Surface urban heat island in China's 32 major cities: Spatial patterns and drivers

Urban heat island (UHI) is a major anthropogenic alteration on Earth environments and its geospatial pattern remains poorly understood over large areas. Using MODIS data from 2003 to 2011, we quantified the diurnal and seasonal surface UHI intensity (SUHII, urban–suburban temperature difference) in China's 32 major cities, and analyzed their spatial variations and possible underlying mechanisms. Results show that the annual mean SUHII varied markedly from 0.01 to 1.87 °C in the day and 0.35 to 1.95 °C at night, with a great deal of spatial heterogeneities. Higher SUHIIs for the day and night were observed in the southeastern and northern regions, respectively. Moreover, the SUHII differed greatly by season, characterized by a higher intensity in summer than in winter during the day, and the opposite during the night for most cities. Consequently, whether the daytime SUHII was higher or lower than the nighttime SUHII for a city depends strongly on the geographic location and research period. The SUHII's distribution in the day related closely to vegetation activity and anthropogenic heat releases in summer, and to climate (temperature and precipitation) in winter, while that at night linked tightly to albedo, anthropogenic heat releases, built-up intensity, and climate in both seasons. Overall, we found the overwhelming control of climate on the SUHII's spatial variability, yet the factors included in this study explained a much smaller fraction of the SUHII variations in the day compared to night and in summer relative to winter (day vs. night: 57% vs. 72% in summer, and 61% vs. 90% in winter, respectively), indicating more complicated mechanisms underlying the distribution of daytime SUHII, particularly in summer. Our results highlight the different diurnal (day and night) and seasonal (summer and winter) SUHII's spatial patterns and driving forces, suggesting various strategies are needed for an effective UHI effect mitigation.