An alternative method of developing landscape strategies for urban cooling: A threshold-based perspective

Increasing the amount of blue-green spaces has been recognized as an effective strategy to mitigate the urban heat island (UHI) effect. Specific quantitative and actionable landscape mitigation strategies for different land-use and land-cover (LULC) types to cool down cities, have been rarely addressed. In this paper, the absolute threshold of cooling (ToCabs) —the specific threshold of a certain influencing factor to ensure the effective cooling of a particular patch type, was proposed and defined for the first time to facilitate the research. The effects of size, shape and neighboring greenspace percentage (NGP) of different types of LULC patches on land surface temperature (LST) were examined and their corresponding ToCabs values were determined if existed, in the metropolitan areas of Beijing, Shanghai and Tianjin using satellite images. Results demonstrate that larger-sized and complex-shaped water bodies and forested areas produce greater cooling effect. The size is irrelevant to the cooling intensity of grass-dominated patches. ToCabs of patch size exist and were identified for water bodies (Beijing, 5 ha; Shanghai, 10 ha; and Tianjin, 5 ha) and forested areas (Beijing, 20 ha; Shanghai, 50 ha; and Tianjin, 20 ha). Besides, NGP is negatively correlated with patch LST for all LULC types, indicating that increasing the amount of neighboring vegetation cover can effectively lower the nearby patch LST. ToCabs of NGP exist and were identified for water bodies (Beijing, 30.3%; Shanghai, 61.6%; and Tianjin, 20%) and grass-dominated areas (Beijing, 89.1%; Shanghai, 76.9%; and Tianjin, 81.5%). Among the three influencing factors, the size accounts for greatest variability in patch LST for forested areas and impervious surfaces, and neighboring greenspace proportion accounts for the most variability in LST for water bodies and grass-dominated areas. The methodology and results of this study could help to orientate specific and actionable landscape strategies of urban cooling.