The Impact of Mountain Waves on an Idealized Barocliniczlly Unstable Large-Scale Flow

The feedback of mountain waves and low-level blocking on an idealized baroclinically unstable wave passing over an isolated ridge are examined through numerical simulation. Theoretical analysis implies the volume-integrated momentum budget is dominated by mean-flow acceleration, vertical fluxes of horizontal momentum, and the Coriolis force acting on the ageostrophic wind. These do indeed appear as the dominant balances in numerically computed budgets averaged over layers containing: (1) wave breaking in the lower stratosphere, (2) flow blocking with modest wave breaking near the surface, and (3) a region of pronounced horizontally averaged mean flow deceleration in the upper troposphere where there is no wave breaking.

The local impact of wave breaking on the jet in the lower stratosphere is dramatic, with winds in the jet core reduced by almost 50% relative to the no-mountain case. Although the upper troposphere is the layer with the strongest average deceleration, the local patches of decelerated flow are weakest in this layer. In the case of a 2-km high ridge, the cross-mountain pressure drag greatly exceeds the vertical momentum flux at mountain-top level, which is more closely related to the pressure drag computed over that portion of the topography above which the flow is unblocked. In contrast to a previous study, the momentum flux for quasi-linear flow over a 500-m-high ridge is not maximized during the period when the mean cross-mountain flow is accelerating. WKB analysis suggests the difference between the current and previous results is due to the presence of significant vertical wind shear.