Turbulence close to a rough urban surface part II: Variances and gradients

Abstract

Measurements of the turbulent wind and temperature fluctuations were carried 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 available. 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 (atz=23 m), i.e., fully within the urban roughness sublayer. The scaled variances of temperature and wind velocity, as well as the non-dimensional gradients of wind speed and temperature, are presented and discussed in terms 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 are presented that are valid for all measurement positions; they compare well with results from other urban studies. The non-dimensional gradient of mean wind speed is found to be well represented by the semi-empirical functions for the inertial sublayer if locally scaled. At 5 m above roof level, however, the variability due to horizontal inhomogeneity becomes very large. The non-dimensional temperature gradient, on the other hand, is height dependent and not well defined over the present rough urban surface.