Simulations of gravity wave-turbulence interactions in the stable boundary layer

The planetary boundary-layer under strong, stable stratification continues to elude quantitative description despite its relatively frequent occurrence and many years of effort. Measurements indicate that stable boundary-layers (SBL) possess complex temperature and wind structures which apparently provide favorable conditions for the development of the many interacting processes that can occur within the SBL. Amongst these are patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets. Rather than being uniformly quiescent regions with little or no mixing, SBLs are subject to sporadic overturning and turbulent episodes.

To provide a numerical testbed for probing these environments, the National Taiwan University/Purdue University (NTU/P) Non-hydrostatic model is employed. This model, which has been successfully applied to the simulation of bubble convection, cloud streets, squall lines and lee and mountain waves, has been modified to include a Reynolds' Stress turbulence closure scheme suitable for stable conditions. The model has been efficiently parallelized for use on the Army Research Lab Major Shared Research Center advanced technology clusters; thus providing the computational resources to conduct very high-resolution, three-dimensional simulations. Recent runs have been able to utilize almost a thousand processors with only losing about 10% to communication overhead.

Two prototypical cases have been explored, both of which center on gravity wave-turbulence interactions and the formation of thin sub-layers of turbulent activity. Taking advantage of a stretched, terrain-following coordinate system, we are able to create relatively large domains, to minimize effects from lateral boundary conditions, with upper-boundary conditions capable of properly trapping gravity waves in a resonant duct rather than quickly propagating out of the system.

The first case involves the production of gravity waves via shear instability, which can transport and deposit energy and momentum into regions below the shear layer. This causes destabilization and turbulence development within these localized regions. A second case explores the generation of gravity waves by idealized terrain features. Through a similar mechanism as above, these waves can also excite turbulent spots within the boundary-layer.