Feasibility of Using Green Roofs and Cool Roofs for Extreme Heat Mitigation in Melbourne, Australia

Heatwaves pose undesirable consequences to human health as a ‘silent killer’ and penalise the environmental, economic and social aspects. Future heatwaves are predicted to be more severe, longer and more frequent. Over the past decade, Melbourne, Australia, has experienced three notable summer heatwaves. This study presents a simulational experiment in Melbourne’s central business district for the 2014 heatwave, using ‘The Air Pollution Model’, a mesoscale urban climate model. The study evaluated the effectiveness of green and cool roofs as heat mitigation strategies, considering various scenarios that balanced their occupancy on city rooftops, with a maximum 50:50 combination. Temperature data for heatwave days, including average (T_ave), maximum (T_max), and minimum (T_min) temperatures, were compared to the average seasonal summer temperatures. The results showed that heatwave temperatures exceeded seasonal averages significantly, with T_max exceeding by 16.1 °C and T_min and T_ave by 5.8 °C and 12.5 °C, respectively. The temperature reduction behaviour of scenarios during heatwaves proved scenario d5 with 25% green roofs and 75% cool roofs with 0.7 albedo to be the best performance for T_max with the highest reduction. For T_min and T_ave, d8, a scenario with 50:50 green and cool roofs (0.7 albedo) offered the best heat reduction. Moreover, d8 recorded the best reduction for the total average air temperature and apparent temperature, a measurement used to evaluate human thermal comfort. The combined scenarios exhibited complementary cooling potential for multiple heat indices, surpassing the individual effectiveness of either green or cool roofs for heat mitigation in Melbourne. Therefore, wise utilisation of total roof area by integrating both green and cool roofs in urban planning can offer a resource-efficient, cost-effective, and realistic strategy, making them a promising solution for addressing the challenges posed by rising heatwaves in urban areas.