The Influence of Midlatitude-Cyclone-Forced Mountain Waves on the Atmospheric Momentum Budget

Idealized studies of the influence of mountains on the atmosphere have primarily focused on the steady state response to horizontally uniform flows encountering an obstacle. In this research, we examine the influence of an isolated north-south ridge on a mid-latitude cyclone developing in the presence of a baroclinically unstable jet. The mountain is located in an initially quiescent part of the flow. The cyclone center eventually passes to the north of the mountain during which time a cold front crosses the topography and strong low-level cross-ridge flow persists for several days.

When the ridge is 500 m high, the pressure drag maximizes at 6.5 days, two days after frontal passage and then decreases rapidly. In contrast, when the ridge is 2 km high, the drag maximizes slightly earlier (at 6 days) and remains large until the simulation terminates at 7.5 days. In both cases, the vertical momentum fluxes slightly above mountain-top level are significantly less than the pressure drag. The difference between the pressure drag and the vertical momentum flux in the momentum budget is primarily balanced by the Coriolis force acting on the ageostrophic wind.

The 2-km high mountain produces extensive wave breaking, primarily (1) at low levels in the lee of the terrain prior to frontal passage and (2) in the stratosphere after frontal passage. This wave breaking causes significant removal of the cross-mountain momentum at both of these levels, a significant fraction of which, is focused in distinct large-scale structures well downstream of the topography.