Jeffrey R. Barnes

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 atmospheric dynamics as simulated by the NASA Ames General Circulation Model (GCM). This paper describes the GCMs 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.

General circulation model simulations of the Mars Pathfinder atmospheric structure investigation/meteorology data

The NASA Ames Mars General Circulation Model is used to interpret selected results from the Mars Pathfinder atmospheric structure instrument/meteorology (ASI/MET) experiment. The present version of the model has an improved soil thermal model, a new boundary layer scheme, and a correction for non-local thermodynamic equilibrium effects at solar wavelengths. We find good agreement with the ASI/MET entry data if the dust observed at the Pathfinder site is assumed to be distributed throughout the lowest five to six scale heights. This implies that the dust is globally distributed as well. In the lower atmosphere the inversion between 10 and 16 km in Pathfinders entry profile is likely due to thermal emission from a water ice cloud in that region. In the upper atmosphere (above 50 km), dynamical processes, tides in particular, appear to have a cooling effect and may play an important role in driving temperatures toward the CO2 condensation temperature near 80 km. Near-surface air temperatures and wind directions are well simulated by the model by assuming a low surface albedo (0.16) and moderately high soil thermal inertia (336 SI). However, modeled tidal surface pressure amplitudes are about a factor of 2 smaller than observed. This may indicate that the model is not properly simulating interference effects between eastward and westward modes.

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 ∼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 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.

Overview of the Mars Pathfinder Mission: Launch through landing, surface operations, data sets, and science results

Mars Pathfinder successfully landed at Ares Vallis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Path-finder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high-resolution orbiter images and in inertial space from two-way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and ∼2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Air-borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by ∼3–7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near-surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.

Forced Stationary Planetary Waves in Mars's Winter Atmosphere

Abstract Mariner 9 and Viking spacecraft observations provided evidence for planetary-scale, wavelike disturbances in the Mars winter atmosphere. Possible sources of the wave activity are dynamical instabilities, for example, barotropic and / or baroclinic instabilities. Other candidate sources are forced. quasi-stationary planetary waves—waves that arise predominantly via zonally asymmetric surface properties. The authors attempt to model aspects of the wave activity, focusing on forced planetary waves in representative wintertime atmospheres for Mars, by applying a spherical linear primitive equations model. Basic states representing relatively “nondusty” and “highly dusty” conditions near winter solstice allow wavenumber 1 and 2 disturbances to propagate meridionally and vertically about the jet. Higher wavenumbers are strongly vertically trapped. Stationary waves during winter in the northern and southern extratropics differ strongly in amplitude, phase, and dominant horizontal wave pattern. Northern ...

Albedo of the south pole on Mars determined by topographic forcing of atmosphere dynamics

The nature of the martian south polar cap has remained enigmatic since the first spacecraft observations. In particular, the presence of a perennial carbon dioxide ice cap, the formation of a vast area of black ‘slab ice’ known as the Cryptic region and the asymmetric springtime retreat of the cap have eluded explanation. Here we present observations and climate modelling that indicate the south pole of Mars is characterized by two distinct regional climates that are the result of dynamical forcing by the largest southern impact basins, Argyre and Hellas. The style of surface frost deposition is controlled by these regional climates. In the cold and stormy conditions that exist poleward of 60° S and extend 180° in longitude west from the Mountains of Mitchel (∼ 30° W), surface frost accumulation is dominated by precipitation. In the opposite hemisphere, the polar atmosphere is relatively warm and clear and frost accumulation is dominated by direct vapour deposition. It is the differences in these deposition styles that determine the cap albedo.

Mars atmospheric dynamics as simulated by the NASA Ames general circulation model: 3. Winter quasi‐stationary eddies

A set of simulations with the NASA Ames Mars general circulation model (GCM) has been analyzed to define the basic properties and dynamics of quasi-stationary eddy circulations in the winter hemisphere. These circulations, differing substantially from those in low latitudes and the summer hemisphere, extend from the surface to the model top at ∼47 km; typically, the largest geopotential and wind perturbations are found at the highest model level. Near the surface the eddies are of largest amplitude in lower latitudes, but above ∼5–10 km the amplitudes are a maximum in middle and high latitudes. The vertical structure of the eddies is nearly equivalent barotropic; the phase variation in latitude is also relatively small. Zonal wavenumbers 1 and 2 dominate the stationary circulations, with wave 3 being of significance only at low levels (below ∼10 km). The quasi-stationary eddies differ substantially in northern and southern winters, and undergo considerable changes with increasing dustiness in northern winter. The southern winter eddy circulation is dominated by zonal wavenumber 1, while the northern eddies have a strong wavenumber 2 component at low levels of dust loading; at high dust levels wavenumber 1 becomes dominant. This change may be at least partly the result of the dusty zonal-mean state becoming more responsive to wave 1 forcing. The standing eddies in a GCM simulation for Ls ∼ 0° exhibit considerable similarity in basic structure and amplitude to those revealed by an recent analysis of Viking IRTM data [Banfield et al., 1996], though there are differences in the phases of the eddy patterns.

Meteorological predictions for the Mars Pathfinder lander

The NASA Ames Mars general circulation and boundary layer models are used as guides to forecast the meteorological environment of the Pathfinder lander site. Based on these models we predict that for a Viking-like atmospheric dust loading, significant vertical wave structure will be seen in the entry temperature profile above 50 km. Temperatures in this region will oscillate by up to 30 K about a mean value of ∼145 K. At the surface during the primary mission, winds are expected to rotate clockwise during the day at 2–10 m/s, while air temperatures will range from overnight lows in the mid 180s K to daytime highs near 250 K. If the atmosphere is dust free at arrival, as appears possible from recent Earth-based observations, then we expect a cooler upper atmosphere (130–140 K) with less wave structure during entry, weaker surface winds, and a slight increase in near surface air temperatures. Regardless of dust loading, we predict that the daily averaged surface pressure will reach its annual minimum about 15–20 sols after landing. During the extended mission, the basic character of Pathfinders meteorology will change from that of a summertime quasi-regular regime to that of a wintertime highly variable regime. This transition is predicted to occur by sol 60. The strongest winds (30 m/s) and lowest daily-averaged temperatures (∼200 K) will be recorded during early winter.

Dynamical modeling of a planetary wave mechanism for a Martian polar warming

Abstract A dynamical mechanism for the Martian (atmospheric) polar warming observed by the Viking IRTM during the 1977 winter solstice dust storm (and a similar one possibly observed by Mariner 9 in 1971) is proposed, and investigated using a simplified nonlinear model. The model is of a type previously used to successfully simulate the essential aspects of terrestrial sudden stratospheric warmings. The dynamical mechanism is, in part, very similar fundamentally to that responsible for these warmings, involving planetary-scale waves. A number of numerical experiments have been conducted to assess the basic viability of such a mechanism for the martian polar warming and to examine its sensitivity to several factors. These experiments demonstrate that a planetary wave mechanism can produce a polar warming with the magnitude and suddenness of that observed. A planetary wave mechanism must primarily involve wavenumber 1, as wavenumber 2 is too strongly vertically trapped to produce a warming like that observed. The necessary wave forcing in the present model can be topographic (mechanical) or thermal (and nonstationary), and is relatively large but certainly plausible. The strong radiative damping in the Mars atmosphere acts to substantially inhibit a warming, through its effects on both the zonal flow and the wave. Dissipation plays a greater role relative to transience in a model Martian warming of the type studied here than in a sudden stratospheric warming. Increasing radiative damping during a warming due to higher temperatures and greater dust loading may play a role in yielding a relatively rapid cooling phase for the Mars warming event. The residual mean meridional circulation during a model warming entails strong poleward and downward flow into high northern latitudes, throughout a very deep region. This probably indicates similar transport of atmospheric dust, as well as water. Such transports are of considerable potential significance for both the dust and water cycles on Mars.

The Martian Zonal-Mean Circulation: Angular Momentum and Potential Vorticity Structure in GCM Simulations

Abstract Analysis of simulations performed with the NASA/Ames Mars GCM shows that under dusty conditions the Northern Hemisphere winter solstice circulation becomes characterized by a zonally averaged state in which the potential vorticity at upper levels is very small outside of high latitudes. The available observational data-in particular the 15-µm observations obtained by the Viking IRTM during the 1977 winter solstice global dust storm-provide evidence for changes in the Martian circulation that are basically like those found in the GCM. In the Mars GCM simulations for dusty solstice conditions, an extremely intense and approximately angular-momentum-conserving Hadley circulation is responsible for creating the low potential vorticity configuration. This can be contrasted with the Venus-Titan numerical simulations discussed by Allison et al. in which quasi-barotropic eddies appear to be largely responsible for the existence of low potential vorticity in lower and midlatitudes. At a near-equinox seaso...