Simulations of the general circulation of the Martian atmosphere: 1. Polar processes

We have conducted numerical simulations of the general circulation of the Martian atmosphere with a three‐dimensional model based on the primitive equations of meteorology. The radiative effects of atmospheric dust on solar and thermal radiation have been incorporated into the model. It has 13 vertical layers that span the altitude range from the surface to approximately 47 km and a horizontal resolution of 7.5° latitude by 9° longitude. A large number of numerical experiments were conducted for alternative choices of seasonal date and dust optical depth. During each experiment the dust optical depth was kept constant in time, and the amount of dust was constant in space as well, except for the effects of topography. Carbon dioxide condensed in the atmosphere as well as at the ground in the winter polar regions over the entire range of dust optical depths considered (0–5). However, the rate of atmospheric CO 2 condensation increased sharply as the dust content of the winter polar region increased. The simulated properties of atmospheric CO 2 condensation imply that CO 2 ice clouds are chiefly responsible for the occurrence of anomalously low brightness temperatures in the winter polar regions and that they manifest themselves at optical wavelengths as polar hoods. A number of hemispherical asymmetries may ultimately be due to the strong seasonal variation in the atmospheric dust content and to feedback relationships between the dust optical depth on the one hand and the magnitude of poleward heat transport and the rate of atmospheric CO 2 condensation on the other hand. These asymmetries include the greater prevalence of polar hoods in the northern polar region during winter, the lower albedo of the northern polar cap during spring, and the total dissipation of the northern CO 2 ice cap during the warmer seasons.