Orographic Modification of Cyclone Development

Abstract

The orographic modification of cyclone development is examined by means of primitive equation model simulations. When a mature baroclinic wave impinges on an east?west oriented mountain ridge, a relatively intense cyclone forms on the south side of the ridge. This cyclone extends throughout the depth of the troposphere and possesses relatively small vertical tilts, large velocities, and strong temperature perturbations compared to classical baroclinic eddies. The vorticity growth in the orographic cyclone center is larger than that of baroclinic eddies that 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 isentropes. This is consistent with earlier studies using uniform slopes that show that the heat flux forced by this terrain enhances the conversion of available potential energy. It is shown that the structure of nonlinear waves is similar to that of linear modes over a mountain ridge with steep slopes, in which the cross-ridge flow and the associated heat flux are partially blocked by the mountain. Simulations of a stationary cold front interacting with a mountain ridge suggest that orographic cyclogenesis is triggered when the mountain ridge locally modifies the frontal circulation as it impinges on the ridge. Warm southerly flow in the front is diverted westward by the mountain ridge, intensifying the strong hydrostatic pressure gradient between the mountain anticyclone 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 produces the characteristic pressure dipole observed in orographic cyclogenesis. It is concluded that mature baroclinic eddies approaching the mountain ridge should have a strong frontal zone with a considerable temperature contrast and strong circulation for an intense response.