AN EVALUATION OF NEUTRAL AND CONVECTIVE PLANETARY BOUNDARY-LAYER PARAMETERIZATIONS RELATIVE TO LARGE-EDDY SIMULATIONS

This paper compares a number of one-dimensional closure models for the planetary boundary layer (PBL) that are currently in use in large-sca le atmospheric models. Using the results of a large-eddy a simulation (LES) model as the standard of comparison, the PBL models are evaluate d over a range of stratifications from free convective to neutral and a range of surface shear stresses. Capping inversion strengths for the convective cases range from weakly to strongly capped. Six prototypic al PBL models are evaluated in this study, which focuses on the accura cy of the boundary-layer fluxes of momentum, heat, and two passive sca lars. One scalar mimics humidity and the other is a top-down scalar en trained into the boundary layer from above. A set of measures based on the layer-averaged differences of these fluxes from the LES solutions is developed. In addition to the methodological framework and suite o f LES solutions, the main result of the evaluation is the recognition that all of the examined PBL parameterizations have difficulty reprodu cing the entrainment at the top of the PBL, as given by the LES, in mo st parameter regimes. Some of the PBL models are relatively accurate i n their entrainment flux in a subset of parameter regimes. The sensiti vity of the PBL models to vertical resolution is explored, and substan tive differences are observed in the performance of the PBL models, re lative to LES, at low resolution typical of large scale atmospheric mo dels.