Simple Initial Value Problems and Mechanisms for Baroclinic Growth

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Abstract
In this paper a simple 2D linear quasigeostrophic model is used to investigate how the development of local confined cyclonic perturbations is dependent on the perturbation scale, location, and tilt in Eady-type basic states. It is found that the initial growth of the perturbation can be maximized by reducing both the vertical and horizontal scale and using a “midtropospheric” vertical location. “Potential vorticity (PV) thinking” suggests the concept of “PV unshielding” to explain this result. Adding a meridional gradient of basic-state PV lowers the vertical location of perturbations that optimally excite sustained growth. This can be understood by considering the behavior of the upward and downward propagating parts of the initial perturbation. It is found that the importance of the initial perturbation tilt is diminished for confined perturbations. It is shown that diabatic heating in a vertically confined region can lead to a perturbation that exhibits rapid growth. The findings in this paper lay some foundations for understanding calculated optimal growth structures, such as the singular vectors produced routinely by the European Centre for Medium-Range Weather Forecasts.

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