Abstract
Entrainment is critical to the development of the atmospheric convective boundary layers (CBLs) and for the exchange of energy and substances between the boundary layer and free atmosphere. In this study, entrainment relationships are systematically evaluated from large‐eddy simulations (LESs) of shear‐free and sheared CBLs under a broad variety of atmospheric conditions. A total of 36 LES runs are conducted for varying free‐atmosphere stratifications, surface heat fluxes, and height‐constant geostrophic winds. It has been found that entrainment flux ratios have a common value of 0.2 regardless of the stratification of free atmosphere and the surface heat‐flux value when analysed within the zero‐order‐model (ZOM) framework for the shear‐free CBLs, whereas the actual (LES) entrainment ratios are not only stratification‐dependent, but also surface heat flux‐dependent. For the sheared CBLs, the entrainment‐flux ratios are substantially increased when the geostrophic winds are strong while they have a slight reduction under weak wind conditions. In the free‐encroachment regime, the dimensionless entrainment rates have been found to be approximately constant and surface heat‐flux independent, with the value in the range from 0.4 to 0.7 depending on the method used to evaluate the CBL depth.
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