Small-Scale Processes and Entrainment in a Stratocumulus Marine Boundary Layer

Abstract

Numerical studies of boundary layer meteorology are increasingly reliant on large eddy simulation (LES) models, but few detailed validation studies of these types of models have been done. In this paper the authors investigate the behavior of an LES model for simulation of a marine boundary layer. Specifically, the authors focus on the mechanisms that control numerical predictions of entrainment into the tops of marine stratus in a moist generalization of the 1995 Global Energy and Water Cycle Experiment Cloud System Studies model intercomparison. For the computational study the authors present a sequence of simulations of varying resolution, from a typical resolution (50 m horizontal and 25 m vertical mesh size) to a fine resolution (8 m horizontal and 4 m vertical mesh size). The authors also explore variations in the model such as different subgrid models and modifications of the advection scheme. It was found that the thickness of the inversion, the depth of entraining eddies, and the shape of vertical velocity spectra were determined mainly by the mesh spacing used. However, the entrainment rate was found to have a distinct dependence on the amount of combined numerical and subgrid-scale mixing. This indicates that the use of large eddy simulation to study mixing in stratocumulus boundary layers needs to account for both sources of mixing.