Effects of urban trees on mean wind, turbulence and momentum exchange within and above a realistic urban canopy

Accurate modeling of flow and turbulence within and above urban canopies is key to properly predict weather, hydrology, air quality and dispersion in urban environments. Trees are an integral part of the urban canopy, but their effects on wind, turbulence and evaporation is not typically accounted for in urban weather, hydrology, air pollution and dispersion models. Large-Eddy Simulations (LES) of flow over a realistic suburban canopy are used to gain insight into the contribution of vegetation to the overall momentum budget in the atmosphere within and above an urban canopy. The simulated areas are representative subsets of an urban surface in the City of Vancouver, BC, Canada where trees are slightly taller than the buildings. In this area, long-term wind and turbulence measurements are available from a 30 m meteorological tower. Data from airborne Light Detection and Ranging are used to represent both buildings and vegetation at high spatial resolution of 1 m in the LES. In the LES algorithm, buildings are ac- counted through a direct-forcing immersed boundary method, whereas vegetation is parameterized through a location-specific leaf area density, which results in an additional drag force balancing the imposed external pressure gradient. LES are performed including and excluding vegetation on different subsets of the surface, different wind directions and different leaf area density. Results compare well against corresponding tower measurements, validating the chosen tower location and the LES approach. Seasonal variations in vegetation result in up to 60% variation in the hydrodynamic roughness length z0 characterizing the surface experienced in the inertial sublayer. Conversely, within the urban canopy, trees effect are twofold: on one hand, they act as a direct momentum sink for the mean flow; on the other, they act as a barrier to vertical fluxes of mean kinetic energy from above, thus reducing the vertical momentum exchange rates and the intensity of mean flow at the heights where people live.