Assessing urban heat island mitigation potential of realistic roof greening across local climate zones: A highly-resolved weather research and forecasting model study

Given their multifold benefits, green roofs are often considered to mitigate the urban heat island (UHI) effect. Most mesoscale studies consider 100 % green roof fraction or the same green roof fraction in each urban land use category while analysing the influence of green roofs on the UHI effect, which can overestimate their impact on UHI. Consequently, the impact of green roofs evaluated in these studies may not be suitable for informing policy decisions. Furthermore, the effect of morphologies on temperature reduction due to green roofs has not been previously studied. To address this gap, in this paper, we evaluate the impact of a realistic fraction of green roofs specific to the respective local climate zones (LCZ) on the UHI effect during a heatwave in Liège, Belgium, employing a high-resolution WRF study using the BEP-BEM parameterisation with LCZ land use classification. The realistic fraction is estimated for every LCZ class based on the average percentage of flat roofs observed in each LCZ class in Liège. Accordingly, distinct realistic fractions of green roofs are assigned to each LCZ class in WRF. We run the WRF simulation for the base scenario (without green roofs), extreme scenario (100 % green roof fraction), and realistic scenario. The results indicate a limited reduction in near-surface air and surface temperature in a realistic scenario, with a nighttime increase in temperature. Additionally, in the extreme scenario, the temperature reduction largely depends on the morphology. However, in a realistic scenario, it depends on the green roof fraction. Other indicators like heat index and UHI intensity also are not reduced considerably with realistic greening. Therefore, realistic roof greening alone will not be sufficient to achieve an impact on a city-scale.