Effect of Urban Built-Up Area Expansion on the Urban Heat Islands in Different Seasons in 34 Metropolitan Regions across China

Abstract

The urban heat island (𝑈𝐻𝐼) refers to the land surface temperature (LST) difference between urban areas and their undeveloped or underdeveloped surroundings. It is a measure of the thermal influence of the urban built-up area expansion (𝑈𝐵𝐴𝐸), a topic that has been extensively studied. However, the impact of 𝑈𝐵𝐴𝐸 on the LST differences between urban areas and rural areas (𝑈𝐻𝐼𝑈−𝑅) and between urban areas and emerging urban areas (𝑈𝐻𝐼𝑈−𝑆) in different seasons has seldom been investigated. Here, the 𝑈𝐻𝐼𝑈−𝑆 and 𝑈𝐻𝐼𝑈−𝑅 in 34 major metropolitan regions across China, and their spatiotemporal variations based on long-term space-borne observations during the period 2001–2020 were analyzed. The 𝑈𝐵𝐴𝐸 quantified by the difference in landscape metrics of built-up areas between 2020 and 2000 and their impact on 𝑈𝐻𝐼 was further analyzed. The 𝑈𝐵𝐴𝐸 is impacted by the level of economic development and topography. The 𝑈𝐵𝐴𝐸 of cities located in more developed regions was more significant than that in less developed regions. Coastal cities experienced the most obvious 𝑈𝐵𝐴𝐸, followed by plain and hilly cities. The 𝑈𝐵𝐴𝐸 in mountainous regions was the weakest. On an annual basis, 𝑈𝐻𝐼𝑈−𝑅 was larger than 𝑈𝐻𝐼𝑈−𝑆, decreasing more slowly with 𝑈𝐵𝐴𝐸 than 𝑈𝐻𝐼𝑈−𝑆. In different seasons, the 𝑈𝐻𝐼𝑈−𝑆 and 𝑈𝐻𝐼𝑈−𝑅 were larger, more clearly varying temporally with 𝑈𝐵𝐴𝐸 in summer than in winter, and their temporal variations were significantly correlated with 𝑈𝐵𝐴𝐸 in summer but not in winter. The seasonal difference in 𝑈𝐻𝐼𝑈−𝑅 was larger than that of 𝑈𝐻𝐼𝑈−𝑆. Both the 𝑈𝐻𝐼𝑈−𝑆 and 𝑈𝐻𝐼𝑈−𝑅 in coastal cities were the lowest in summer, decreasing the fastest with 𝑈𝐵𝐴𝐸, while those in mountain cities decreased the slowest. The change in the density of built-up lands was the primary driver affecting the temporal variations in 𝑈𝐻𝐼𝑈−𝑆 and 𝑈𝐻𝐼𝑈−𝑅 during 𝑈𝐵𝐴𝐸, followed by changes in proportion and shape, while the impact of the speed of expansion was the smallest, all of which were more obvious in summer than in winter. The decreased density of built-up lands can reduce 𝑈𝐻𝐼. These findings provide a new perspective for a deeper understanding of the effect of urban expansion on LST in different seasons.