Numerical Simulation of Katabatic Flow with Changing Slope Angle

A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.