Evaluation of entrainment velocity from a First-order model and a Zero-order model with Large-eddy simulations and observations

The entrainment is a crucial process in the growth of the convective boundary layer (CBL). This process drives the exchanges of heat, humidity and of other scalars, such as ozone or dimethyl sulphide, between the boundary layer and the free troposphere. By definition the entrainment velocity estimates the engulfment within the CBL of air from the troposphere. This study focuses on the entrainment velocity parameterization. Several LES simulations and observations from various field experiments combined to compare two jump-models parameterization for various types of boundary layers. To parameterize the entrainment-layer, the most basic approach represents the entrainment zone as a sharp discontinuous inversion. This representation is referred to as a zero-order jump model (ZOM). Nevertheless, the use of large-eddy simulation (LES) models has revealed that the ZOM approach is often insufficient to reproduce real conditions and that the interface layer can be described in a more realistic way by the so-called first-order jump model (FOM). The entrainment region is then assumed to have a finite thickness. The investigation presented here is based on a set of 7 LES cases (from ARM, BOMEX, AMMA, IHOP, FIRE cases) and 4 field experiments (AMMA, IHOP, Cabaw, DYCOMS-II), covering various meteorological boundary layer types. It shows that the FOM jump model is appropriate for all convective boundary layers used in this study. The ZOM can fairly be used in case of sharp thin inversion, like those found in stratocumulus-topped boundary layers.