Analysis of Turbulence Kinetic Energy Dynamics in Complex Terrain

Boundary layer processes and turbulence in a complex terrain are influenced by thermally driven flows, as well as dynamical or non-local winds. We investigate the variability in turbulence kinetic energy (TKE) with elevation, and topography in a shallow high mountain valley in the Canadian Rockies. The Fortress Mountain Research Basin in the Kananaskis Valley, Alberta, was chosen for this study. Data from three high-frequency eddy-covariance systems at a north-west-facing slope location, and at two ridgetops at the south and north valley side walls were used for the analysis, and combined with large-eddy simulations (LES). The observed data and simulations focused on a sunny summer day when turbulence was well-developed, and cross-ridge flows interacted with thermally driven circulations. The observed TKE time series compared reasonably well with simulations at the north-west-facing slope and southern ridgetop. The model was then used to evaluate the vertical and horizontal TKE budget equation. Analysis of the TKE budget showed that horizontal shear driven by interactions of cross-ridge flows with the up-valley flow could be an important source of TKE production on the north-west-facing slope station in the Fortress Valley. At the northern ridgetop, both model and observations showed no contributions from the vertical production terms, while model showed a significant contribution from the horizontal shear production to TKE at this location. The correlation between the TKE at the valley station and the wind speed at a different location above the valley suggests that both horizontal and vertical exchange processes are an important part of TKE production mechanisms in this high mountain valley.