Assessing the Urban Climate Effects of Roof Strategies in a Subtropical Urban Agglomeration

Rooftop mitigation strategies (RMSs) have been extensively implemented worldwide to reduce urban heat. However, the effectiveness of different RMSs varies under diverse climatic conditions. In this study, we conducted RMSs observation experiments in Guangzhou, a subtropical city in China, to assess their cooling potential during heatwaves and explain it using the physiological characteristics of vegetation. We found that there is a temperature threshold for the cooling of green roofs to follow the transpiration of vegetation. When the temperature is below 33°C, transpiration gradually increases, and the cooling effect is significant. However, when the temperature exceeds 33°C, transpiration gradually decreases, and the dragging effect of vegetation leaves on wind speed leads to an increase in temperature at 0.3~0.6 m near the ground. Considering the high temperature and humidity environment of the subtropical region, cool roofs may be a better option due to their lower cost and more significant cooling effect. Based on observational data and existing literature, we modified the physical parameterization scheme of RMSs within the Weather Research and Forecasting (WRF) model. This included incorporating mixed vegetation green roofs, bifacial photovoltaic panels, and their coupled roofs with cool and green roofs. The improved model was then used to estimate the urban climate impacts of various RMSs when applied to the urban agglomeration of the Pearl River Delta region in China.