On the Basic Dynamics of Regional Cyclogenesis

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This paper investigates the dynamics of regional cyclogenesis from the perspective of local instability of a zonally inhomogeneous baroclinic jet streak in a two-layer quasi-geostrophic beta-plane channel model. When such a representative jet streak is embedded in a background uniform vertical shear UT, there are both local and global unstable normal modes. In the absence of such a background shear (UT = 0), only the local modes are unstable. The shorter the jet is, the fewer local modes would there be. A local mode consists of a group of dominant waves that jointly give rise to a maximum local energy downstream of the jet core. Its existence is independent of the cyclical boundary condition. The growth rate of a local mode diminishes rapidly when the constant part of the basic zonal wind U0 is increased. A global mode, on the other hand, largely consists of a single wave and its growth rate is much less sensitive to U0. These properties are qualitatively similar to those in the WKB solution. The structural characteristics of these modes are identifiable with those of three classes of unstable modes of an observed atmospheric flow reported in Frederiksen and Bell. Our nonmodal analysis shows that a localized disturbance naturally emerges from a zonally unbiased initial state in a relatively short time. The excitation of a local mode within a few days from an initially isolated disturbance also depends strongly upon its initial position relative to the jet core. The two processes that locally generate the perturbation energy depend upon the structural properties of the disturbance relative to the basic thermal and deformation fields. The two processes that redistribute the perturbation energy are the advection of energy by the basic flow and the convergence of energy flux associated with the ageostrophic component of the perturbation. These four processes are comparably important and greatly counteract one another resulting in a net intensification of a disturbance centered downstream of the jet core. The feedback effects of the most unstable mode on the basic flow resemble the observed geopotential tendencies induced by the transient eddies. The feedback results of this analysis differ noticeably from the WKB counterparts.