Comparisons of urban-related warming for Shenzhen and Guangzhou

Urban-related warming in two first-tier cities(Guangzhou and Shenzhen)in southern China with similar large-scale climatic backgrounds was compared using the nested weather research and forecasting regional climate model.The default urban data in the model were replaced by reconstructed annual urban data retrieved from satellite-based images for both coarse-(including all of China)and fine-resolution domains(eastern China and three city clusters in China:Beijing– Tianjin–Hebei(BTH),the Yangtze River Delta(YRD),and the Pearl River Delta(PRD)),which reproduced urban surface expansion during the past few decades.The results showed that the 37-year(1980–2016)area-averaged annual urban-related warming was similar(0.69°C/0.64°C)between the urban areas of Guangzhou/Shenzhen;however,the values across the entire area of the two cities varied(0.21°C/0.45°C).Seasonal characteristics could be detected for mean surface air temperatures(SAT)at 2 m,SAT maximum and minimum,and diurnal temperature range(DTR).Both the SAT maximum and minimum generally increased,especially over urban areas;however,changes in the SAT minimum were larger,which induced a decrease in DTR.The DTR in summer decreased by-0.25°C/-0.86°C across the entire area of the two cities and decreased by-0.93°C/-1.15°C over urban areas.The contributions of urban surface expansion to regional warming across the entire area of the two cities were approximately 17%/35%of the overall warming and much greater over Shenzhen.However,the values over urban areas were much closer to the values from total warming(35%/44%).

Evaluating the contributions of urban surface expansion to regional warming in Shanghai using different methods to calculate the daily mean temperature

The contributions of urban surface expansion to regional warming over subregions of Shanghai and Shanghai as a whole using different methods to calculate the daily mean surface temperature(SAT),including the averages of four daily time-records(0000,0600,1200,and 1800 UTC;T4),eight daily time-records(0000,0300,0600,0900,1200,1500,1800,and 2100 UTC;T8),and the averages of the SAT maximum(Tmax)and minimum(Tmin),Txn,were compared based on simulated results using nested numerical intergrations with the Weather Research and Forecasting regional climate model,where only the satellite-retrieved urban surface distributions differed between two numerical experiments.The contributions from urban-related warming expressed similar intensities when using T8 and Txn,while the smallest values occurred when using T4 over different subregions of Shanghai(with the exception of areas that were defined as urban for both time periods(U2U))and Shanghai as a whole.Similar values for the changing trends could be detected over different subregions when no urban surface expansion(EX1)was detected for both T4 and Txn.The corresponding values increased under urban surface expansion(EX2)and varied over different subregions,revealing much stronger intensities over urban-surface expansion areas;the weakest intensities occurred over U2U areas.The increasing trends for EX2 and relative contributions when using T4 were smaller than those when using Txn,with the exception of those over U2U areas,which could be explained by the changing trends in Tmax and Tmin due to urban surface expansion,especially during intense urban expansion periods.

Regional warming induced by urban surface expansion in Shanghai

To detect the impacts of urban surface expansion on surface air temperature at 2-m（SAT） in Shanghai, China, nested numerical integrations based on satellite-derived urban data between the 1980 s and 2010 s were performed using the Weather Research and Forecasting（WRF） model. Urban surface expansion induced an annual-averaged warming of 0.31 °C from 1980 to 2016 across the whole of Shanghai, showing the greatest intensity between 2010 and 2016. The values were 0.36, 0.78, and 0.75 °C over grids that were classified as urban in both time periods（U2 U）, landuse grids that changed from non-urban to urban（N2 U）, and urban areas（including U2 U and N2 U）, respectively, and revealed weak warming over the inner-ring areas because the urban surfaces had been there since the 1980 s, whereas warming areas were coincident with the outward expansion of the urban surface. Meanwhile, marked seasonal variations could be detected, which were greater in spring and summer but less in autumn and winter. The approximately homogenously distributed SAT maximum（weaker） and heterogeneously SAT minimum（stronger） contributed to the decreased diurnal temperature range. Regional warming induced by urban surface expansion was approximately 0.12 °C per decade, which accounted for 19% of the overall warming across the whole of Shanghai. The values were 0.11 °C per decade and 0.39 °C per decade over U2 U and N2 U, which accounted for approximately 17% and 42% of the overall warming, respectively, and resulted in approximately 41% of the overall warming over urban areas.