Evaluating the Cooling Performance of Green Roofs Under Extreme Heat Conditions

The local rise in urban temperature is increasingly exacerbated due to the combined effect of urban heat islands and global climate change. Numerous studies have shown that green roofs (GRs) have great potential for facilitating urban heat mitigation. However, little is known about whether such cooling effects can be achieved under extreme heat conditions. With the expected occurrence of more extreme heat events under climate change, such understanding is crucially important for the effective design of heat mitigation. This study aims to fill this gap by investigating the pedestrian-level cooling effect of GR under two weather conditions (i.e., typical summer weather conditions and extreme heat conditions). This research employed a three-dimensional simulation model, ENVI-met, to simulate pedestrian-level air temperature for three typical residential areas with different roof heights in Beijing. We conducted the simulations in two different roof scenarios, conventional roofs versus green roofs. The results showed that green roofs could provide large cooling exceeding 0.2 degrees C on downwind sides and in the daytime, although the average cooling intensity was small. The pedestrian-level cooling intensity of GR decreased significantly under extreme heat conditions compared to typical summer weather conditions. It varied diurnally following an inverted W-shape for both weather conditions. Results also showed that the pedestrian-level cooling intensity of GR decreased with the increase in roof height in a nonlinear way and became 0 when roof height reached similar to 50m for both weather conditions. The results of our research can provide important insights for cooling-oriented urban design in the future, as we are expecting such extreme weather conditions nowadays may be the new normal in the future.