Flow over a mesoscale ridge: pathways to regime transition

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 (R_o = 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 R_o 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 R_o~ 1), and thereafter to the forcing induced by the adjustment that accompanies the scale contraction of balanced flow (for R_o < 1). In effect this change highlights the various pathways to regime transition within the intermediate domain.