Simulations Of The Impacts Of Urban Expansion And Future Climate Change On Regional Atmospheric Thermal Environment

Urban expansion and climate change are the two most important factors affecting atmospheric thermal environment.Therefore,quantifying the impacts of urbanization and assessing global climate forcings at regional scale can provide essential data for urban development and provide decision-making reference for climate change mitigation.Urban expansion mainly manifests two characteristics:the changes in land surface properties and the emission of a large amount of anthropogenic heat(AH),which modulates the energy exchange process between land surface and atmosphere and influences the urban ecological environment and public health.In addition,under the background of continuous global warming,the response of urban atmospheric thermal environment to greenhouse gas emission is also a major public concern.In the study of urban expansion and climate change at regional scale,the acquisition of localized datasets with high spatial resolution are critical to the accuracy of final results.Although a series of studies on urbanization and climate change have been conducted,previous investigations still have some deficiencies in the updating of surface vegetation coverage and the coupling of gridded AH data due to insufficient consideration of land surface parameters and difficulties in calculating high-resolution AH emission data.In addition,studies on the climatic effects induced by future urbanization have failed to take into account the response of atmospheric thermal environment to different heatwaves currently.In this study,the WRF-UCM(Weather Research and Forecasting Model coupled with the Single-Layer Urban Canopy Model)model is used to quantify the impacts of past urbanization,future urban expansion and future climate change on atmospheric thermal environment in Hefei,a megacity in eastern China.Firstly,the static geographic datasets in WRF model,including land cover types,green vegetation fraction and leaf area index,were replaced based on high-resolution satellite observations,and the gridded AH emission data with a spatial resolution of 1 km in Hefei city has also been incorporated.From the perspective of old urban area and new urban area,it has been analyzed that the respective and combined effects of land surface changes and AH emission induced by urbanization in past 20 years on atmospheric thermal environment.The results show that the contribution of land surface changes to the urban heat island in July is about 0.76℃,and that of AH emission is about 0.17℃,which is slightly smaller than other megacities in the Yangtze River Delta.The influence of land surface changes on daily averaged 2 m air temperature and 2 m relative humidity are 0.71℃ and-4.78%,respectively.During the daytime,the impact of land surface changes in new urban area is greater than that in old urban area,but the effect is comparable over old and new urban areas at night.AH emission has very little influence on atmospheric thermal environment in new urban area,but in old urban area,the impact of AH emission on the daily average 2 m temperature,2 m relative humidity and heat index is up to 0.41℃,-1.78%and 1.14℃,respectively,and the effect on night is about twice that of day.Secondly,two heat-wave events in Hefei were selected to explore the impact of future urban expansion under different weather backgrounds and their difference from average state,using the future urban land cover type dataset with 1 km spatial resolution trained through night lights.The results show that in the expanded area,the maximum change in surface sensible heat flux induced by future urban expansion is about 100 W/m2,the increased energy stored in the ground during the daytime is up to 140 W/m2,and the increased energy released from the ground at night is as high as 105 W/m2.Under the weather background of heat-wave event 1,there is a synergistic effect between urban expansion and heat wave,while heat-wave event 2 weakened the warming effect caused by urban expansion,which is related to the fact that high wind speed in event 2 leads to heat dispersion.The above evidence shows that the response of air temperature to urban expansion is different in different heat-wave events.Finally,the influence of climate change under different emission scenarios on atmospheric thermal environment in Hefei was discussed.The results show that the daily averaged 2 m temperature over urban area under RCP 4.5 and RCP 8.5 scenarios is as high as 29.9 ℃ and 32.2℃ respectively,which is 0.7℃ and 3℃ higher than that at present,respectively.It is noted that the effects of global warming on rural and urban near-surface air temperature are comparable.The 2 m relative humidity in the future will not decrease with the increase of near-surface air temperature because there will be more water vapor in the atmosphere.The heating effect of future greenhouse gases on land surface at night can cause a greater temperature difference between land surface and atmosphere,leading to the increasing of planetary boundary layer height(PBLH).However,future PBLH will decrease in the daytime,which may be related to the fact that increased atmospheric aerosol in the future can scatter more solar short-wave radiation and thus lead to the reduced solar radiation reaching the ground.In the future,the intensity and frequency of high temperature in Hefei will increase,especially at 1400 LST,the probability of 2 m temperature above 38.0℃ is up to 26.5%.