- Advanced mitigation technologies under heat-wave conditions.
- Maximum cooling effect in the case of ‘cool/reflective’ roofs and roads.
- Increase of latent heat by green roofs and a cooling effect during the night.
- Cool/reflective and green roofs result in changes of air motion and circulation.
- Deciduous broadleaf street trees have an evident reduction in surface temperature.
We assess the impact of advanced heat-mitigation technologies in a coastal temperate city under heatwave conditions. For the first time urban-heating mitigation scenarios that refer to ‘cool/reflective’ roofs and roads, ‘green/living’ roofs and shading by replacing low urban vegetation with deciduous broadleaf trees are considered at the highly dense-populated city of Athens (Greece). Numerical simulations are performed for a typical see-breeze and a heatwave day with the Weather Research and Forecasting (WRF) model coupled to an urban-canopy model. Highresolution data on vegetation and urban land use, derived from satellite image analysis, are considered. All scenarios show a cooling effect, with the maximum mean daytime temperature reduction in the case of ‘cool/reflective’ roofs and roads. During daytime, the mean ambient-temperature reduction reaches up to 1 °C while for the surface-temperature up to 9.5 °C and 11.5 °C, on the see-breeze and heatwave day respectively. In the case of ‘green/living’ roofs, the mean daytime latent-heat flux is increased (e.g. up to 140 W/m2 on the heatwave day) due to increased evapotranspiration while the surface temperature is more affected during nighttime. Both scenarios result in a sea-breeze attenuation of 0.5–1 m/s. The presence of deciduous broadleaf street trees has a minor impact on mean ambient temperature but an evident reduction in surface temperature. The mean urban-heating reduction ranges from 0.1 °C to 0.8 °C and from 0.3 °C to 1.7 °C during the sea breeze and heatwave day respectively, with the maximum reduction shown in ‘cool/reflective’ roofs and roads and the minimum in ‘shading trees’ scenarios.