The Observed Cooling Potential of Rooftop Strategies During Heatwaves in a Subtropical City

Abstract

Rooftop mitigation strategies (RMSs) have gained global recognition as effective measures to mitigate the urban thermal environment. However, the cooling efficacy of green roofs (GRs) and cool roofs (CRs) remains a subject of debate, particularly when considered against diverse climatic contexts. This study undertook RMS observation experiments within the subtropical urban landscape of Guangzhou, China, to assess the cooling potential of these strategies during periods under both normal and heatwave weather conditions. Our experiments identified a pivotal temperature threshold that influences the cooling potential of GRs, a phenomenon rooted in vegetation transpiration. Below 33°C, transpiration gradually intensifies, yielding a notable cooling effect. However, as temperatures exceed this threshold, transpiration diminishes. Combined with the drag imposed by vegetation leaves on wind flow, this nuanced dynamic leads to a temperature rise at heights ranging from 0.3m to 0.6m above the roof. In the context of the subtropical climate characterized by high temperatures and humidity, prudent consideration is warranted when selecting appropriate strategies. In this regard, our observations advocate for CRs as a potentially more efficacious choice due to their lower costs and substantial cooling potential.

Keywords: Rooftop experiment, Green roof, Cool roof, Stomatal conductance, Transpiration, Weather condition