Analysis of the Daytime Urban Heat Island Mechanism in East China

Urban heat islands (UHIs) are one of the most typical changes to surface climate induced by human activities, which manifest as higher temperatures in urban areas than in their surroundings. In this study, we synthetically analyzed spatiotemporal features of daytime surface UHI intensity in east China over the period from 2009 to 2018 carried out with several offline simulations coupled with four land-use data sets, and identified the causes of UHIs with two attribution methods. The results indicate that spatial variations of summer daytime UHI intensity in east China are dominated by variations in the efficiency of evaporating water (similar to 3 degrees C) between urban and rural areas. Geographic variations of winter UHI intensity mainly depend on the urban-rural contrast of heat storage capacity (similar to 1 degrees C). Additionally, we found that forest-covered rural areas have an extremely higher convection efficiency than urban areas and nonforest rural landscapes, resulting in a remarkable contribution to summer daytime UHI intensity (similar to 6 degrees C). However, rural areas largely comprising croplands and grasslands evaporate water more efficiently, expanding summer daytime UHI intensity by approximately 3 degrees C. The prominent capacity of heat storage in nonforest vegetation rural areas accounts for 1 degrees C warming of winter daytime UHI intensity. The major discrepancy between the two attribution methods lies in the evapotranspiration performances of forest-covered rural lands. Furthermore, the inter-comparison of several modeling experiments for land-use data sets suggests that the disparity in UHI intensity could increase by up to 0.4 degrees C, which may exceed 1 degrees C at the local scale.