Quasi-geostrophic Lee Cyclogenesis

The passage of a low-level baroclinic zone toward an elongated ridge is simulated in the limit of the nonlinear quasi-geostrophic dynamics of a uniform potential vorticity atmosphere. It is demonstrated that lee cyclogenesis can be realized for this combination of flow setting and idealized model. Moreover, the temporal growth rate, the spatial structure, the genesis region, and the subsequent movement of the cyclone bear comparison with observed features of Alpine lee cyclogenesis. A series of numerical experiments serve to demonstrate the sensitivity of the lee event to external parameters and help to pinpoint its dynamical essence. In particular it is shown that the simulated development exhibits a two-stage character. The first phase is accompanied by growth rates that clearly exceed those of classical baroclinic instability and it is associated with the retardation, distortion, and frontogenetic development of the baroclinic zone as it impinges upon the orography. The resulting low-level perturbation is trapped in the close vicinity of the obstacle by effects due to localized baroclinicity. The second phase is a baroclinic development. It is characterized by the relative propagation of upper- and lower-level anomalies that result in the interaction of the preexisting upper-level vorticity strip and the incipient low-level vortex. In relation to the various theories for lee cyclogenesis, it is noted that the horizontal scale of the first phase thermal perturbation is in agreement with the predictions of the baroclinic lee–wave theory, while the development in the second phase is more compatible to that of a type-B cyclogenesis than to an orographically modified modal baroclinic wave development.