Vertical transport of heat and momentum across a sheared density interface at the top of a horizontally evolving convective boundary layer†

The turbulence regime in a quasi-stationary, horizontally evolving, and sheared boundary layer with bottom buoyancy forcing has been studied numerically by means of large eddy simulations (LES) in conjunction with its experimental investigation in a laboratory wind tunnel. The atmospheric prototype of the investigated boundary layer is commonly observed in the earth’s atmosphere during daytime conditions. In meteorology, a boundary layer of this kind is usually called the convective boundary layer (CBL). The case studied of a horizontally evolving CBL corresponds to the boundary layer flow that develops in a stably or neutrally stratified air mass advected over a heated underlying surface. A characteristic feature of the CBL case studied is the presence of wind shear across the capping temperature inversion (density interface) at the CBL top. This elevated wind shear affects the CBL turbulence structure in combination with the shear and buoyancy sources at the underlying surface. This study has focused on the convective entrainment and the CBL growth dynamics under the combined influence of all mentioned forcings. It has been found that entrainment of momentum across the sheared inversion can accelerate or decelerate the mean flow in the main portion of the CBL depending on the sign of elevated shear. The induced flow convergence/divergence below the inversion leads to local organized ascending/descending motions that considerably modify the CBL growth rate. The turbulence structure throughout the whole CBL is also noticeably affected by these motions. PACS numbers: 92.60.Fm, 47.27.Eq, 47.27.Nz, 47.27.Te † This article was chosen from selected Proceedings of the Eighth European Turbulence Conference (Advances in Turbulence VIII (Barcelona, 27–30 June 2000) (Barcelona: CIMNE) ed C Dopazo. ISBN: 84-89925-65-8). c ©2001 IOP Publishing Ltd PII: S1468-5248(01)22931-2 1468-5248/01/000001+17$30.00 oT 2 (201) 07 Vertical transport of heat and momentum