Stratified flow over topography: models versus observations

Essential mechanisms identified in the authors' observations of flow establishment over topography are restated, with emphasis on those which are at variance with numerical simulations. Specifically, small-scale instabilities were observed to lead to upwards transport of fluid from the primary flow so as to form a nearly stationary intermediate layer, and boundary-layer separation significantly delays establishment of the downslope flow. Simulations that force the streamlines to follow the topography generate a large-amplitude breaking wave, which we did not observe. Our observations show that small-scale instability and separation act in concert to determine the time-dependent evolution of the stratified flow response.