Simple initial value problems and mechanisms for baroclinic growth

In this paper a simple 2D linear quasigeostrophic model is used to investig ate how the development of local confined cyclonic perturbations is depende nt 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 b y reducing both the vertical and horizontal scale and using a "midtroposphe ric'' vertical location. "Potential vorticity (PV) thinking'' suggests the concept of "PV unshielding'' to explain this result. Adding a meridional gr adient of basic-state PV lowers the vertical location of perturbations that optimally excite sustained growth. This can be understood by considering t he behavior of the upward and downward propagating parts of the initial per turbation. It is found that the importance of the initial perturbation tilt is diminished for confined perturbations. It is shown that diabatic heatin g in a vertically confined region can lead to a perturbation that exhibits rapid growth. The findings in this paper lay some foundations for understan ding calculated optimal growth structures, such as the singular vectors pro duced routinely by the European Centre for Medium-Range Weather Forecasts.