The role of the shear on the growth rate of a convective boundary layer over land is studied with detailed observations and large-eddy simulations (LES). Typically, it is found that under pure convective conditions, without shear, the entrainment buoyancy flux at the inversion is about -20% of the surface buoyancy flux. This "universal" value is widely used for entrainment rate closures in general circulation models (GCMs). In this paper the role of wind shear at the surface and inversion is investigated using observations and LES. The observations were obtained on 20th June 1997 at the Southern Great Plains (SGP) Atmospheric Radiation Measurements (ARM) site.

Based on the observations initial vertical profiles of potential temperature and specific humidity and (time depended) surface fluxes of heat and moisture for the simulations were constructed. In the observations high wind speeds were observed with significant shear at the inversion. Therefore, three LES simulations were performed with different geostrophic winds: a pure surface buoyancy driven case (B) without geostrophic wind, a case with constant geostrophic (BG) wind with height and a case with geostrophic shear (BGS) near the inversion. The B and BGS cases are compared in detail with the observed boundary layer parameters.

We have compared the evolution of vertical profiles and boundary layer height for the three simulations with the observed values. The role of the wind shear can be quantified by calculating the ratio of the entrainment buoyancy flux to the surface buoyancy flux, which is -0.2, -0.25 and -0.33 for the B, BG and BGS simulations, respectively. The analysis of the Turbulence Kinetic Energy (TKE) budget shows that the inclusion of wind shear at the surface and entrainment zone modifies the vertical profile of all terms in the TKE budget. In particular, for the BGS simulation, the turbulent transport term at the inversion almost vanishes and is replaced by the shear production term. With the LES results we also validate parameterizations of the entrainment buoyancy flux in terms of the surface buoyancy flux, friction velocity at the surface and the wind shear at the inversion.