Prediction of regional-scale fluxes of momentum and heat in atmospheric simulation models (e.g., numerical weather prediction and climate models) is subject to substantial errors associated with our limited ability to account for the combined effects of subgrid-scale topography and atmospheric stability. In particular, enhancement of turbulent fluxes due to topographically-induced form drag and gravity waves cannot be properly captured by commonly-used similarity-based parameterizations, which are strictly only valid for boundary layer flow over homogeneous flat surfaces. In order to produce physically realistic results these models require stability corrections, which are "inspired in model performance" and have little or no scientific foundation. In this project, a novel combination of wind tunnel experiments, field measurements and large-eddy simulations (LES) will be carried out to gain physical insight concerning the complex interactions between natural topography and stably stratified atmospheric boundary layer turbulence. First, a framework will be created to evaluate the performance of the new generation LES (including recent advances in tuning-free dynamic subgrid-scale models) over complex terrain. The new-generation LES will then be used to simulate stable boundary layers over different topographies to obtain high-resolution 3-D transient information needed to study the physics governing land-atmosphere interaction. Multiscale wavelet analysis will be used to quantify the dominant scales of interaction between topography, atmospheric stability and turbulent fluxes. This will be aimed at developing physically-based modifications to similarity theory that include relevant temporal and spatial scales associated with both topography and thermal stratification. These results have the potential to substantially improve the parameterization of regional-scale fluxes and, therefore, the accuracy of weather and climate models.
The intellectual merit of the research lies in its use of a unique and comprehensive combination of wind tunnel experiments, field measurements, state-of-the-art large-eddy simulation techniques, and statistical tools needed to better understand the effects of topography on turbulent fluxes in stable boundary layers. The research has the potential to shed new light on our understanding of, and ability to parameterize, land-atmosphere interactions.
The broader impacts include substantial improvement in the parameterization of atmospheric boundary layer fluxes and, as a result, improved accuracy of atmospheric simulation models. A new web-based educational tool will be developed to take advantage of the educational potential of visualizations/animations of large-eddy simulation fields. The new tool will be used in undergraduate and graduate courses taught by the principal investigator as well as outreach activities for high school students. Furthermore, two graduate students will be mentored as research assistants and will also participate in the development and implementation of the educational tool.
