The idea that extratropical atmospheric variability on time scales of the order of several days is due to an intrinsic instability of the atmospheric circulation is widely accepted in dynamic meteorology. The most evident manifestation of this variability is the development and movement of cyclones and anticyclones (Blackmon et al. 1984). The leading process has been identified, after Charney (1947) and Eady (1949), in the baroclinic instability of a vertically sheared current. The basic formulation of the theory has been substantially improved over the past thirty years. While the linear problem has been generalized to more “realistic” basic state flows, the nonlinear problem has been tackled with an increasing degree of complexity, including dynamical analysis of chaotic regimes (Malguzzi et al. 1988; Buzzi et al. 1990). Baroclinic instability depends in an essential way upon boundary conditions, and orography enters the problem as a lower boundary condition. We shall see that orographic cyclogenesis is a phenomenological manifestation of the sensitivity of the baroclinic atmosphere to surface relief.
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