Flow over a mesoscale ridge: pathways to regime transition

Abstract

A theoretical and modelling study is undertaken of an airstream of uniform flow (U) and stratification (N) impinging normally upon a two-dimensional bell-shaped ridge of half-width (L) and height (H) that is located on an f-plane. The associated Rossby number (Ro = U/fL) and inverse Froude Number (F = NH/U) are taken to be such that the effects of both the earth’s rotation and non-linear processes can significantly influence the nature of the flow response. It is shown that within this intermediate meso-α/β scale sub-domain of (Ro, F) parameter space there are several distinctive signatures to the flow response, and each signature is associated with the generation of appreciable buoyancy wave energy. As Ro decreases across the sub-domain, the source for this wave energy changes continuously from the conventional direct orographic forcing of vertically propagating waves (for Ro > 1), to that linked with the cusping of a leeward train of low-level near-inertial waves (for Ro~ 1), and thereafter to the forcing induced by the adjustment that accompanies the scale contraction of balanced flow (for Ro < 1). In effect this change highlights the various pathways to regime transition within the intermediate domain.