Sensitivity Experiments Of Impacts Of Urban Land Cover Changes And Anthropogenic Aerosol Emissions In East China On The East Asian Monsoon

Impacts of urban land cover changes and aerosol emissions in East China on the East Asian monsoon(EAM)are investigated using the Community Atmosphere Model(CAM5.1)?Evaluation has been made on the simulation of summer and winter air temperature,precipitation,circulation in East Asia.Impacts of urban land cover changes and aerosol emissions in East China on summer and winter air temperature,precipitation,surface energy balance are assessed to further understand the detailed mechanisms of East Asian monsoon changes.Ensemble experiments with different initial states and sensitivity experiments driven by different sea surface temperature(SST)backgrounds are utilized to reduce the uncertainty of the results.The impact of urban land cover changes over a key zone on EAM is also assessed.The results are as follows:(1)The increase of urbanization causes large surface warming and an urban frictional drag,leading to a northeasterly wind anomaly in the lower troposphere over East China(EC).This weakens the southerly winds and causes a convergence anomaly in southern China(SC)with increased ascent,latent heating and cloudiness.The enhanced latent heating reinforces surface convergence and upper-level divergence over SC,leading to more northward advection in the upper-level into northern China(NC)and descending between 30°N and 50°N over East Asia.Cloudiness reduction,adiabatic heating and warm advection over NC all enhance the urban heating there,causing anomalous tropospheric warming over East Asia.(2)Anthropogenic aerosols cause widespread cooling at the surface and in the troposphere over EC,which decreases the summer land-ocean thermal contrast,leading to a weakened East Asian summer monsoon(EASM)with reduced moisture transport to NC.This results in wetter and drier conditions over SC and NC,respectively.When both the urbanization and anthropogenic aerosols are considered,aerosols' cooling is partially offset by the urban heating;and their joint effect on the circulation is dominated by the aerosols' effect with a reduced magnitude.In the combined experiment,surface and tropospheric temperatures are also altered by the decrease(increase)in cloudiness over NC(SC)with most of the cooling confined to SC,which further weakens the EASM circulation.(3)The impact of urbanization and anthropogenic aerosols on East Asian winter monsoon(EAWlM)was investigated.The increase of urbanization causes large surface warming anomaly in the lower troposphere over EC and leads to a weakened meridional gradient over SC.This reduces cold northerly winds from the high latitude continent over SC,which favors the increase of warm air from South China Sea(SCS),leading to a significant warming over south Tibetan Plateau(TP)and Indo-China Peninsula.A warming anomaly over TP enhances temperature advection downstream,and causes the increased air temperature in mid-upper troposphere over East China and therefore the elevated isobaric surfaces in the troposphere.This weakens the northerly winds and causes a weakened EAWM.The increased anthropogenic aerosols reduce shortwave radiation in the atmosphere.A reduction in air temperature is found over most East China in lower troposphere.However,the BC deposit on snow over TP tends to reduce the surface albedo and increase the surface air temperature over TP.The TP warming tends to weaken EAWM and prevent cold air from transportation from Siberia to South China.(4)The sensitivity experiments are conducted to investigate the response of the EASM circulation to urban land cover when sea surface temperature in 1960 and 1979(negative phase of PDO)is used to force the CAM5.1.Ensemble experiments with different initial states are assessed to make the results more robust.The impacts of urbanization on the EASM of both experiments with different initial states and with different SST backgrounds are similar to the decisive experiments when the CAM5.1 is forced by sea surface temperature in 1988 and 2007.Air temperature increases significantly to the north of 40°N.The change of precipitation shows the anomalous pattern of "southern-flood and northern-drought".The urban agglomeration areas in 30-40°N in East China are also the fringe regions of the EASM.The sensitivity experiments showed that the increase of urban land cover in 30-40°N causes a similar anomalous pattern of the response of the EASM in CTL.However,urbanization to the north of 40°N and to the south of 30°N significantly increases the precipitation over southern China,which indicates that the increase of urban land cover in 30-40°N plays an important role in modifying EASM.