Modelling the influence of the car-induced momentum and heat on the microscale wind dynamics in the street canyon

The well-being of citizens in urban areas is significantly affected by the air quality and thermal comfort in street canyons. The air quality is strongly dependent on local transportation emissions and street-canyon ventilation. This ventilation is affected by street-canyon characteristics, e.g., the presence of trees, and by meteorological conditions and local forcings, including the wind flow and turbulence affected by moving cars, as well as the heat produced by the cars which promotes the convective flow. The human thermal comfort, which can be expressed using thermal comfort indices such as the Universal Thermal Comfort Index (UTCI), is also affected by the speed of the wind which aids in cooling via perspiration. The microscale meteorological model PALM can simulate both the air quality and the thermal comfort at street-canyon scale, and its recently added PALM Traffic Module provides a simple, parameterised way to incorporate car-related processes. Based on traffic flow characteristics, it calculates the induced drag forces and adds them to the model dynamics as additional airflow tendencies, without the need to resolve the exact shape and spatiotemporal position of each individual car. It also considers the heat produced by the vehicles and transforms it into additional tendencies of air potential temperature. In this study, we present the influence of car-related processes on street-level pollutant concentrations under different meteorological conditions. We also show that these processes have a significant impact on air quality under stable conditions, although their influence is negligible under windy and convective conditions. We will present preliminary results of the comparison of the modelled wind tendency with field observations in a real street canyon.