Radiative and Photochemical Processes in Mesospheric Dynamics: Part IV, Stability of a Zonal Vortex at Mid-Latitudes to Baroclinic Waves

Abstract

The models developed in Part I for radiative transfer and ozone photochemistry in the mesosphere are incorporated into a two-level model for baroclinic flow, and the effect of radiative and photochemical processes on the stability of the flow is separately investigated for radiative and photochemical conditions obtaining at 30 km and 52.5 km. In each case it is found that the flow is unstable for all non-zero values of shear, in contrast to the adiabatic case where instability required that the shear exceed some critical shear. At 30 km the instabilities at low shears differ considerably from the instabilities for higher shears near the critical shear of the adiabatic theory. The latter have a dominant wavelength of the order of 10,000 km and a phase speed relative to the mean zonal wind of about ?20 m sec?1. The former have a dominant wavelength of about 5000 km and a relative phase speed of about ?2 m sec?1. The effect of the advection of ozone on the heating appears to be responsible for the low shear mode. This effect is negligible at 52.5 km where there are no significant differences (apart from growth rate) between low and high shear instabilities. The instabilities at this level have a dominant wavelength of about 7900 km and a relative phase speed of about ?20 m sec?1.