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Enhanced Summertime Surface Warming Effects of Long‐Term Urbanization in a Humid Urban Agglomeration in China

In this study, we examined the summertime climatic effects of urban expansion during 1990–2010 in the Yangtze River Delta (YRD) region by analyzing station observations and performing numerical simulations with the Weather Research and Forecast (WRF) model. Long‐term observations showed that urban area experienced a larger increase in summertime 2‐m air temperature than rural part during 1980–2018, and the influence of urbanization on the urban‐rural contrast was greater in the late stage (after 2000) than the early stage (before 2000). We applied the WRF model incorporated with historical land surface cover data (year 1990, 2000, and 2010) to further evaluate the climatic effects of long‐term urbanization. On average, urban expansion over 1990–2010 led to 0.75°C increase in daily average temperature (1.06°C in daily minimum and 0.45°C in daily maximum) during the summer. The summertime daily temperature range decreased by 0.61°C in urban environment during the same period. Compared to the warming effect of urbanization in the 1990s, both the magnitude and affected area have increased after the millennium. Also, urban expansion reduced moisture in low‐level atmosphere, and this urban dry island (UDI) effect was enhanced in the late stage. Less moisture in the atmosphere offset heat stress index induced by the warming temperature. We also found that the partitioning of net radiation between sensible and latent heat was the key factor that controlled urban warming effect.