Numerical and Laboratory Study of Horizontally Evolving Convective Boundary Layer. Part II: Effects of Elevated Wind Shear and Surface Roughness

Abstract

Modifications of turbulence regime in the sheared convective boundary layer (CBL) by a number of external nonbuoyant forcings are studied experimentally in a thermally stratified wind tunnel and numerically by means of large eddy simulation. This type of CBL is observed in the atmosphere when an originally neutral or stable air mass is advected over a heated underlying surface. Emphasis in the present study is laid on the effects of elevated wind shear and surface roughness on the structure and evolution of the CBL. For the flow cases, for which both numerical and wind tunnel results are available, the numerical predictions of mean flow parameters and turbulence statistics are found to be in good agreement with the experimental results. In the case of wind shear across the inversion layer, the authors distinguish between positive shear, when the flow above the inversion possesses a higher momentum than mean motion in the mixed layer, and the opposite case of negative shear. For the case of positive shear the growth of the CBL is found to be impeded compared to the shear-free case. Negative shear has an opposite effect on the CBL evolution. In this case, the damping of thermals by stable stratification in the inversion layer is weakened compared to the shear-free case and consequently entrainment is activated. A physical explanation for such a directional effect of elevated shear is suggested. In the case of enhanced bottom roughness, both experiments and numerical simulations provide the evidence of slightly larger CBL growth rate compared to the CBL over a relatively smooth surface with a 10 times smaller roughness length.