Climate-dependent impact of vegetation on thermal comfort in urban neighborhoods through resolved CFD simulations

Given the urban heat island effect and extreme events induced by climate change, studying the urban microclimate and mitigation strategies is essential, particularly to enhance pedestrian thermal comfort.
Vegetation has demonstrated its effectiveness in mitigating urban heat at local scale but its effect on thermal comfort has still to be accurately studied across various climates and urban configurations. Vegetation improves pedestrian thermal comfort through shading and evapotranspiration. However, vegetation can deteriorate thermal comfort by increasing the relative humidity and impeding wind flow and nocturnal heat removal.
In this study, the urban microclimate is simulated using the high-fidelity CFD model urbanMicroclimateFoam based on OpenFOAM. This solver solves successively turbulent air flow, heat and moisture transport in solid materials, and radiation exchanges. Environmental boundary conditions are dynamically downscaled from Weather Research and Forecasting (WRF) mesoscale results. The impact of vegetation is analyzed across three distinct climates: a continental climate in Montreal, Canada, a tropical climate in Singapore, and an arid climate in Morocco. 
Trees yield local improvements in all three cases, while the highest cooling potential is observed for the arid climate context. Conversely, in tropical climate, vegetation air cooling is offset by the increase in humidity, resulting in a reduced thermal comfort impact. On a daytime average, vegetation in Montreal reduces UTCI locally by up to 6°C, with a non-local adverse heating effect of 2°C. In Singapore, the local cooling effect evaluated reaches 5°C UTCI, with non-local increases up to 3°C, while in Morocco, vegetation achieves a local improvement of UTCI by 7°C, with non-local adverse effects limited to 1°C. The primary factor contributing to pedestrian thermal comfort improvement in all climates is shading provided by trees.
Urban planners and stakeholders can integrate such valuable insights to harness the benefits and challenges of vegetation across diverse climatic contexts.