Retrofitting parking lots to mitigate heat stress in an urban park: A numerical case study at microscale

Urban forests and vegetated parks in cities, such as in the St. Helene Island in St. Lawrence River in Montreal, may help to mitigate heat stress during heatwaves. In this paper, we study how retrofitting scenarios that increase canopy cover in parking lots via tree planting can improve the thermal comfort for pedestrians. Specifically, we investigate the influence of tree size and placement within the parking lot sites on the thermal comfort in four retrofitting scenarios. We use the numerical suite of models urbanMicroclimateFoam, which simulates microclimate conditions at a local scale (submeter scale) solving Reynolds-averaged Navier-Stokes (RANS) equations for non-isothermal air flow, heat and moisture transport and storage in porous urban materials, solar radiation and longwave radiative exchanges between sky and urban surfaces and vegetation. The canopy of the park is reconstructed from LiDAR data allowing for simulation of transpirative cooling, shading and wind drag effects due to vegetation. We base our simulation on a 2020 heat wave that occurred in the Montreal region. Simulation of the current situation shows strong thermal heat stress with UTCI (Universal Thermal Climate Index) values up to 37°C. The retrofit scenarios explore the effect of replacing the pavement of two parking lots with grass, and the addition of trees in different arrangements, varying their size and position. We found that replacement of part of the asphalt pavement with grass yields an overall UTCI reduction of 1.5 °C. Scenarios adding trees in different arrangements improve the thermal comfort locally by cooling up to 6.5 °C. The reduction in UTCI is shown to be most importantly caused by a reduction of mean radiant temperature due to shading from the added trees in the area.