Baroclinic Instability of Easterly Jets with Applications to the Summer Mesosphere

Abstract

anally and seasonally averaged zonal wind fields in the summer mesosphere are unstable in the Charney-Stern sense, with a primarily baroclinic component. Two-dimensional stability analyses show peaks in the unstable wave growth spectrum at zonal wavenumbers 2?4, with periods of 1.4?3 days, for a variety of basic state flows. Wavenumbers of peak growth are consistently lower for the two-dimensional analyses than for the one-dimensional analyses, with the lowest wavenumber peaks found for the basic state flows with the smallest latitudinal scale. For strongly baroclinic basic state flows, wave growth was inhibited by introducing stronger latitudinal variations in the basic state flow; however, for marginally unstable flows, stronger latitudinal variations slightly enhance the peak growth rates. The strong thermal damping in the mesosphere reduces growth rates by 20?60% of the damping rate, but has little effect on the wave structure. The primary significance of the thermal damping is that basic state flows cannot be substantially less unstable than the weakly unstable flows considered here in order to allow growing waves. The growth spectrum peak at zonal wavenumber 3 with a period close to two days is a good match for the observed two-day wave phenomenon, but the calculated temperature structures have their maximum amplitude at 40?60° latitude, rather than the observed 20°. Other periodicities ranging from 1.7 to 4 days have also been observed in the meteor region at middle and high latitudes, and these could be explained by baroclinic instability as well.