Dynamics of sheared convective boundary layer entrainment. Part I Methodological background and large-eddy simulations

The reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer (CBL) when wind shear is present. The sheared CBL can be studied by means of direct measurements in the atmosphere, laboratory studies, and numerical techniques. The advantages and disadvantages of each technique are discussed in the present paper, which also describes the methodological background for studying the dynamics of entrainment in sheared CBLs. For the reported study, large-eddy simulation (LES) was chosen as the primary method of convective entrainment investigation. Twenty-four LES runs were conducted for CBLs growing under varying conditions of surface buoyancy flux, free-atmospheric stratification, and wind shear. The simulations were divided into three categories: CBL with no mean wind (NS), CBL with a height-constant geostrophic wind of 20 m s-1 (GC), and CBL with geostrophic wind shear (GS). In the simulated cases, the sheared CBLs grew fastest, relative to the NS CBLs, when the surface buoyancy flux was weak and the atmospheric stratification was moderate or weak. Three fundamental findings resulted from the investigated CBL cases: (i) the entrainment zone shear is much more important than the surface shear in enhancing CBL entrainment, although entrainment zone shear is indirectly affected by surface shear; (ii) the sheared entrainment zone features a sublayer of nearly constant flux Richardson number, which points to a balance between shear production and buoyancy consumption of turbulence kinetic energy (TKE) that regulates entrainment; and (iii) the fraction of entrainment zone shear-generated TKE spent on the entrainment is lower than suggested by earlier studies.