Large-Eddy Simulations of the Atmospheric Boundary Layer Using a New Subgrid-Scale Model – II. Weakly and moderately stable cases

Abstract

A series of simulations under weakly to moderately stable boundary layers (SBLs) have been performed using the proposed subgrid-scale (SGS) model implemented into the Terminal Area Simulation System (TASS). The proposed SGS model incorporates some aspects of the two-part eddy viscosity SGS model of Sullivan et al. (1994) and further refinements which include the dependence of SGS mixing length on stratification, two-part separation of the SGS eddy diffusivity of heat, and more realistic empirical forms of Monin–Obukhov similarity functions. The potential temperature profiles from simulations clearly show a three-layer structure: a stable surface layer of strong gradients, a middle layer of small gradients, and an inversion layer on the top. The wind speed profiles show the formation of low level jet (LLJ). However, the sub-layer structures under moderately SBLs differ from those under weakly SBLs. Both the momentum and heat fluxes decrease almost linearly in the lower part of the SBL. The near surface values of the normalized turbulent kinetic energy (TKE/u*2) in all simulations are about 4 which is much less than the typical value of 5.5 under the neutral condition. The decay of turbulence first occurs in the area with large values of Richardson number (Ri<0.2). Generally, instantaneous values of the TKE and Ri at the various grid points are negatively correlated, but there is not a unique relationship between the two parameters.