Stratified flow over topography: time-dependent comparisons between model solutions and observations

Detailed observations of stratified flow over a sill in Knight Inlet, British Columbia are compared with numerical solutions from a two-dimensional primitive equation model. The comparison focuses on the establishment of the high drag state over the sill in response to a time-varying barotropic forcing associated with an ebbing tidal flow. The results show that the model solution evolves to a hydraulically controlled, high drag state that is similar to the observed one. However, the model evolution to this state occurs rapidly through an overturning internal wave response with subsequent convective mixing. On the other hand, the observations show a more gradual evolution with no apparent overturning. It is suggested that flow separation in the lee of the sill crest, which typically is not well represented in models of stratified flow, likely exerts a strong control on the rate and manner in which the high drag state becomes established. (C) 2000 Elsevier Science B.V. All rights reserved.