A Theory of Deep Cyclogenesis in the Lee of the Alps. Part I: Modifications of Baroclinic Instability by Localized Topography

Observational and numerical studies on Alpine cyclogenesis have shown that a developing baroclinic wave approaching the mountain region gives rise to a disturbance of dipolar structure, extending throughout the troposphere with horizontal scales comparable to the Rossby deformation radius. It is possible to interpret such disturbances as modifications of baroclinically unstable modes, induced by localized topography. In the present approach, the effect of the mountain is introduced in a perturbative sense, in the framework of quasi-geostrophic theory. Even in this simple approach the spatial structure of the unstable modes is modified by a localized topography in the direction required in order to explain the observed features. In the case of a continuously stratified fluid, the basic characteristics of the observed vertical structure are also reproduced.