Urban tree cooling and thermal stress mitigation in a dry mediterranean climate: Coupled CFD–SOLWEIG modelling with UTCI assessment

Urban heat stress is an escalating concern in Mediterranean cities, where dense morphology and limited vegetation increase surface heat storage, reduce evaporative cooling, and intensify thermal stress. This study quantifies the microclimatic impact of urban trees on pedestrian thermal stress in the historical centre of Nicosia (Cyprus), focusing on vegetation-induced cooling effects under thermally critical clear-sky hot-summer conditions. The analysis combines Unsteady Reynolds-Averaged Navier-Stokes simulations with detailed urban geometry and porous-media representations of tree canopies, coupled with the SOLWEIG radiative model. Vegetation-induced shading and evapotranspirational cooling were explicitly parameterized. Validation against high-resolution field observations yielded mean absolute deviations of 1.48 °C for air temperature ( R 2 = 0.89 ) , and 0.68 m/s for wind speed ( R 2 = 0.67 ) . Results show that vegetation reduces Universal Thermal Climate Index (UTCI) by an average of 5.1 °C during peak solar hours (14:00–16:00), through the combined effects of shading and evapotranspiration. During building-shaded morning hours UTCI reductions of 2.4 °C were attributed to evapotranspirational cooling, while under direct solar exposure (11:00–13:00) reductions decreased to 1.7 °C due to increased radiative loading. Vegetation created localized cooling zones extending downstream up to three crown widths, depending on wind conditions. Diurnal thermal stress patterns revealed transitions from moderate to extreme heat stress with increasing solar exposure. These findings highlight the capacity of vegetation to significantly reduce thermal stress, with notable thermal stress relief observed even from isolated trees. The validated CFD–SOLWEIG-UTCI framework provides a transferable tool for quantifying vegetation-induced cooling and supports the evidence-based design of climate-resilient, nature-based solutions in dense urban environments.