How the city shapes the shade: the influence of urban and tree morphology on modelled daily mean radiant temperature

Shading is a key factor for mitigating heat stress on hot summer days. This study evaluates how modelled street tree cooling efficiency (CE) is shaped by interactions between urban morphology and tree characteristics. CE is defined as the reduction in mean radiant temperature (ΔTmrt) relative to street tree canopy coverage, enabling standardized comparisons across diverse urban configurations. Using the SOLWEIG model, Tmrt is simulated at meter-scale resolution during 24 h of the hottest day of 2023 in a modified street canyon in Ghent, Belgium.

A distribution for three urban morphological parameters is derived from city-center inventories (street widths 10 - 20 m; building heights (0 - 20 m);  4 orientations). Similarly, realistic tree structural variability is determined for three parameters: canopy heights (10 - 16 m, derived from an inventory of Terrestrial LiDAR scans of 346 street trees), trunk heights (0 - 8 m), and shortwave transmissivities (0 - 40%). A chosen street and tree were modified across these morphological ranges and repeatedly used as simulation input.

Preliminary results indicate for urban morphology, daily averaged Tmrt decreased exponentially with higher street height-to-width (H/W) ratios. North-South oriented streets exhibit 2.5x the decay rate of East-West streets. Moreover, by adding the same singular tree to different street canyons, the resulting tree’s CE correlated strongly positively with street width (R² = 0.97). Regarding tree morphology, vertical canopy distribution significantly impacts CE: extending the lower canopy (the space between the ground and start of canopy) has a 3.4 larger impact on CE compared to equivalent height extensions of the upper canopy. Finally, increasing vegetation transmissivity (i.e., less dense foliage) by 1% reduced CE by 1.3%.

We demonstrate a sensitivity analysis framework applied to the urban morphology of Ghent, revealing how tree parameterization affects Tmrt and the implications for thermal comfort and urban tree management.