Mesoscale Simulation of Supercritical, Subcritical, and Transcritical Flow along Coastal Topography

Abstract

A mesoscale atmospheric model is used to address the characteristics of stratified flow bounded by a side wall along a varying coastline. Initial Froude number values are varied through alteration of marine inversion strength, permitting examination of supercritical, subcritical, and transcritical flow regimes encountering several coastal configurations. Consistent with shallow water models, sharp drops in boundary layer depth and flow acceleration occur in flow rounding convex bends; however, significant flow response occurs in the stratified layer aloft, which is unexplained by conventional shallow water theory. The strongest flow acceleration occurs in the transcritical 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 horizontal force balance. A simulation having several coastline bends exhibits a detached, 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 with the mesoscale model, in the boundary layer and aloft. Horizontal structure functions for these similarity solutions satisfy a set of equivalent shallow water equations. This comparison provides a new perspective on previous shallow water models of supercritical flow around coastal bends and suggests 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.