Unstable Radiative - Dynamical Interactions

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 usually unstable when the basic state absorber mixing ratio decreases with height. Upward motion, high absorber concentration and warm temperatures 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 normally 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 gradient in each case. Complicating factors such as the effects of dissipative processes, vertical shear, and scattering of sunlight are also considered. The competition between vertical transport of absorber due to the modal instability mechanism described above and that due to initial horizontal patchiness of an absorber injection is also examined. For patches with horizontal scales much larger than the deformation radius the modal form of absorber transport is shown to be important, while for smaller patch scales the nonmodal form dominates. Thesis Supervisor: Peter H. Stone Title: Professor of Meteorology