Effects of nature-based green and cool roof mitigation strategies on the thermal environment: A case study of Sendai city, Japan

: This study examines the effectiveness of green and cool roof strategies for improving the thermal environment in Sendai, Japan. Nine high-resolution numerical simulations were conducted using the Weather Research and Forecasting (WRF) model, including a control case, multiple green roof configurations (sedum and grass with different irrigation schedules), and cool roofs with moderate (0.50) and high (0.80) albedo. The results clearly contrast between the green and cool roof mitigation mechanisms. Grass (Sedum)-based green roofs induce stronger (weaker) daytime cooling, with maximum reductions in the daytime surface air temperature of 0.6 °C (0.3 °C). However, both the grass- and sedum-based green roof scenarios exhibited weak nighttime warming, resulting in limited net cooling. Among the green roof configurations, grass roofs consistently outperformed sedum roofs, whereas variations in irrigation timing yielded only insignificant differences in cooling effectiveness. The moderate-albedo roof reduced the daytime (nighttime) surface air temperature by 0.27 °C (0.12 °C), whereas the high-albedo roof achieved a reduction of 0.63 °C (0.22 °C). The heat index reduction under cool roofs was 5–15 times greater than that under green roofs, with peak decreases exceeding 1.7 °C and spatial coverage exceeding 80–99% of the urban area. Surface energy balance analysis revealed that green roofs primarily redistributed energy from sensible to latent heat during the day, whereas cool roofs strongly suppressed sensible and ground heat fluxes via enhanced solar reflectance. The results indicate that across different urban forms, cool roofs provide stronger and more sustained urban heat mitigation in Sendai.