Entrainment into sheared convective boundary layers as predicted by different large eddy simulation codes

There is no consensus in the boundary-layer research community regarding the role played by wind shears in the dynamics of turbulent entrainment – a complex physical process that is the driving mechanism of convective boundary-layer (CBL) development. Previous numerical studies of sheared CBLs were primarily focusing on the turbulence regimes associated with quasi-stationary CBL evolution, when entrainment and the CBL growth dynamics were not regarded as main issues. In the present study, the emphasis is specifically laid on the dynamics of entrainment as related to the CBL growth.

Some existing theories predict attenuation of vertical turbulent transport across the entrainment layer in the presence of mean flow shear. On the other hand, most of numerical simulation studies conducted to date pointed to the overall enhancement of CBL growth in the presence of either surface or elevated wind shear, or with combination of both shears. For the present study, we have brought together several numerical codes built on the large eddy simulation (LES) methodology in an attempt to investigate basic features of entrainment dynamics in sheared CBLs as reproduced by LES codes of different architecture, with different numerical algorithms, and using different subgrid-turbulence and near-wall parameterizations.

All codes participated in the study were run in the simulation domain of 10 by 10 by 2 km for three CBL cases with the same buoyancy flux at the surface and the same thermal stratification in the free atmosphere above the CBL, but with three different shear forcings: (1) with no mean shear (NS case, which was the reference case), (2) with height-constant geostrophic wind (GC case), and (3) with geostrophic wind linearly growing away form the surface (GS case). All codes have indicated that wind shears in the CBL and across the entrainment zone (i) induce vertical transport in the upper portion of the CBL, (ii) modify the CBL turbulence regime in general, and (iii) enhance boundary-layer growth compared to the case of shear-free CBL. However, particular turbulence statistics, especially those of higher orders, have been differently represented by different codes. In the paper, modifications of particular entrainment quantities by wind shears will be analyzed, and shear-induced changes in turbulence statistics of different kinds will be demonstrated, also in relation to available experimental data on turbulence parameters in sheared CBLs.