Seasonal Spatiotemporal Dynamics and Gradients of the Urban Heat Island Effect in Subtropical Furnace Megacity

Urban heat island (UHI) effect significantly influences the urban sustainability and health of cities and varies seasonally. However, spring and autumn have received less attention. Furthermore, research on long-term seasonal UHI changes and impacts is insufficient. This study examines the seasonal spatiotemporal dynamics and gradient characteristics of UHI in spring, summer, autumn, and winter in Changsha, a typical subtropical "furnace city" from 2006 to 2022. (1) Spatiotemporal dynamics: The high-temperature UHI (relatively high-temperature zone and high-temperature zone) range expands most significantly in spring and least in autumn. Additionally, the UHI migrates northward within the study area, and proximity to the urban core results in multiple high-temperature UHI effects. (2) Gradient characteristics: The proportion of the high-temperature UHI in spring, summer, autumn, and winter decreases to varying degrees within the 5 km gradient from the central point, but increases within the 6-8 km and 11-13 km gradients, especially in spring and autumn. Additionally, within the 8 km gradient in spring, the aggregation index (AI), contagion index (CONTAG), and largest patch index (LPI) decreased, with UHI patches more affected by these metrics in autumn. Overall, this study offers new insights into the seasonal effects and development of UHI, which are crucial for addressing climate change, promoting sustainability, and improving human well-being.