Highly-resolved LES of the stable boundary layer over terrain

One of the most important scenarios for atmospheric modelers is the case of the stably stratified boundary layer (SBL). For example, pollutants are more likely to be trapped near the surface in the stable boundary layer. Unfortunately, the stable case is also extremely challenging to understand and model correctly. Large-eddy simulation (LES) has become a promising approach to study the SBL because much of the dynamical structure is explicity resolved and allowed to develop according to the full equations of motion. However, most of the LES research to date has been focused on simple canonical cases such as flow over an infinite flat plane. Therefore, a gap exists between current state-of-the-art LES of the SBL and many of the scenarios we are most interested in modeling (e.g. CASES-99). Difficult challenges remain in making the LES approach more realistic. For example, several improvements would include: 1) increased resolution to better capture localized events, 2) terrain effects , 3) in some cases, radiation flux divergence, 4) the effects of mesoscale meandering or supergrid motions propagating into the domain, and 5) improved subgrid (subfilter) scale models. This research is an effort to begin to bridge the gap by focusing on 1 and 2 above, although we have made progress recently on promising new subgrid-scales models as well. We perform highly-resolved LES of the SBL over terrain. In particular, we report on progress to include the effects of several scales of topography. We investigate the effect of small scale topography on the near surface flow and look for small, intermittent drainage events in a flow forced by larger-scale topography. Small-scale simulations of flow over one mode of topography have shown the tendency of the flow to fluctuate quasi-periodically in the recirculation zones behind the crests of the terrain.

This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract No. W-7405-ENG-48. RLS was supported in part by NSF Grant ATM-952646 (Physical Met. Prog.: R. R. Rogers, Program Director)