Mesoscale simulation of supercritical, subcritical, and transcritical flowalong coastal topography

A mesoscale atmospheric model is used to address the characteristics of str atified how bounded by a side wall along a varying coastline. Initial Froud e number values are varied through alteration of marine inversion strength, permitting examination of supercritical, subcritical, and transcritical fl ow regimes encountering several coastal configurations. Consistent with sha llow water models, sharp drops in boundary layer depth and flow acceleratio n occur in flow rounding convex bends; however, significant flow response o ccurs in the stratified layer aloft, which is unexplained by conventional s hallow water theory. The strongest flow acceleration occurs in the transcri tical case while, regardless of inversion strength, the deformation of the isentropes aloft shows general structural similarity. Advection of horizontal momentum is an important component of the horizonta l force balance. A simulation having several coastline bends exhibits a det ached, oblique hydraulic jump upwind of a concave bend that strongly blocks the flow. For the single-bend case, a shallow water similarity theory for stratified flow provides qualitative, and partial quantitative, agreement w ith the mesoscale model, in the boundary layer and aloft. Horizontal struct ure functions for these similarity solutions satisfy a set of equivalent sh allow water equations. This comparison provides a new perspective on previo us shallow water models of supercritical flow around coastal bends and sugg ests that the existence of the supercritical flow response may depend more on the presence of a low-level jet than on a sharp boundary layer inversion .