Numerical models of entrainment into sheared convective boundary layers evaluated through large eddy simulations

Atmospheric convective boundary layer (CBL) entrainment is the downward mixing of free atmospheric air into the CBL as the CBL grows. Primarily, the CBL entrainment is driven by the buoyancy production of turbulence kinetic energy (TKE) due to heating of the lower surface or cooling at the CBL top. The effects of shear-generated turbulence in the CBL are secondary. However, cases of a purely buoyancy-driven CBL are rare, and in many situations, the flux of buoyancy is relatively weak, allowing the effects of shear-generated turbulence to become equal to those of buoyancy-generated turbulence.

Large eddy simulation (LES), which can resolve most of the energy-containing motions in the CBL, has become a standard tool for fundamental studies of the CBL dynamics. However, the spatial and temporal resolution needed in the LES for an adequate reproduction of the CBL turbulence properties is still beyond the limits of numerical models commonly employed for weather prediction purposes, as well as in applied climate and air pollution research. In the present study, we use LES to evaluate predictions of entrainment into sheared CBL by numerical models of two types: (i) models with turbulence closure schemes based on Reynolds averaging, which resolve some vertical structure of the CBL and are commonly applied in numerical weather prediction (NWP) and (ii) models based on vertically integrated momentum, buoyancy, and TKE balance equations assuming a parameterized CBL vertical structure (the integral budget approach). The latter approach is widely used to predict integral CBL parameters (e.g., depth of convectively mixed layer) in applied atmospheric dispersion studies.

Focusing on the horizontally homogeneous CBL, the effects of shear on the CBL entrainment have been evaluated from LES of 24 CBL cases with various combinations of wind shear, atmospheric stratification, and surface buoyancy flux. The results have been analyzed with regard to the dynamics of entrainment as predicted by a variety of models with different turbulence closure schemes, models based on the integral budget approach, and atmospheric observations of the CBL entrainment.