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 (T_max) and minimum (T_min), T_xn, 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 T_xn, 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 T_xn. 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 T_xn, with the exception of those over U2U areas, which could be explained by the changing trends in T_max and T_min due to urban surface expansion, especially during intense urban expansion periods.