The Downstream and Upstream Development of Unstable Baroclinic Waves

Abstract The response of a baroclinically unstable atmosphere to a localized initial perturbation is examined. In a preliminary numerical experiment, using a nonlinear primitive-equation model with spherical geometry, new disturbances grow regularly downstream in broad agreement with a particular type of observed development. The initial disturbance moves downstream, while smaller scale disturbances develop upstream of it—each forming at about the same longitude. Later downstream disturbances have upper level amplitudes significantly larger than found in nonlinear integrations using normal-mode initial conditions. Similar development is found in a quasi-geostrophic model with spherical geometry, and the rate of spreading of the instability is insensitive to the amplitude of the initial perturbation. For the baroclinic instability model considered by Eady the downstream disturbances appear first at the upper surface at a position which moves with a speed close to that of the zonal-mean flow at this surface...