TURBULENCE CLOSE TO A ROUGH URBAN SURFACE .2. VARIANCES AND GRADIENTS

Measurements of the turbulent wind and temperature fluctuations were c arried out in the vicinity of the roof level, over an urban surface at a site where mean gradients of wind speed and temperature were also a vailable. The measurement heights were about 23 and 28 m above ground, the local roof level being 18 m. Measurements were taken on top of a building (at z = 23 and 28 m) and over a street canyon (at z = 23 m), i.e., fully within the urban roughness sublayer.The scaled variances o f temperature and wind velocity, as well as the non-dimensional gradie nts of wind speed and temperature, are presented and discussed in term s of departures from Monin-Obukhov similarity theory. Local scaling is found to be a useful concept for the description of turbulence within a roughness sublayer. Expressions for the scaled velocity variances a re presented that are valid for all measurement positions; they compar e well with results from other urban studies. The non-dimensional grad ient of mean wind speed is found to be well represented by the semi-em pirical functions for the inertial sublayer if locally scaled. At 5 m above roof level, however, the variability due to horizontal inhomogen eity becomes very large. The non-dimensional temperature gradient, on the other hand, is height dependent and not well defined over the pres ent rough urban surface.