Baroclinic Instability of the Zonally Averaged Flow with Boundary Layer Damping

Abstract

In this paper we show that the observed zonally averaged atmosphere is unstable to normal mode type perturbations even when the effects of surface friction as modeled by an Ekman layer or Rayleigh damping are included. Growth rates are reduced and short wavelengths stabilized. Further, we show that thermal damping generally has a weaker effect and in some circumstances can destabilize the flow. The atmosphere is baroclinically unstable, and normal mode baroclinic instability theory provides an invaluable guide to the mechanisms responsible for and the structure of middle latitude weather systems. However there is no doubt that the initial value problem and the growth of nonnormal mode disturbances is crucial to the development of individual systems. Linear calculations are presented that exhibit large initial transient growth. The growth rate can be more than twice that of the normal mode rates, but only for one or two days. Thereafter the normal mode dominates the evolution.