Boundary layer development over a heterogeneous urban canopy

Due to the heterogeneous nature of an urban surface, the form of the vertical profile of windspeed and turbulence measured at any point is highly dependent on the extent of the upstream fetch over a consistent type of surface roughness. The roughness sublayer extends to several times the building height, and the inertial sublayer may not be well-developed due to close proximity to a roughness change. Detailed measurements of the wind profile are desirable when considering the dispersion of pollutants from the urban surface and how this depends on wind direction.

During the Salfex urban observation campaign, which took place in Salford, Greater Manchester, in May 2002, a sodar was deployed alongside radiosonde ascents, Doppler lidar, and surface turbulence measurements within a street canyon using sonic anemometers. The aim was to investigate the urban boundary layer at a wide range of scales, and to determine the strength of coupling between flow at street level and the boundary layer above. Sodar profiles of wind and turbulence up to 200m were made and analysed in terms of the surface cover upstream of the measurement site.

Vertical profiles of mean windspeed showed a logarithmic layer extending up to approximately 70m, which is consistent with the development of an internal boundary layer from the start of the urban fetch, approximately 7.5km upstream. However, the measured angle of the flow indicated a downward tilt which decays with height, with a heightscale of 40m. Given that the measurements were made in an area of no significant topography, this suggests that another mechanism was causing perturbation to the flow. An analysis of surface form revealed that the urban canopy was “patchy”, on a lengthscale of several hundred metres. Whilst existing blending height theory suggests that changes in roughness of this scale would be rapidly blended out by diffusive processes, changes in canopy on this scale appear to cause perturbations to up to several times the canopy height due to displacement of the flow, as a consequence of the canopy drag.