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.

No abstract

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.

Anthropogenic sulfate (SO 4) aerosol particles play two potential roles in the radiative climate of the earth. In cloud-free air, SO 4 particles scatter sunlight, some of which is lost to space, thereby reducing solar irradiance at the ground. The same particles can act as cloud condensation nuclei (CCN), the number concentration of which is an important determinant of cloud albedo. This albedo effect, in turn, also influences incoming short-wave solar radiation. Development of a three-dimensional global model for estimating the SO 4 aerosol mass concentration, along with previously-acquired information on the scattering and back-scattering coefficients per unit mass concentration allow calculation of the effects of anthropogenic SO; aerosol on clear-sky optical depth. Subsequently, this can be used to estimate the change in hemispheric and global average reflected solar radiation. The conclusion is that the change of reflected solar flux due to anthropogenic S04 averaged over the Northern Hemisphere is ca. -1.1 wm-2 , which is comparable but opposite in sign to the present-day radiative forcing by anthropogenic C0 2 , + 1.5 Wm -2 • Because of the spatial variability of the anthropogenic SO; distribution, its meteorological effects must be studied regionally. That is, global models with regional resolution and regional data are required. Unlike the direct effect on solar irradiance, the relationship of CCN number concentration to mass concentration is not known. Thus it is not yet possible to make quantitatively reliable statements about anthropogenic forcing of cloud albedo, although there is qualitative evidence that the CCN effect may also be substantial.

No abstract

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 ∼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 (∼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 baroclincally 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 (∼25–50 km) there are strong transient eddy motions with structures resembling those characteristic 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 incr...