On the value of the von Kármán constant in the atmospheric surface layers over urban surfaces

Abstract

Large-eddy simulations of turbulent boundary flow over different urban-like surfaces in neutrally stratified conditions are performed to study the atmospheric surface layer characteristics over urban areas. A simple method based on the vertical profile of the dimensionless velocity gradient is used to identify the inertial sublayer (ISL) in all cases. The results show that the mean wind profiles in the different urban ISLs can all be generally described by the logarithmic wind profile. However, for the idealized urban surfaces composed of regular building block arrays and the realistic urban surface case, the von Kármán constant (κ) in the ISL is found to be significantly smaller (in the range of 0.21–0.27) than the typically adopted value of 0.4. As κ is a key constant in the parametrizations of urban surface flows and is usually assumed to be ≈0.4 in existing surface parametrization models, a significantly smaller κ implies that large uncertainties may exist in the predictions of the momentum, heat, and scalar turbulent fluxes over urban surfaces using these models.