Cyclogenesis and frontogenesis

Detailed linear analysis of the stability of realistic atmospheric frontal structures using the full hydrostatic primitive equations has demonstrated that such localized baroclinic zones support instability at two distinct and well-separated length scales, even when the frontal mean states have uniform potential vorticity. The first scale is associated with the well-known Charney-Eady mode of baroclinic instability with a wavelength of 3000–4000 km that is responsible for the generation of observed mid-tropospheric long waves. The second scale is defined by an apparently new mode of baroclinic instability, that we have previously called the “cyclone mode”, which has a wavelength near 1000 km and is boundary confined rather than being troposphere filling as is the case of the Chamey-Eady mode. The most important characteristic of the cyclone mode is that it is completely filtered by both the quasi-geostrophic and semi-geostrophic approximations to the equations of motion. In this paper, we demonstrate explicitly through the analysis of a specific case of polar low development that this new mode is an important actor in at least one recurrent atmospheric dynamical process. Besides reviewing and extending these linear stability analyses, we describe a number of rather interesting results that we have obtained from a new series of f-plane analyses of the nonlinear life cycles of frontal baroclinic waves. These have already provided considerable new insight into the detailed dynamics of the processes through which frontal waves “break”.