The action of high and steep mountains on baroclinic instability and Alpine cyclogenesis

The modification induced on unstable baroclinic waves by high and steep bottom obstacles of limited horizontal extent is studied in some, mathematically tractable, limiting cases (obstacles with vertical lateral walls blocking the horizontal motion of the fluid). The model atmosphere is assumed quasi-geostrophic and the stability problem is dealt with in the two-level approximation. The results of such analysis show that the action of the obstacle can extend to the entire baroclinic wave unless strongly non-geostrophic effects, relaxing the “stiffness” of the quasi-geostrophic flow near the obstacle, are postulated. The weaknesses of this representation with respect to the problem of Alpine lee cyclogenesis are discussed.