Microscale Urban Flow Simulations with Realistic Distributions of Surface Thermal Forcing

Better understanding of the physical processes within the urban canopy is important for diverse applications related to urban climate, air quality and pedestrian thermal comfort. Urban micrometeorology is determined by interactions between the atmosphere and urban surfaces. These interactions result in complex flow fields and heterogeneous distributions of temperature and pollutants within the cities. Surface-atmosphere interactions may be classified as mechanical (blocking, deviating and slowing down the flow) or thermal (buoyancy forces due to heat exchange between the atmosphere and street and building surfaces). Urban surface heat fluxes are responsible for the temperature distribution in the canopy air, which is heterogeneous and depends on several factors such as urban geometry, solar position, etc. Microscale simulations usually neglect thermal interaction, or it is included in a simple way by setting only one facet to a different (but constant) temperature with respect to the other surfaces. In this study, microscale simulations using a RANS model are carried out over a periodic array of cubes with a packing density of 0.25. Thermal effects are analysed by imposing realistic distributions of heat fluxes as boundary conditions at building and street surfaces. The microscale heat flux distributions are computed by the TUF3D model (Krayenhoff and Voogt, BLM 2007). Several scenarios are studied to explore the effects of different solar positions and ratios of buoyancy to dynamical forces. The main objective is to determine the impacts of “realistic” distributions of urban surface heat fluxes on airflow properties. Flow and temperature distributions within the street canyon are analysed. In addition, the impact of the microscale thermal forcing on the spatially averaged flow properties is also discussed. This information can be useful for air pollution dispersion models and urban canopy models to parameterize processes that are subgrid relative to typical mesoscale model grid resolutions (e.g., drag forces induced by buildings).