How do green roofs mitigate urban thermal stress under heat waves?

As the climate warms, heat waves (HWs) are projected to be more intense and to last longer, with serious implications for public health. Urban residents face higher health risks because urban heat islands exacerbate HW conditions. One strategy to mitigate negative impacts of urban thermal stress is the installation of green roofs (GRs) given their evaporative cooling effect. However, the effectiveness of GRs and the mechanisms by which they have an effect at the scale of entire cities are still largely unknown. The Greater Beijing Region is modeled for a HW scenario with the Weather Research and Forecasting model coupled with a state-of-the-art urban canopy model (Princeton Urban Canopy Model) to examine the effectiveness of GRs. The results suggest that GR would decrease near-surface air temperature (Delta T2(max) = 2.5 K) and wind speed (Delta UV10(max) = 1.0 m s(-1)) but increase atmospheric humidity (Delta Q2(max) = 1.3 g kg(-1)). GRs are simulated to lessen the overall thermal stress as indicated by apparent temperature (Delta AT2(max) (=) 1.7 degrees C). The modifications by GRs scale almost linearly with the fraction of the surface they cover. Investigation of the surface-atmosphere interactions indicates that GRs with plentiful soil moisture dissipate more of the surface energy as latent heat flux and subsequently inhibit the development of the daytime planetary boundary layer (PBL). This causes the atmospheric heating through entrainment at the PBL top to be decreased. Additionally, urban GRs modify regional circulation regimes leading to decreased advective heating under HW.