Combined Dynamics of Boundary and Interior Perturbations in the Eady Setting

Abstract

A theoretical study is undertaken of the relative influence of intermediate-level, near-surface, and tropopause-level perturbations upon flow development and cyclogenesis. The study is based upon two complementary quasigeostrophic models with consideration given to different types of perturbations of Eady-like basic states. First, the standard linear Eady model is refined to include an interior, vertically localized potential vorticity (PV) wave perturbation, and the wave's influence upon the growth of the boundary waves is examined as a function of its vertical location and its initial phase and amplitude. It is shown that the pseudoresonance arising from the interaction of the interior PV wave with a surface-confined wave is highly transient and comparatively weak. In contrast, a PV wave located at midlevel can have supraexponential sustained impact upon the perturbation growth that results from a combination of its direct forcing effect and its ability to maintain the two Eady edge waves near quadrature for longer, rather than asymptoting to the normal mode setting. Second, an extended nonlinear configuration is adopted that comprised a baroclinic jetlike basic state of uniform PV perturbed by a localized upper-boundary thermal perturbation or an interior point PV vortex, and the vortex's influence upon the baroclinic growth and cyclogenesis is examined in terms of its vertical and horizontal vertical location. It is shown that the interior perturbation can exert an appreciable and sustained influence upon the growth when it is located on or near the so-called critical surface, and the attendant flow transits smoothly to modal behavior. The derived results point to the possible influence of low- and midtropospheric perturbations upon cyclone development, and are in accord with and can account for specific aspects of the structure and properties of singular vector (SV) perturbations.