China’s Greater Bay Area (GBA) is one of the fastest urbanizing regions in the world, featured by its complex land surface and unique geography. In this study, heatwaves (HWs) in the GBA during the summers (June, July, and August) of 1961–2020 are analyzed by using observational and reanalysis datasets. The results indicate that 70% of daytime HWs occur in the northern forested areas of the GBA, 65% of nighttime HWs are observed in the cropland and forest areas around the GBA, and 75% of compound HWs occur in the urban and southern coastal areas of the GBA. Daytime HWs are featured by lower near-surface specific humidity and drier soil moisture, while nighttime HWs are often accompanied by relatively wetter conditions. For compound HWs, they are jointly affected by the conditions of the above two types. During daytime and compound HWs, soil moisture dries and recovers quickly, exacerbating the high temperatures of HWs through strong soil moisture–air temperature coupling that far exceeds the climatology. Nighttime HWs lack this coupling and are primarily driven by atmospheric factors, with high temperatures maintained mainly by increased water vapor.

The combined effects of global warming and the urban heat islands exacerbate the risk of urban heat stress. It is crucial to implement effective cooling measures in urban areas to improve the comfort of the thermal environment. In this study, the Weather Research and Forecasting Model (WRF), coupled with a single-layer Urban Canopy Model (UCM), was used to study the impact of heat mitigation strategies. In addition, a 5-km resolution land-cover dataset for China (ChinaLC), which is based on satellite remote sensing data, was adjusted and used, and 18 groups of numerical experiments were designed, to increase the albedo and vegetation fraction of roof/ground parameters. The experiments were conducted for four heatwave events that occurred in the summer of 2013 in the Yangtze River Delta urban agglomeration of China. The simulated results demonstrated that, for the single roof/ground schemes, the mitigation effects were directly proportional to the albedo and greening. Among all the experimental schemes, the superposed schemes presented better cooling effects. For the ground greening scheme, with similar net radiation flux and latent heat flux, its storage heat was lower than that of the roof greening scheme, resulting in more energy flux into the atmosphere, and its daytime cooling effect was not as good as that of the roof greening scheme. In terms of human thermal comfort (HTC), the improvement achieved by the ground greening scheme was better than any other single roof/ground schemes, because the increase in the relative humidity was small. The comprehensive evaluation of the mitigation effects of different schemes on the thermal environment presented in this paper provides a theoretical basis for improving the urban environment through rational urban planning and construction.