Atmospheric turbulence over cities

This paper presents a comprehensive, critical review of turbulence observations over cities and is the first review of its kind. Over 60 studies spanning more than 5 decades have been examined. The main results are based on 14 high quality experiments which met criteria based on stringent experimental requirements. The observations are presented as non-dimensional statistics to facilitate comparison between urban studies and work conducted over other rough, inhomogeneous surfaces. Wake production associated with bluff bodies and the inhomogeneous distribution of sources and sinks of scalars, result in a roughness sub-layer which for the studies reviewed extends to about 2.5 to 3 times the height of the buildings. It is shown that within this region the basis of several traditional micrometeorological approaches to describe the turbulent exchange is in doubt.

The present contribution explores the possibilities of interpreting the urban turbulence characteristics within the framework of a plane mixing-layer. The latter was successfully used by Raupach et al. (1996) to explain the flow over plant canopies which differs in several ways from turbulence in the surface layer. A mixing layer forms when two co-flowing streams of different velocities mix (e.g. flow within and above a canopy). Variations observed between urban and vegetation data are probably related to differences in the density of the individual roughness elements.