MARS ATMOSPHERIC DYNAMICS AS SIMULATED BY THE NASA AMES GENERAL-CIRCULATION MODEL .2. TRANSIENT BAROCLINIC EDDIES

A large set of experiments performed with the NASA Ames Mars general circulation model (GCM) have been analyzed to determine the properties, structure, and dynamics of the simulated transient baroclinic eddies. The Mars GCM simulations span a wide range of seasonal dates and dust loadings and include a number of special sensitivity experiments (e.g., with flat topography). There is strong transient baroclinic eddy activity in the extratropics of the northern hemisphere during the northern autumn, winter, and spring seasons. The eddy activity remains strong for very large dust loadings, though it shifts northward. The eastward propagating eddies are characterized by zonal wavenumbers of 1-4 and periods of approximately 2-10 days. In several simulations, the eddy variance is dominated by a single zonal wavenumber and a narrow range of periods. The longer (wavenumbers 1 and 2) transient eddies have a very deep vertical structure, exhibiting a maximum kinetic energy density at the model top (approximately 45-50 km) in many of the simulations. In a simulation for early northern spring the eddies are quite shallow however. The transient eddies generate the bulk of their energy baroclinically via large meridional and vertical heat fluxes, at both lower and upper levels. This is despite the fact that their vertical structure is typically close to equivalent barotropic above the lowest 10 km. The eddies also appear to generate a substantial amount of energy barotropically, via large meridional momentum fluxes at both lower and upper levels. In the tropics and northern subtropics at upper levels(approximately 25-50 km) there are strong transient eddy motions with structures resembling those characterisitic of inertially unstable modes. This eddy activity appears to be a response to the forcing of a region of marginal inertial stability by the extratropical transient baroclinic eddies, as the wavenumbers and periods are the same as those in the extratropics. A major surprise is the presence of very weak transient eddy activity in a number of the southern winter simulations. It appears that this is partly a consequence of the stabilizing effects of the zonally symmetric topography in the GCM, but it also must be associated with certain aspects of the zonal-mean circulation in southern winter. This is indicated by the presence of relatively large amplitude eddies in simulations for early southern autumn and spring and in a southern winter solstice simulation incorporating a different topography (derived from the Mars Digital Terrain Model). This topography differs from that used in most of the GCM simulations in not being characterized by steep symmetric slopes (which are stabilizing to baroclinic instability) in southern high latitudes. It is hypothesized that the very large extent of the southern seasonal polar cap and high elevations in the south both might contribute to weakening the transient eddy activity. Large zonally symmetric topography in the northern hemisphere of the Mars GCM also appears to have a strong impact on the transient eddies, acting to increase the dominant zonal wavenumbers and phase speeds. The properties of the GCM baroclinic eddies in the northern extratropics are compared in detail with analogous properties inferred from Viking Lander meteorology observations. The GCM eddies are found to be very similar to those observed in most respects. A notable exception is that the eddy amplitudes in the highly dusty GCM simulations are much larger than those observed by Viking during the 1977 winter solstice dust storm. This is almost certainly at least partly due to the relatively small latitudinal expansion of the Hadley circulation in the highly dusty GCM experiments.