Quantifying the Cooling Effects of Urban Trees on Pedestrian Heat Stress in a Dense Mediterranean Urban Environment

Urban heat stress is an escalating concern in Mediterranean cities, which are climate-change hotspots, and is associated with increased mortality and adverse health outcomes. These risks highlight the need for evidence-based strategies to reduce thermal exposure in dense urban environments, with urban trees representing a key nature-based mitigation measure. Computational Fluid Dynamics (CFD) is widely used to study urban microclimates; however, many studies rely on idealized canyon geometries, and only a limited number incorporate thermophysiological indices to assess how urban microclimates influence human thermal perception¹. This study uses a coupled CFD–SOLWEIG framework with UTCI-based assessment to enable integrated evaluation of urban microclimate and pedestrian thermal stress. High-resolution simulations were conducted and validated in a dense Mediterranean urban fabric in the historical centre of Nicosia (Cyprus) using ANSYS Fluent. Unsteady Reynolds-Averaged Navier–Stokes simulations were performed over a 24-hour summer period using detailed urban geometry to resolve airflow and air temperature, while mean radiant temperature was calculated using the SOLWEIG² radiative model. These variables were combined in the UTCI-Fiala model to estimate UTCI at pedestrian level³. Urban trees were represented as porous canopy volumes to account for aerodynamic drag, shading, and evapotranspirational cooling effects and to quantify their mitigation role. Simulation results were validated using field observations, yielding mean absolute deviations of 1.48°C for air temperature (R²=0.89) and 0.68m s⁻¹ for wind speed (R²=0.67). UTCI exhibits a pronounced diurnal cycle driven by solar radiation, increasing from below 30°C in the early morning to above 45°C around midday. Even isolated urban trees can locally mitigate thermal stress by reducing radiative loading and air temperature through shading and evapotranspiration. UTCI reductions reach up to 5.1°C during peak solar hours at pedestrian level beneath the tree canopy, while evapotranspiration alone contributes reductions of up to 2.4°C under shaded conditions. Vegetation also generates localized cooling zones extending downstream up to three times the tree crown width, depending on local wind conditions. These results highlight the capacity of urban trees to significantly reduce pedestrian thermal stress and demonstrate the potential of the CFD–SOLWEIG–UTCI framework to support climate-responsive urban planning in dense built environments, providing quantitative evidence to inform heat mitigation strategies and urban design.

1. Antoniou, N., Montazeri, H., Blocken, B. & Neophytou, M. K. A. On the impact of climate change on urban microclimate, thermal comfort, and human health: multiscale numerical simulations. Build. Environ. (2024).
2. Lindberg, F., Holmer, B. & Thorsson, S. SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and mean radiant temperature in complex urban settings. Int J Biometeorol 52, 697–713 (2008).
3. Fiala, D., Havenith, G., Bröde, P., Kampmann, B. & Jendritzky, G. UTCI-Fiala multi-node model of human heat transfer and temperature regulation. Int J Biometeorol 56, 429–441 (2012)