This is the first in a series of papers that will discuss Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model (GCM). This paper describes the GCM's zonal‐mean circulation and how it responds to seasonal variations and dust loading. The results are compared to Mariner 9 and Viking observations, and the processes responsible for maintaining the simulated circulation are discussed. At the solstices the zonal‐mean circulation consists of a single cross‐equatorial Hadley circulation between 30°S and 30°N. For relatively modest dust loadings (τ=0.3), the associated peak mass flux is 100 × 108 kg s−1 at northern winter solstice and 55 × 108 kg s−1 at southern winter solstice. At both seasons, westerlies dominate the winter hemisphere, and easterlies dominate the summer hemisphere. Maximum zonal winds occur near the model top (∼47 km) and are about the same at both seasons: 120 m s−1 in the winter hemisphere and 60 m s−1 in the summer hemisphere. Mean surface westerlies of 10–20 m s−1 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. With increasing amounts of dust (up to τ=5), the zonal mean circulation 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 circulation is the main dynamical contributor to the heat and momentum balance; the eddies play a relatively minor role. There is no evidence in these simulations for a polar warming. At the equinoxes the zonal mean circulation is more Earth‐like and consists of two roughly symmetric Hadley cells with westerly winds in the mid‐latitudes of each hemisphere and easterlies in the tropics. The simulated zonal winds are about half as strong as they are at solstice. However, the strength of the mean meridional circulation is much less than at solstice and averages between 5 and 10 × 108 kg s−1. At these seasons, the eddies and mean circulation make comparable, but opposing, contributions to the heat and momentum balances.

[1] Against a backdrop of intensive exploration of the Martian surface environment, intended to lead to human exploration, some aspects of the modern climate and the meteorology of Mars remain relatively unexplored. In particular, there is a need for detailed measurements of the vertical profiles of atmospheric temperature, water vapor, dust, and condensates to understand the intricately related processes upon which the surface conditions, and those encountered during descent by landers, depend. The most important of these missing data are accurate and extensive temperature measurements with high vertical resolution. The Mars Climate Sounder experiment on the 2005 Mars Reconnaissance Orbiter, described here, is the latest attempt to characterize the Martian atmosphere with the sort of coverage and precision achieved by terrestrial weather satellites. If successful, it is expected to lead to corresponding improvements in our understanding of meteorological phenomena and to enable improved general circulation models of the Martian atmosphere for climate studies on a range of timescales.

We deal here primarily with the surface meteorological data for both Viking landers during the nominal missions (44 sols for lander 1 and 61 sols for lander 2). The diurnal patterns of wind, temperature, and pressure were strongly similar from sol to sol, as was expected in the summer. The chief characteristics of the wind data are that winds were light (a few meters per second), with a complex hodograph at VL‐1 dominated by counterclockwise turning of the wind and a simpler hodograph at VL‐2 marked by clockwise turning of the wind. This repetitive pattern of wind has begun to break down at VL‐2 with advancing season, and several episodes of protracted northeasterly winds have occurred. Some of these are associated with lower than normal temperatures. Examples are given of wind and temperature traces over short periods, illustrating the effects of convection, static stability, and lander interference. We present a theoretical argument based upon the horizontal scale dictated by heating of slopes and upon vertical mixing of momentum to explain the different sense of rotation of the wind vectors at the two sites. Analysis of the semidiurnal pressure oscillation suggests that absorption of solar radiation is an important thermal drive but that convective heat flux from the surface is also significant. The seasonal variation of pressure extending past the end of the nominal missions shows a decrease of pressure to a minimum at Ls ≈ 149° with a rapid rise thereafter. This is clearly due to condensation and sublimation of CO2 on and from the southern polar cap.

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The first systematic observations of the middle atmosphere of Mars (35km–80km) with the Mars Climate Sounder (MCS) show dramatic patterns of diurnal thermal variation, evident in retrievals of temperature and water ice opacity. At the time of writing, the dataset of MCS limb retrievals is sufficient for spectral analysis within a limited range of latitudes and seasons. This analysis shows that these thermal variations are almost exclusively associated with a diurnal thermal tide. Using a Martian General Circulation Model to extend our analysis we show that the diurnal thermal tide dominates these patterns for all latitudes and all seasons.