying surface properties and the increases in pollutant emissions and hence aerosol loading in the atmosphere,which has brought tremendous environmental pressure to the economic and social development.Located in the most active East Asian monsoon region,East China is known as one of the places in the world that always suffer from the devastatingly synoptic disasters.Recent years have witnessed numerous climate extremes such as heavy rainfall events,which have brought tremendous material and property losses to the city.In this paper,numerical simulations were conducted by using the WRF,WRF-Chem model coupled with a single-layer Urban Canopy Model(UCM)to investigate urbanization-induced changes in land cover and pollutants emission on weather and climatic characteristics of summer precipitation.The conclusions are as follow:(1)The WRF model that couples with a single-layer UCM is used for the ensemble simulation on a summertime heary rainfall event in the Great Beijing Metropolitan Area(GBMA)in 2008.Generally,the model can well reproduce the large-scale circulation in this heavy rainfall event,as well as some important meteorological parameters such as temperature,moisture and wind pattern.The southwest-northeast extending rain belt along with two high precipitation centers in the south and southwest of Beijing is also well represented in the simulation.The effect of urbanization on precipitation is investigated by using the sensitivity experiments,we found that the modification of land-cover tends to reflect less shortwave radiation and increase surface temperature(UHI),reduces moisture and surface wind speed,and increases sensitive flux and reduces latent flux.Due to the land-use change,more precipitation can be induced over and in the area downwind of Beijing while less rainfall is found in the upwind region of central urban areas.In addition,we explored the potential mechanism of urban effect on the frontal system based on a density current speed equation.One the one side,the abrupt pressure jump during the frontal passage has a significant influence on the cold front movement.The weaker-than-normal cold advection and subsequent smaller pressure jump during the frontal passage over the GBMA are the major mechanisms behind the urbanization effects,which lead to the retardation of the cold front movement and late onset of precipitation over the GBMA.One the other hand,the underlying urban surface not only retards the cold front movement but also favors the development of a wet and unstable atmosphere as the movement of frontal system.This explains why the precipitation can be sustained a longer time with higher rainfall intensity over urban areas.Therefore,the urbanization effects delay the onset of precipitation over Beijing but keep it sustained for a longer period with stronger rainfall intensity.(2)Convection-resolving ensemble simulations using the WRF-Chem model coupled with a single-layer Urban Canopy Model are conducted to investigate the individual and combined impacts of land use and anthropogenic pollutant emissions from urbanization on a heavy rainfall event in the GBMA in 2008.The expanded urban land cover and increased aerosols have an opposite effect on precipitation processes,with the latter playing a more dominant role,leading to suppressed convection and rainfall over the upstream(northwest)area,and enhanced convection and more precipitation in the downstream(southeast)region of the GBMA.In addition,the influence of aerosol indirect effect is found to overwhelm that of direct effect on precipitation in this rainfall event.Increased aerosols induce more cloud droplets with smaller size,which favor evaporative cooling and reduce updrafts and suppress convection over the upstream(northwest)region in the early stage of the rainfall event.As the rainfall system propagates southeastward,more latent heat is released due to the freezing of larger number of smaller cloud drops that are lofted above the freezing level,which is responsible for the increased updraft strength and convective invigoration over the downstream(southeast)area.(3)The WRF-Chem model coupled with a single-layer Urban Canopy Model(UCM)is integrated for 5 years at convection-permitting scale to investigate the individual and combined impacts of urbanization-induced changes in land cover and pollutants emission on regional climate in the Yangtze River Delta(YRD)region in eastern China.The impact of urbanization plays an important role in the climatic characteristics in the YRD region.Results of the simulations with the urbanization effects indicate that the general features of both temperature and precipitation over this region are well described.Urbanization over the YRD induces the Urban Heat Island(UHI)effect,which increases the surface temperature by 0.53 ? in the summer.Meanwhile,annual heat wave days increase by 3.7 d/yr over major megacities in the YRD,accompanied by intensified heat stress.In the winter,the near-surface air temperature increases by approximately 0.7 ? over commercial areas in the cities but decreases over the surrounding areas.Radiative effects of aerosols tend to cool the surface air by reducing net shortwave radiation at the surface.Compared with the more localized UHI effect,the aerosol effects on solar radiation and temperature affect a much larger area,especially the downwind area of the city-cluster in the YRD.(4)The 5-year simulation of the present study indicates that the urbanization-induced UHI and the elevated aerosol loading have opposite effects on extreme precipitation in the summer,i.e.,the UHI(aerosol)effect increases(decreases)the frequency of extreme precipitation over urbanized areas.The UHI effect strengthens the convergence and updrafts over urbanized areas in the afternoon,which are favorable for the development of deep convection that can trigger extreme rainfall events.In contrast,the radiative forcing of aerosols results in surface cooling and upper atmospheric heating,thereby potentially enhances the atmospheric stability and decreases the vertical velocity.Such effects apparently suppress the development of extreme rainfall in the afternoon.The combined effects of the UHI and aerosol on precipitation,however,vary from case to case under different synoptic conditions.In the present study,three typical rainfall events under different synoptic weather patterns are further analyzed.Results suggest that the impacts of the UHI and aerosols on precipitation highly depend on synoptic circulation pattern,environmental condition,and the timing of rainfall events.