Nonlinear Flow Past an Elliptic Mountain Ridge

Abstract

The hydrostatic flow over an elliptical mountain of aspect ratio 5 is explored by numerical experiments. The upstream profiles of wind and stability are constant, the Coriolis effect is ignored, and there is free slip at the lower boundary. In these conditions, the, flow characteristics depend mainly on the nondimensional mountain height, Nh/U. The authors have conducted experiments with Nh/U varying from 0.500 to 6.818. For low values of Nh/U, the results confirm the linear theory of Smith, which predicts stagnation aloft, leading to wave breaking and, on the upstream slope, leading to flow splitting. For higher values of Nh/U, the authors find that wave breaking ceases on the axis of symmetry but continues on each side of this axis. Even for the highest value of Nh/U used (6.818), significant areas of wave breaking and wave activity aloft are found. For all values of Nh/U, a substantial part of the flow is diverted vertically above the mountain. The detailed study of the kinematic pattern within the upstream blocking reveals an increasing tendency to small vortex creation when Nh/U increases. This, however, does not affect the main flow features. Finally, the authors observe the generation of potential vorticity in the wake of the mountain, leading to the creation of lee vortices. The potential vorticity pattern is very similar to the vorticity pattern shown by Schär and Smith for shallow-water flow. It is found to be insensitive to the turbulence parameterization in our model, as well as the general flow pattern. On the other hand, comparison with an experiment using a circular mountain reveals large differences in the elliptical case.