Unstable Radiative–Dynamical Interactions. Part I. Basic Theory

Abstract

The interaction between trace shortwave radiative absorbers and the dynamical circulation is shown to be linearly unstable for horizontally uniform basic states with a vertical gradient in the basic-state absorber mixing ratio. Two types of instability are identified, described as the advective mode and the propagating mode. The advective mode is unstable when the basic-state absorber mixing ratio decreases with height. Upward motion, high absorber concentration and warm temperature are typically in phase for this mode. Growth rates, which can be competitive with those associated with baroclinic instability, are largest for perturbations that are much shorter than the internal deformation radius. Thus, the requirement that the basic state be horizontally uniform is often satisfied for the advective mode. The propagating mode is unstable when the basic-state absorber mixing ratio increases with altitude. Propagating waves such as Rossby and inertia-gravity waves are amplified by the feedback with absorber transport and radiative heating. Growth rates for the propagating mode are usually bounded by the frequency of oscillation of the ambient wave, an important limitation for slowly propagating waves such as Rossby waves. Vertical transport of the absorber by the amplifying mode is down the basic-state absorber mixing ratio gradient in each case.