MARS ATMOSPHERIC DYNAMICS AS SIMULATED BY THE NASA AMES GENERAL-CIRCULATION MODEL .1. THE ZONAL-MEAN CIRCULATION

This is the first in a series of papers that will discuss Mars atmosph eric dynamics as simulated by the NASA Ames General Circulation Model (GCM). This paper describes the GCM's zonal-mean circulation and how i t responds to seasonal variations and dust loading. The results are co mpared to Mariner 9 and Viking observations, and the processes respons ible for maintaining the simulated circulation are discussed. At the s olstices the zonal-mean circulation consists of a single cross-equator ial Hadley circulation between 30-degrees-S and 30-degrees-N. For rela tively modest dust loadings (tau=0.3), the associated peak mass flux i s 100 x 10(8) kg s-1 at northern winter solstice and 55 x 10(8) kg s-1 at southern winter solstice. At both seasons, westerlies dominate the winter hemisphere, and easterlies dominate the summer hemisphere. Max imum zonal winds occur near the model top (approximately 47 km) and ar e about the same at both seasons: 120 m s-1 in the winter hemisphere a nd 60 m s-1 in the summer hemisphere. Mean surface westerlies of 10(-2 0) m s-I are predicted at the middle and high latitudes of the winter hemisphere, as well as in the summer hemisphere near the rising branch of the Hadley circulation. The latter has the structure of a ''jet'' and is particularly strong (>20 m s-1) at northern winter solstice. Wi th increasing amounts of dust (up to tau=5), the zonal mean circulatio n at northern winter solstice intensifies and gives no indication of a negative feedback. Dust can easily double the mass flux of the Hadley circulation. In the solstice simulations, the mean meridional circula tion is the main dynamical contributor to the heat and momentum balanc e; the eddies play a relatively minor role. There is no evidence in th ese simulations for a polar warming. At the equinoxes the zonal mean c irculation is more Earth-like and consists of two roughly symmetric Ha dley cells with westerly winds in the mid-latitudes of each hemisphere and easterlies in the tropics. The simulated zonal winds are about ha lf as strong as they are at solstice. However, the strength of the mea n meridional circulation is much less than at solstice and averages be tween 5 and 10 x 10(8) kg s-1. At these seasons, the eddies and mean c irculation make comparable, but opposing, contributions to the heat an d momentum balances.