Heat waves (HW) are projected to be more intense and to last longer as the climate warms, with serious implications for public health. Urban residents face higher health risks because urban heat islands (UHIs) exacerbate heat wave conditions. One strategy to mitigate negative impacts of urban thermal stress is the installation of green roofs (GRs) given their evaporative cooling effect. The effectiveness of GRs in mitigating the UHI effect is well documented in a variety of cities under different climates. 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 (GBR) is modeled for a HW scenario with the Weather Research and Forecast (WRF) model coupled with a state-of-the-art urban canopy model (PUCM) to examine the effectiveness of GRs. PUCM can take the heterogeneity of urban surfaces into account and thus allows for GRs to be a roof facet alongside more conventional ones.

From simulations with six GR cover fractions (0, 10, 20, 50, 80, 100%), modifications of the near-surface micrometeorology (indicated by T2, Q2 and UV10) and low-level atmospheric conditions (indicated by Ta, Qa and UVa) were documented. With a 100% vegetation cover of roofs: T2 (Ta) is reduced by a maximum of 2.5 K (1.6 K), Q2 (Qa) is increased by a maximum of 1.31 g kg^-1 (1.28 g kg^-1), and UV10 (UVa) is reduced by a maximum of 1.0 m s^-1 (1.1 m s^-1). Under the full GR scenario, a cool island rather than heat island effect is evident. The modifying effects of GRs extend to the surroundings with the regional temperature and wind regimes significantly affected. Reductions in air temperature and wind speed, and increase in air humidity increase linearly with GR coverage fraction. A 41.5% increase in GR coverage would cause a 1 K decrease in the daily maximum air temperature in this study, which could lessen the mortality risk by 4.5% under heat waves.

Further investigations of land-atmosphere interactions shows that GRs can alter the surface energy balance (SEB) by dissipating more turbulent energy as latent heat flux and subsequently inhibit the development of the daytime planetary boundary layer (PBL), with the result that the atmospheric heating through entrainment at the PBL top is decreased. In addition, regional circulation regimes are affected by the urban GRs, leading to decreased advective heating under HW.