The dynamics of nonstationary entrainment in the shear-free convective boundary layer (CBL) developing in discretely stratified atmosphere has been recently studied by Fedorovich and Conzemius (2001). Other preceding studies, such as Moeng and Sullivan (1994), have investigated the turbulence statistics in the sheared CBL developing in a homogeneously (linearly) stratified atmosphere. The current study extends both of these analyses by examining the dynamics of convective entrainment in the atmosphere, which is vertically heterogeneous with respect to both thermal stratification and wind shear. The main tool of investigation employed is the numerical large eddy simulation (LES), however some relevant laboratory and atmospheric data are also considered. The conducted simulations encompassed much of the natural variability of stratification and shear in the lower troposphere. The paper will discuss inter-relationships between parameters of entrainment for shear cases, compared to those of shear-free cases and will focus on departures of entrainment from equilibrium entrainment in cases in which wind speed and/or temperature stratification rapidly change at a specified level in the atmosphere. The presence of shear greatly alters the structure of temperature and wind shear within the entrainment zone, and these changes in structure will be examined. Effects that could be related to shear sheltering (Hunt and Durbin 1999) are observed in the simulations at early stages of the CBL development.

Fedorovich, E., and R. Conzemius, 2001: Large-eddy simulation of convective entrainment in linearly and discretely stratified fluids. Direct and Large-Eddy Simulation IV, B. J. Geurts et al., Eds., Kluwer, 435-442. Hunt, J. C. R. and P. A. Durbin, 1999: Perturbed vortical layers and shear sheltering. Fluid Dyn. Res., 24, 375-404. Moeng, C. H., and P. P. Sullivan, 1994: A comparison of shear- and buoyancy-driven planetary boundary layer flows. J. Atmos. Sci., 51, 999-1022.