Numerical studies of barotropic stability of stratospheric states

The linear stability properties of basic states relevant to the stratosphere are studied in a barotropic nondivergent spectral model on a sphere. The stability of realistic zonally symmetric jets is examined in the first part, and the second part a deals with zonally asymmetric basic states;In the first part attention is focused on unstable modes that are associated with a region of negative basic state absolute vorticity gradient on the poleward side of the jet. These modes are approximately nondispersive. Study of realistic analytical jet profiles shows that broader jets, and jets which peak at higher latitudes produce poleward modes that are less dispersive. Jet profiles derived from observational data are studied in detail for three Southern Hemisphere winter months, and the results are compared with quasi-nondispersive features which have been observed in satellite data in the Southern hemisphere winter stratosphere. Characteristics of the barotropically unstable modes agree remarkably well with those of the observed features. The appearance of westward moving modes in the summer hemisphere during June, and in analytical profiles with realistic global structure, is noted;The second part focuses on the stability of zonally symmetric basic states which include a realistic jet profile such as previously studied, and a traveling wave resembling observed features in the winter stratosphere. This is of interest because many planetary scale traveling waves are observed in the winter stratosphere. Basic state waves are chosen to resemble observed features. Results for growth rates and energy conversion for the most unstable disturbances are presented as a function of the amplitude and frequency of the basic state wave. These results show ranges of basic state wave amplitude where the total basic state is more stable that a zonally symmetric basic state with the same jet profile, and also ranges where it is more unstable. At very small basic state wave amplitudes, the stability characteristics differ markedly from those for the zonally symmetric problem only when the phase speed of the basic state wave is near that of a free mode of the zonally symmetric problem. Suggestions are given as to how the results may apply to the winter stratosphere