Subgrid-scale physics under strongly stable atmospheric stratification: the SnoHATS experiment (Formerly 18A.2)

Stably stratified atmospheric boundary layers present particular challenges both from an experimental and a modeling perspective. On the modeling side, many of the characteristics of stable flows (weak and highly anisotropic turbulence, gravity waves, instabilities, meandering motions, …) complicate the formulation of effective parameterizations for unresolved physics, such as subgrid scale (SGS) turbulence in Large Eddy Simulation (LES). To address these concerns, a field study (SnoHATS) was held at the extensive "Plaine-Morte" glacier in the Swiss Alps (3000 m) from February to April 2006. The snow cover provided stable stratification of the flow over long periods. Two horizontal arrays of vertically separated 3D sonic anemometers were deployed to allow two dimensional filtering and computation of the full three-dimensional stress and strain tensors. This enables the investigation of the dynamics of small turbulence scales under stable conditions.

Results from SnoHATS indicate that increasing stability reduces turbulent transport efficiencies. The efficiency of momentum transport is reduced faster that that of heat transport. Another important effect of stability was to increase the fraction of the total Reynolds stresses that fall in the SGS range of turbulence, effectively indicating a reduction in the integral scale of turbulence. The vertical fluxes had the highest fraction of unresolved contributions; up to 40 percent of the vertical fluxes are performed by the subgrid scales.

Despite these pronounced effects of stability, the variations of the SGS model coefficients with stability are consistent with previous results over different surfaces (HATS experiment, Kleissl et al., JAS, 2003; 2004). In addition, the orientation of the stress tensors relative to the strain tensors are remarkably similar to previously obtained results under considerably different flow conditions (Tao et al., JFM, 2002; Higgins et al., BLM, 2003).