Abstract
Abstract This paper investigates the generation and propagation of spiral bands on an axisymmetric base-state vortex. A linear model is used to study the formation of bands from internal gravity-inertia waves in a barotropic atmosphere. Spiral bands form random perturbations placed on a vortex with unstable static stability that is equal for ascending and descending motion. This growing mode assumes the characteristics of pseudoadiabatic motion in a conditionally unstable atmosphere due to the coarse vertical resolution of the linear model. Implicit diffusion from the centered finite-difference scheme shifts the preferred growth modes from infinite wavenumbers, characteristic of inviscid analytical solutions, to 4Δλ and 4Δr,ar wavelengths in numerical experiments. Here Δλ and Δr are the angular and radial distances between grid points. Explicit diffusion representing subgrid-scale eddies shifts preferred modes to longer wavelengths. Rotation in the basic state is a necessary condition before the unstable ...
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