Impact of High-Density Urban Development on the Urban Heat Island Effect: A Case of Macau, China

In recent years, an increasing number of scholars have focused on the study of urban heat island (UHI) effects in high-density urban environments based on two-dimensional (2D) and three-dimensional (3D) built environments. However, few scholars have investigated whether the influence of high-density urban built environment factors on the UHI is subject to thresholds and characteristic constraints of the built environment. Therefore, this paper takes the high-density reclamation city of Macau as an example to analyze the thresholds and characteristics of the built environment affecting UHI variations in such urban typologies. The results indicate that among the factors of a 2D built environment, the alleviating effect of the normal difference vegetation index on the UHI is prone to be ineffective due to its characteristic influences. While considering the cooling effect of vegetation cover on the city, the type of vegetation cover also needs to be taken into account. Compared with grassland and shrubs, the vegetation cover of tall trees is more likely to produce shading and cooling effects. The alleviating effect of water bodies on the UHI is subject to a threshold limitation, with the limiting distance being 200 m. Road density and the normalized difference built-up index are not subject to threshold or characteristic attribute limitations. Among the factors of a 3D built environment, building density does not exert any alleviating effect on the UHI due to threshold constraints. The sky view factor (SVF) does not have threshold or self-characteristic limitations on the UHI. In summary, the study concludes that regulating building density has little effect on mitigating the UHI in high-density cities. Urban policymakers should focus on the SVF, namely the depth of urban canyons, to enhance the shading effect of impermeable urban surfaces and hence alleviate the UHI. In terms of vegetation cover, attention should be paid not only to the area of vegetation cover but also to the cooling effects of different vegetation types.