Theory of Deep Cyclogenesis in the Lee of the Alps: Modification of Baroclinic Instability by Topography

摘要

A theory modeling Alpine cyclogenesis in terms of orographically modified normal modes of the baroclinic instability of a zonal flow is proposed. It is shown how the free unstable modes growing on a baroclinic zonal wind are modified by a small (perturbative) mountain extending in the east-west direction in a quasigeostrophic atmosphere. Such modification is antisymmetric with respect to the mountain crest; in the case of limited mountain it essentially consists of a high-low dipole bound to the mountain, extending throughout the troposphere and having a horizontal scale comparable to the Rossby radius of deformation. There are two limitations to the theory: the Alps are not small, and their horizontal scale is too short for the quasigeostrophic theory to be consistent.