A study is made of the effects of stable stratification on the fine-scale features of the flow in an evolving stable boundary layer (SBL). Large-eddy simulation (LES) techniques are used so that spatially and temporally varying and intermittent features of the turbulence can be resolved; traditional Reynolds-averaging approaches are not well suited to this. The LES model employs a dynamic, two-parameter subgrid-scale (SGS) turbulence submodel (see Cederwall and Street, 12th AMS BLT Sympos., 1997) that provides upscale (backscatter) of turbulent energy and incorporates effects of buoyancy through the use of a time-evolving SGS turbulent kinetic energy (TKE) scheme. The meteorological scenario is a developing convective boundary layer (CBL) during the day, followed by an evolving SBL after sunset due to surface cooling.
Once the surface heating is reduced and replaced by surface cooling, the turbulence structure in the upper part of the CBL collapses, as seen in other studies, but not as rapidly as in a previous study using an earlier version of this model (Cederwall, 11th AMS BLT Sympos., 1995). In that study, the SGS submodel (Deardorff, Bound.-Layer Meteorol., 1980) was too dissipative. The new SGS submodel with backscatter provides a more realistic simulation of the turbulence damping by stable stratification. In the presence of strong stability, periods of intermittent turbulence are simulated, with the enhanced velocity fluctuations seen in the streamwise component, consistent with direct numerical simulation (Hartel and Kleiser, J. Fluid Mech., 1998). The turbulence becomes highly anisotropic, with the reduction of vertical velocity fluctuations. As the ground is approached, the flow field shows reduced vorticity and the presence of wave-like motions.