OROGRAPHIC MODIFICATION OF CYCLONE DEVELOPMENT

The orographic modification of cyclone development is examined by mean s of primitive equation model simulations. When a mature baroclinic wa ve impinges on an east-west oriented mountain ridge, a relatively inte nse cyclone forms on the south side of the ridge. This cyclone extends throughout the depth of the troposphere and possesses relatively smal l vertical tilts, large velocities, and strong temperature perturbatio ns compared to classical baroclinic eddies. The vorticity growth in th e orographic cyclone center is larger than that of baroclinic eddies t hat grow over flat terrain. However, there is no absolute instability associated with this orographic enhancement. A longer ridge produces a more intense eddy. The behavior of small-amplitude normal modes on a zonally symmetric mountain ridge shows that baroclinic development is enhanced where the topography slopes in the same direction as the isen tropes. This is consistent with earlier studies using uniform slopes t hat show that the heat flux forced by this terrain enhances the conver sion of available potential energy. It is shown that the structure of nonlinear waves is similar to that of linear modes over a mountain rid ge with steep slopes, in which the cross-ridge flow and the associated heat flux are partially blocked by the mountain. Simulations of a sta tionary cold front interacting with a mountain ridge suggest that orog raphic cyclogenesis is triggered when the mountain ridge locally modif ies the frontal circulation as it impinges on the ridge. Warm southerl y flow in the front is diverted westward by the mountain ridge, intens ifying the strong hydrostatic pressure gradient between the mountain a nticyclone and the developing cyclone to the south. In contrast, cold northerly flow is diverted eastward as it approaches the mountain and effectively broadens the mountain anticyclone toward the north. This p roduces the characteristic pressure dipole observed in orographic cycl ogenesis. It is concluded that mature baroclinic eddies approaching th e mountain ridge should have a strong frontal zone with a considerable temperature contrast and strong circulation for an intense response.