On high-drag states of nonlinear stratified flow over an obstacle

First we examine the behaviour of the system as a function of the height of a zero-wind line imposed in the ambient flow. The character of the high-drag states conforms well to the predictions of the internal hydraulic analysis of Smith, and cannot be explained in terms of linear resonance. However, a high-drag state emerges even when the initial critical level height is below the lowest predicted resonant height. In this case an upstream-propagating bore is generated which adjusts conditions so as to allow a high-drag state. In the second series of experiments, the numerical model is initialized with the idealized high-drag states yielded by Smith's theory, subject to uniform upstream wind conditions. When the mountain is high enough to produce wavebreaking in uniform flow, an overturning region develops at the theoretical level of no motion and a vertically propagating wave emerges aloft; nevetherless, the flow near the ground remains substantially unaltered. When the mountain is too low to support wavebreaking, the mixed region in the lee collapses, and the flow reverts to a nonhydraulic Long's model solution subject to a radiation upper boundary condition. -from Authors