Effects of cooling roofs on mitigating the urban heat island and human thermal stress in the Pearl River Delta, China

The Weather Research and Forecasting model was employed to simulate the effects of cooling roofs on the urban heat island effect and human thermal stress in the Pearl River Delta region, China, in summer (June-August) 2014. It was found that cooling roofs can reduce the 2-m air and ground-surface temperature, wind speed, and planetary boundary layer height, while increasing the specific and relative humidity, in urban areas. The effects can influence the entire urban boundary layer, especially in commercial/industrial areas in the daytime. Cooling roofs can effectively mitigate the urban heat island effect, and the mitigation of white roofs is more significant than that of green roofs. The mitigation of the urban heat island effect is stronger in commercial/industrial areas than in low-intensity residential areas. The city-wide 2-m and ground-surface urban heat island intensity induced by white roofs can reduce by 1.5 degrees C and 4.8 degrees C at noon, while the corresponding decreases induced by green roofs can reach 0.7 degrees C and 1.9 degrees C, respectively. White roofs improve human thermal comfort both in the daytime and at nighttime. With white roofs, three city-wide thermal-stress indices (wet-bulb globe temperature, apparent temper-ature, and humidity index) can be reduced at noon by 0.8 degrees C, 1.3 degrees C and 1.5 degrees C, respectively. Green roofs mostly improve human thermal comfort at nighttime but even deteriorate it in the daytime. The above three indices decrease by less than 0.5 degrees C in different urban categories at nighttime with green roofs.