High-fidelity simulation of the effects of street trees, green roofs and green walls on the distribution of thermal exposure in Prague-Dejvice

We investigate the heat stress mitigation potential of greening strategies in Prague using a configuration of the PALM-4U model that has been rigorously evaluated with measurements. Three greening scenarios were evaluated using the Universal Thermal Climate Index (UTCI). The UTCI reduction effect of broad-leaf or coniferous trees in a complex urban environment was found to be strongly local, with minor domain-average UTCI reductions; −4.1K under tree crowns and −0.6K on average in the neighbourhood as a day-time average, peaking at about twice these values near midday. During daytime the UTCI reduction potential of trees increases with the intensity and duration of solar exposure; −15.1K is the spatial maximum across all scenarios. For trees fully shaded by buildings, UTCI reduction was low (−0.5K as maximum). Tree planting reduces air temperature by more than 5K in some locations under trees, and reduces neighbourhood-average air temperature by up to 0.3K, with cooling peaking in the early evening about 8 h after the corresponding peak in UTCI reduction. Results emphasize the highly localized microclimate effects of trees for pedestrian thermal exposure reduction. The combination of green walls and roofs yielded negligible results in terms of UTCI reduction and only small air temperature effects.