Dynamics of Sheared Convective Boundary Layer Entrainment. Part I: Methodological Background and Large-Eddy Simulations

Abstract

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 Ϫ1 (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.

Convective boundary layer wind dynamics and inertial oscillations: the influence of surface stress

“…This can also be seen in large-eddy simulations (LES) of such boundary layers. For cloudless CBLs see Pino et al (2006) and Conzemius and Fedorovich (2006), and for shallow cumulus convection see Siebesma and Cuijpers (1995).…”

“…Assuming a well-mixed layer, both processes -surface friction and momentum entrainment -can be considered as being driven by both CBL convection and local shears in the surface layer and the entrainment zone, respectively (Conzemius and Fedorovich, 2006;Pino and Vilà-Guerau de Arellano, 2008). This suggests that the shear dependence of turbulent surface friction and entrainment can be characterized in a similar way (Conzemius and Fedorovich, 2007).…”

Sheared convective boundary layers: turbulence kinetic energy and entrainment dynamics

“…Indeed, also quasistationary sheared CBLs or fully neutral BLs 60 have been frequently observed and analyzed (more recently by, e.g. Conzemius and Fedorovich, 2006a;Jonker et al, 2013). Therefore we can, in analogy to the pure CBL, break down our analysis in at least two different parts.…”

“…However a more recent paper by Conzemius and Fedorovich (2006b) showed that this is not yet or at least not yet fully the case. In this paper the authors reviewed a couple of models on basis of a rather complete set of LESs (Conzemius and Fedorovich, 2006a). Many of the models performed surprisingly poor 61 and none was fully convincing.…”

“…Except for the purely convective reference simulations, the atmosphere in all our LESs is driven by a horizontal pressure gradient that is constant in time and space 7 , and a Coriolis force that mimics the constant planetary rotation (as in e.g. Moeng and Sullivan, 1994;Conzemius and Fedorovich, 2006a). As common in BL-meteorology we consider the pressure forcing in terms of the geostrophic wind U g , which is the equilibrium velocity of the frictionless horizontal wind in the case of straight isobars (Eq.…”