Atmospheric boundary layers are usually classified into three types: neutral, convective and stable, based on atmospheric stability (buoyancy effects) and the dominant mechanism of turbulence generation. The boundary layer becomes stably stratified whenever the underlying surface is colder than the air. Under this atmospheric condition, turbulence is generated by shear and destroyed by negative buoyancy and viscosity. Because of this competition between shear and buoyancy effects, the strength of turbulence in the stable boundary layer is much weaker in comparison to the neutral and convective boundary layers. As a result, the stable boundary layer is also much shallower and characterized by smaller eddy motions (see Figure 2).

Figure 1: Velocity isosurface simulated by our locally-averaged scale-dependent dynamic SGS model (Basu and Porté-Agel, 2005). The nocturnal jet formation at around 170 m is clearly noticeable.

Figure 2: Weakly (top) and very stable (bottom) boundary layers observed in a wind-tunnel experiment (Ohya 2001, BLM).

In order to improve our understanding of stable boundary layer turbulence and to explore its inherent characteristics, in our research we make use of a contemporary numerical modeling approach known as large-eddy simulation (LES). Until now, LES models have not been sufficiently faithful in reproducing the characteristics of moderately and strongly stable atmospheric boundary layers. The main weakness of LES is associated with our limited ability to accurately account for the dynamics that are not explicitly resolved in the simulations. Under stable conditions - due to flow stratification - the characteristic size of the eddies becomes increasingly smaller with increasing atmospheric stability, which eventually imposes an additional burden on the LES subgrid-scale (SGS) models. The recent GABLS (Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study) LES intercomparison study (www.gabls.org) highlights that LESs of moderately stable boundary layers are quite sensitive to SGS models even at a relatively fine resolution of 6.25 m.

Our recent research shows that:

Figure 3: Locally computed non-dimensional gradients of velocity (Basu and Porté-Agel, 2005).

Our current research objectives are:

• Basu S., and F. Porté-Agel (2005), Large-edy simulation of stably stratified atmospheric boundary layer turbulence: a scale-dependent dynamic modeling approach, Journal of the Atmospheric Sciences, under review. Available at arxiv.org/abs/physics/0502134

• Basu S., F. Porté-Agel , E. Foufoula-Georgiou, J.-F. Vinuesa, M. Pahlow (2005), Revisiting the local scaling hypothesis in stably stratified atmospheric boundary layer turbulence: an integration of field and laboratory measurements with large-eddy simulations, Boundary-Layer Meteorology, in revision.