The observed cooling potential of rooftop strategies during heatwaves in a subtropical city

Rooftop Mitigation Strategies (RMSs) have garnered global recognition as effective measures for mitigating urban thermal environments. However, the cooling effectiveness of Green Roof (GR) and Cool Roof (CR) remains a subject of ongoing debate, especially when considered within diverse climatic contexts. This study conducted RMSs observation experiments within the subtropical urban landscape of Guangzhou, China, to assess the cooling potential of these strategies during both normal weather conditions and heatwaves. Our experiments identified a critical temperature threshold that influences the cooling potential of GR, a phenomenon rooted in vegetation transpiration. Below 33 °C, transpiration gradually intensifies, resulting in a noticeable cooling effect. However, as temperatures exceed this threshold, transpiration diminishes. Coupled with the aerodynamic drag imposed by vegetation leaves on wind flow, this complex dynamic leads to a temperature increase at heights ranging from 0.3 m to 0.6 m above the roof. Given the subtropical climate's characteristics of high temperatures and humidity, prudent consideration is warranted when selecting appropriate mitigation strategies. In this context, our observations suggest that CR may be a more cost-effective and potentially more efficacious choice due to their lower costs and substantial cooling potential. Our research provides a significant contribution to the reevaluation of RMSs' cooling potential and its role in reducing urban energy consumption and lowering building carbon emissions. •Green roofs with mixed plants have a more significant cooling effect.•Heatwave weather leads to the failure of green roof cooling.•Plant transpiration and wind resistance cause near-surface warming during heatwaves.•Cool roof is an effective cooling strategy in subtropical climates.