The Cooling Potential and Mechanism of Rooftop Mitigation Strategies in a Subtropical Urban Agglomeration

Heatwave is one of the main health threats faced by urban residents in the context of climate change. Rooftop mitigation strategies (RMSs) have been widely used globally for cooling urban thermal environments. However, the cooling effect of green roofs (GRs) and cool roofs (CRs) is controversial under different background climate. In this study, we established RMSs observation experiments in a subtropical city, Guangzhou, China, to evaluate their cooling/warming potential at extremely high temperatures and explain it using the vegetation’s physiological characteristics. It is found that there is a temperature threshold for the cooling of GRs to follow the transpiration of vegetation. When the temperature is lower than 33°C, the transpiration gradually increases, and the cooling effect is remarkable. When the temperature is higher than 33°C, the transpiration gradually weakens, with the dragging effect of vegetation leaves on the wind speed, resulting in an increase in temperature at 0.3~0.6m near the ground. We demonstrated that GRs could lead to additional warming under certain extreme heat conditions. Considering the subtropical region’s high temperature and high humidity environment, the CRs may be a better choice with lower cost, and a more significant cooling effect. Experimental data is further used to validate the Weather Research and Forecasting (WRF) coupled with urban canopy models. Finally, the optimized model is applied to estimate the cooling potential of various RMSs when they are largely applied to the urban agglomeration of the Pearl River Delta (PRD) region in China.