Quantifying morphology evolutions of urban heat islands and assessing their heat exposure in a metropolis

Taking effective measures to solve thermal environment problems in summer and reduce their impacts on citizens requires a more comprehensive understanding of urban heat islands (UHI) morphology and exposure. Therefore, selecting Beijing as the study area, a framework was created to portray UHI morphology evolution in terms of distribution, expansion type, and topological structure from 2000 to 2020. Then, using the population-weighted model and relative exposure risk model, the UHI exposure was explored at the city and raster scales. The results showed that the longtime-series and multi-perspective analysis on morphology can provide a deeper understanding of UHI evolution characteristics and help propose targeted mitigation strategies. Specifically, the UHI scale in Beijing grew significantly, dominated by the edge-expansion type, and the topological structure of the main heat island performed more complicated from single-core to multi-core. Moreover, the cross-scale assessment of UHI exposure can derive the overall exposure conditions and the local heat risk levels. The overall UHI exposure in Beijing deteriorated significantly from 2005 to 2010, with a slight improvement after 2010, and the higher risk areas were mainly concentrated within the urban center region. Our study can provide governments and planners important insights and help propose scientific UHI mitigation measures.