This work investigates the turbulent boundary layer through a realistic urban geometry, representing a 1:200 scale model of the Marylebone area in central London (UK). The work forms part of a larger research effort by the DAPPLE¬ consortium involving field and wind-tunnel experiments, computational modelling, as well as operational model evaluation. Computational Fluid Dynamics (CFD) is used in this work as a fit-to-purpose application to identify the salient features of the wind flow within the urban area and to provide appropriate parameterizations for use in operational models. Profiles of mean velocity and individual turbulence components are examined. The time-averaged flow field in sheltered regions was found to be dominated by large vortical flow patterns whose form varied with the local packing density of the urban geometry. It is suggested that this form of vortices may influence the effectiveness of upward mixing within the canopy. It is further shown that the use of an exchange velocity between the in-canopy and above-canopy air flows is also applicable in the case of complex geometries, and that the definition of a “breathability” property of the canopy may be used for sustainable design purposes.