Effect of roughness inhomogeneities on the development of the urban boundary layer

Abstract

Wind tunnel measurements over a regular array of cubes were carried out to investigate the influence of surface inhomogeneities on the development of the urban boundary layer. Two idealised cases were investigated. The first one was in terms of a strong step change in roughness from very smooth to very rough with a low turbulent approach flow and a thin approach boundary layer. For this case it was found that a very long fetch was needed until the flow reaches equilibrium with its underlying surface. In the second case the approach boundary flow was turbulent and equilibrium was reached much faster, but the necessary fetch was still significant. The results were compared and conclusions for roughness parameterisations in numerical models were drawn.