Linear Baroclinic Instability in the Martian Atmosphere

Abstract

The linear baroclinic instability of zonal-mean flows like those in the wintertime Martian atmosphere under both relatively nondusty and highly dusty conditions is examined using a spherical quasi-geostrophic model. The basic states are idealized, but based closely upon Mariner 9 and Viking observations. Zonal wavenumbers 3 and 4 are found to be most unstable and phase speeds are ?10?20 m s?1 in middle latitudes generally consistent with observations as well as with previous results obtained with simpler models. Growth rates are not greatly reduced by the rapid Martian radiative relaxation, though the growth at higher wavenumbers is significantly inhibited by Ekman friction. Even with this dissipation minimum e-folding times of ?2 days are obtained. For a dust storm basic state characterized by enhanced static stability growth rates are substantially decreased, but the most unstable wavenumber is essentially not altered. This behavior differs from that found in simple models, but is consistent with that expected in a Charney-type model. The most unstable scale is shown to be sensitive to the vertical distribution of static stability, rather than the mean stability. The structures of the spherical modes are similar to those for terrestrial zonal flows, if similar zonal wavelengths are compared. Wavenumber 2 exhibits considerable vertical penetration. The modes for the dust storm state are situated farther poleward than the others, significantly reducing the relative amplitudes in middle latitudes. Several aspects of the meridional and vertical structure of the modes are discussed in relation to Viking lander observations and Mariner 9 IRIS data. Zonally symmetric topography like that in the northern hemisphere of Mars is found to decrease the linear growth rates substantially, without significantly changing the most unstable scale, and to increase the phase speeds.