Responses of the Urban Atmospheric Thermal Environment to Two Distinct Heat Waves and Their Changes With Future Urban Expansion in a Chinese Megacity

This study investigates the responses of the urban atmospheric thermal environment to two distinct heat waves in Hefei, China, and explores potential changes associated with future urban expansion. During the Event 1, characterized by clear and dry conditions, the western Pacific subtropical high limits water vapor influx, resulting in a significant cooling effect in rural area due to higher surface latent heat flux. The urban heat island (UHI) intensity, calculated using surface temperature and 2-m temperature, reaches 5.2 degrees C and 1.7 degrees C during the Event 1, respectively. Although Event 2, characterized by cloudy and humid conditions, exhibits weaker UHI and urban dry island effects, it remains highly unfavorable for human comfort. During distinct heat waves, the vertical extent of the warming effect induced by future urban expansion varies, which can be attributed to environmental factors, such as atmospheric stability and near-surface wind speed. Plain Language Summary The study investigates the interactions between the urban atmospheric thermal environment and two distinct heat waves in Hefei, China, as well as the changes induced by future urban expansion. During the first event, which has clear and dry weather conditions, the influx of water vapor is limited by a high-pressure system. Consequently, rural area experiences a significant cooling effect due to higher surface latent heat flux. The urban heat island (UHI) intensity, measured by surface temperature and 2-m temperature, reaches 5.2 degrees C and 1.7 degrees C, respectively, during this event. In contrast, the second event, characterized by cloudy and humid weather conditions, exhibits weaker UHI and urban dry island effects but remains highly uncomfortable for humans. The vertical extent of the warming effect caused by future urban expansion varies during distinct heat waves. This variation can be attributed to environmental factors, such as atmospheric stability and near-surface wind speed.