Local similarity relationships in the urban boundary layer

Abstract

To investigate turbulent structures in an urban boundary layer (UBL) with many tall buildings, a number of non-dimensional variable groups based on turbulent observations from a 325-m meteorological tower in the urban area of Beijing, China, are analyzed in the framework of local similarity. The extension of surface-layer similarity to local similarity in the stable and unstable boundary layer is also discussed. According to local similarity, dimensionless quantities of variables: e.g., velocity and temperature standard deviations σi/u*l (i = u,v,w) and σT/T*l, correlation coefficients of uw and wT covariance, gradients of wind and temperature Φm and Φh, and dissipation rates of turbulent kinetic energy (TKE) and temperature variance Φε and ΦN can be represented as a function only of a local stability parameter z/Λ, where Λ is the local Obukhov length and z is the height above ground. The average dissipation rates of TKE and temperature variance are computed by using the u spectrum, and the uw and wT cospectra in the inertial subrange. The functions above were found to be in a good agreement with observational behaviour of turbulence under unstable conditions, but there were obvious differences in the stable air.