Scaling of Mean Flow and Turbulence in the Urban Canopy Layer

Flow and turbulence data collected during a yearlong experiment in a street-canyon configuration located in suburban terrain are analyzed. The instrumentation included 13 sonic anemometers deployed on two masts within the street canyon and on three masts at the building roofs. The results confirm the strong dependence of in-canyon flow structure on the above-roof wind direction. While channeling along the street dominates for most wind directions, recirculation patterns develop for narrow sectors with above-roof wind directions close to perpendicular to the street. For these cross-flow scenarios different scaling velocities are tested and the influence of stability is investigated in more detail. Similar to previous studies, our findings confirm that a single velocity scale that unifies both mean flow and turbulence properties inside the canyon does not exist. Turbulence properties scale best with the friction velocity at the upwind roof but scaling with mean wind speeds measured at the upwind roof or at an operational meteorological station 5-km away from the study area, resulted in comparable or even better statistics for the mean flow parameters. Turbulence kinetic energy levels in the shear-layer region at roof layer varied depending on the upwind fetch and larger scale stability. As turbulence is actively transported from the shear-layer into the canyon region, the in-canyon flow structure also varies as a function of upwind fetch and stability.