Ames Mars General Circulation Model Research Articles

Diurnal waves forced by horizontal convergence of near-surface winds on Mars

Boundary-layer winds near steeply sloped topography excite a distinct type of atmospheric wave. Basic properties of the waves were determined through analysis of 7 Mars years of temperature profiles from the Mars Climate Sounder on Mars Reconnaissance Orbiter. A more complete understanding of the waves was derived from a simulation with the NASA Ames Mars General Circulation Model. This type of wave appears in the tropics in the vicinity of conspicuous surface features, such as large volcanoes and the rim of Isidis Planitia. The amplitude is typically 5–10 K, the vertical wavelength is about 40 km, the zonal wavelength is about 1400 km, and the zonal phase speed relative to the surface is about 25 m s−1. The wave forcing comes from horizontal convergence of the near-surface winds, which generates a daily surge in the vertical winds. These convergence zones were not previously recognized as an important source of atmospheric waves on Mars. Strong forcing occurs only within a limited range of local time, producing wave packets (diurnal pulses) rather than steady oscillations. Waves are present throughout the year and their properties vary with season. Near the solstices, the wave amplitude is largest in the tropics of the summer hemisphere, where the convergence zones are stronger than in the winter hemisphere. The diurnal-mean zonal winds, which vary with latitude and season, control the direction of propagation of the predominant waves through filtering by critical levels.

Modeling the effects of martian surface frost on ice table depth

Ground ice has been observed in small fresh craters in the vicinity of the Viking 2 lander site (48°N, 134°E). To explain these observations, current models for ground ice invoke levels of atmospheric water of 20 precipitable micrometers – higher than observations. However, surface frost has been observed at the Viking 2 site and surface water frost and snow have been shown to have a stabilizing effect on Antarctic subsurface ice. A snow or frost cover provides a source of humidity that should reduce the water vapor gradient and hence retard the sublimation loss from subsurface ice. We have modeled this effect for the Viking 2 landing site with combined ground ice and surface frost models. Our model is driven by atmospheric output fields from the NASA Ames Mars General Circulation Model (MGCM). Our modeling results show that the inclusion of a thin seasonal frost layer, present for a duration similar to that observed by the Viking Lander 2, produces ice table depths that are significantly shallower than a model that omits surface frost. When a maximum frost albedo of 0.35 was permitted, seasonal frost is present in our model from Ls=182° to Ls=16°, resulting in an ice table depth of 64cm – which is 24cm shallower than the frost-free scenario. The computed ice table depth is only slightly sensitive to the assumed maximum frost albedo or thickness in the model.

Initial results from radio occultation measurements with the Mars Reconnaissance Orbiter: A nocturnal mixed layer in the tropics and comparisons with polar profiles from the Mars Climate Sounder

The Mars Reconnaissance Orbiter (MRO) performs radio occultation (RO) measurements on selected orbits, generally once per day. We have retrieved atmospheric profiles from two subsets of data, yielding a variety of new results that illustrate the scientific value of the observations. One set of measurements sounded the tropics in northern summer at a local time ∼1h before sunrise. Some of these profiles contain an unexpected layer of neutral stability with a depth of ∼4km and a pressure at its upper boundary of ∼160Pa. The mixed layer is bounded above by a temperature inversion and below by another strong inversion adjacent to the surface. This type of structure is observed near Gale Crater, in the Tharsis region, and at a few other locations, whereas profiles in Amazonis Planitia and Elysium Planitia show no sign of a detached mixed layer with an overlying inversion. We supplemented the measurements with numerical simulations by the NASA Ames Mars General Circulation Model, which demonstrate that water ice clouds can generate this distinctive type of temperature structure through their influence on radiative transfer at infrared wavelengths. In particular, the simulations predict the presence of a nocturnal cloud layer in the Tharsis region at a pressure of ∼150Pa (∼10km above the surface), and the nighttime radiative cooling at cloud level is sufficient to produce a temperature inversion above the cloud as well as convective instability below the cloud, consistent with the observations. The second set of measurements sounded mid-to-high northern latitudes in spring, when carefully coordinated observations by the MRO Mars Climate Sounder (MCS) are also available. The differences between the RO and MCS temperature profiles are generally consistent with the expected performance of the two instruments. Within this set of 21 comparisons the average temperature difference is less than 1K where the aerosol opacities are smaller than 10-3km-1, at pressures of 10–50Pa, whereas it increases to ∼2K where the aerosol opacities exceed this threshold, at pressures of 50–300Pa. The standard deviation of the temperature difference is ∼2K, independent of pressure. The second set of RO measurements also provides unique information about the stability of the annual CO2 cycle and the dynamics near the edge of the seasonal CO2 ice cap.

Characterization of the nighttime low‐latitude water ice deposits in the NASA Ames Mars General Circulation Model 2.1 under present‐day atmospheric conditions

This effort advances the exploration of the current Martian water cycle by analyzing the nighttime equatorial water ice deposits in the NASA Ames Mars General Circulation Model (version 2.1). The possibility that the current Martian water cycle plays a role in the generation or maintenance of the longitudinally confined tropical maxima of water‐equivalent hydrogen (WEH) in the near‐surface regolith is also investigated. Mars Odyssey Orbiter Gamma Ray suite observations indicate that tropical latitudes exhibit longitudinally distinct maxima of the presence of near‐surface hydrogen, likely in the form of hydrated minerals, in the Arabia and Tharsis regions. Atmospheric numerical simulations with the NASA Ames Mars General Circulation Model produce an annual water cycle that faithfully reproduces the cycle derived from Mars Global Surveyor Thermal Emission Spectrometer measurements. This simulated water cycle produces maxima of near‐surface tropical water condensation at longitudes consistent with the WEH maxima regions, though these maxima are shifted north of the Odyssey Gamma Ray suite WEH regions. While the simulated pattern of near‐surface tropical water condensation does exhibit a longitudinal structure consistent with the observed WEH maxima, it is concluded that the current water cycle is not playing a dominant role in the formation or retention of those maxima. Thus, atmospheric involvement in the formation of the WEH‐rich regions must occur during past climatic conditions, if at all.

Dissecting the polar dichotomy of the noncondensable gas enhancement on Mars using the NASA Ames Mars General Circulation Model

The atmospheric processes underlying the observed spatial and temporal enhancement of noncondensing gases in Mars' atmosphere are investigated. The Gamma Ray Spectrometer (GRS) on board Mars Odyssey has obtained measurements indicating that the absolute and relative column abundance of noncondensing gases (primarily argon and nitrogen) maximizes at high latitudes in both hemispheres during winter as CO2 gas condenses and forms the seasonal polar ice cap. This condensing CO2 “leaves behind” noncondensing gases whose local absolute and relative column abundances can increase at a rate controlled by mixing with less‐enhanced air from lower latitudes. Understanding the processes responsible for the magnitude and seasonal variations of these enhancement values is an aid in understanding atmospheric transport processes. The NASA Ames Mars General Circulation Model is employed to help understand the atmospheric thermodynamical mechanisms that give rise to the observed temporal and magnitude variations in the polar enhancement values. The model produces a threefold noncondensable gas enhancement in the south polar region and an approximate 1.4‐fold increase in noncondensables in the north polar region. These model results are temporally consistent with observed values, but the observed enhancement magnitudes exceed those modeled by up to a factor of two. The difference in strength and the season of formation between transient eddies in the southern and northern hemispheres may play a large role in determining the different character of the two polar enhancements. Model simulations also illuminate the effect that topography, orbital eccentricity, and atmospheric dust opacity have on producing the north versus south polar enhancement dichotomy.

Modeling the Martian dust cycle and surface dust reservoirs with the NASA Ames general circulation model

We employ the NASA Ames Mars general circulation model (GCM) to investigate the dust lifting mechanisms responsible for the observed Martian dust cycle and the net surface response to the combined influence of dust lifting and deposition. This GCM includes lifting, transport, and deposition of radiatively active dust. Two dust lifting mechanisms are accounted for: wind stress lifting and dust devil lifting. A “baseline” simulation is presented and shown to compare well to available spatial and temporal observations of atmospheric opacity, wind stress dust lifting events, and atmospheric temperatures recorded during a nonglobal dust storm year. Multiple simulations were conducted to explore the model's sensitivity to a wide range of dust lifting parameters (the functional dependence of surface dust flux on wind stress, the wind stress threshold for lifting, etc.) Model results robustly suggest that wind stress lifting produces the peak in atmospheric dust load during southern spring and summer and that dust devils maintain the background haze of atmospheric dust during northern spring and summer. These results are consistent with previously published conclusions. Dust devil and wind stress lifting contribute equally to the simulated total amount of dust lifted annually during nonglobal dust storm years. The simulated spatial pattern of annual net deflation/deposition suggests that the low thermal inertia regions (Tharsis, Arabia, and Elysium) are not currently net dust accumulation regions. This net deflation is the result of dust devil dust lifting, suggesting that dust devils could play an important role in the present‐day pattern of surface dust reservoirs.

Validation of Mars Global Reference Atmospheric Model (Mars-GRAM 2001) and planned new features

For altitudes below 80 km, Mars Global Reference Atmospheric Model (Mars-GRAM 2001) is based on output climatology from NASA Ames Mars General Circulation Model (MGCM). At COSPAR 2002, results were presented of validation tests of Mars-GRAM versus data from Mars Global Surveyor Thermal Emission Spectrometer (TES) and Radio Science (RS) experiment. Further validation tests are presented comparing Mars-GRAM densities with those from the European Mars Climate Database (MCD), and comparing densities from both Mars-GRAM and MCD against TES observations. Throughout most of the height and latitude range of TES data (0–40 km and 70S to 70N), good agreement is found between atmospheric densities from Mars-GRAM and MCD. However, at the season and latitude zone for Mars Phoenix arrival and landing ( L s = 65°–80° and latitude 65N to 75N), Mars-GRAM densities are about 30%–45% higher than MCD densities near 40 km altitude. Further evaluation is warranted concerning potential impact of these model differences on planning for Phoenix entry and descent. Three planned features for Mars-GRAM update are also discussed: (1) new MGCM and Thermospheric General Circulation Model data sets to be used as a revised basis for Mars-GRAM mean atmosphere, (2) a new feature to represent planetary-scale traveling waves for upper altitude density variations (such as found during Mars Odyssey aerobraking), and (3) a new model for effects of high resolution topographic slope on winds near the surface (0–4.5 km above Mars Orbiter Laser Altimeter topography level). Mars-GRAM slope winds will be computed from a diagnostic (algebraic) relationship based on Ye et al. [Ye, Z.J., Segal, M., Pielke, R.A., A comparative study of daytime thermally induced upslope flow on Mars and Earth, J. Atmos. Sci. 47(5), 612–628, 1990]. This approach differs from mesoscale models (such as Mars Regional Atmospheric Modeling System and Mars Mesoscale Model Version 5), which use prognostic, full-physics solutions of the time- and space-dependent differential equations of motion. As such, slope winds in Mars-GRAM will be consistent with its “engineering-level” approach, and will be extremely fast and easy to evaluate, compared with mesoscale model solutions. Mars-GRAM slope winds are not being suggested as a replacement for sophisticated, full-physics Mars mesoscale models, but may have value, particularly for preliminary screening of large numbers of candidate landing sites for future Mars missions, such as Phoenix and Mars Science Laboratory. Test output is presented from Mars-GRAM slope winds in the area of Gusev Crater and Valles Marineris.

Observing the martian surface albedo pattern: Comparing the AEOS and TES data sets

High spatial resolution images of Mars were acquired with the Advanced Electro-Optical System (AEOS) 3.63-meter telescope at the Maui Space Surveillance System (MSSS) during both the 2001 and 2003 Mars apparitions. Comparisons are made of the surface albedo patterns obtained from these AEOS images to the surface albedo maps constructed from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data taken during the same time periods. These comparisons demonstrate that the images provide albedo information in a limited area surrounding the sub-Earth point that is consistent with the TES-derived albedo field. Additionally, it is shown that by employing adaptive optics (AO), the typical ground-based observing season of Mars can be extended. This is the only known published AO data set of Mars with temporal coverage over an entire apparition. Changes in the surface albedo affect the local ground temperature, which impacts the depth of the planetary boundary layer (PBL) above the surface. Since it is the state of the PBL that controls surface/atmospheric interaction, albedo variations have the power to alter the amount of dust that is lifted. A one-dimensional radiative/convective version of the NASA Ames Mars General Circulation Model is used to demonstrate that the measured albedo variations can alter the daytime ground temperatures by as much as 5 K, which in turn alters the structure of the planetary boundary layer (PBL). Therefore, albedo changes are thermodynamically important, and the ability to characterize them, should orbital observations become unavailable, is a valuable capability.

Global MGS TES data and Mars-GRAM validation

Mars Global Reference Atmospheric Model (Mars-GRAM 2001) is an engineering-level Mars atmosphere model widely used for many Mars mission applications. From 0 to 80 km, it is based on NASA Ames Mars General Circulation Model (MGCM), while above 80 km it is based on University of Michigan Mars Thermospheric General Circulation Model. Mars-GRAM 2001 and MGCM use surface topography from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). Validation studies are described comparing Mars-GRAM with a global summary data set of Mars Global Surveyor Thermal Emission Spectrometer (TES) data. TES averages and standard deviations were assembled from binned TES data which covered surface to ∼40 km, over more than a full Mars year (February 1999–June 2001, just before start of a Mars global dust storm). TES data were binned in 10° × 10° latitude–longitude bins (36 longitude bins, centered at 5°–355°, by 18 latitude bins, centered at −85° to +85°), and 12 seasonal bins (based on 30° increments of L s angle). Bin averages and standard deviations were assembled at 23 data levels (temperature at 21 pressure levels, plus surface temperature and surface pressure). Two time-of-day bins were used: local time near 2 or 14 h. Two dust optical depth bins were used: infrared optical depth, either less than or greater than 0.25 (which corresponds to visible optical depth less than or greater than about 0.5). For interests in aerocapture and precision entry and landing, comparisons focused on atmospheric density. TES densities versus height were computed from TES temperature versus pressure, using assumptions of perfect gas law and hydrostatics. Mars-GRAM validation studies used density ratio (TES/Mars-GRAM) evaluated at data bin center points in space and time. Observed average TES/Mars-GRAM density ratios were generally 1 ± 0.05, except at high altitudes (15–30 km, depending on season) and high latitudes (>45°N), or at most altitudes in the southern hemisphere at L s ∼ 90° and 180°. Compared to TES averages for a given latitude and season, TES data had average density standard deviation about the mean of ∼2.5% for all data, or ∼1–4%, depending on time of day and dust optical depth. Average standard deviation of TES/Mars-GRAM density ratio was 8.9% for local time 2 h and 7.1% for local time 14 h. Thus standard deviation of observed TES/Mars-GRAM density ratio, evaluated at matching positions and times, is about three times the standard deviation of TES data about the TES mean value at a given position and season.

Mars-GRAM validation with Mars global surveyor data

Mars Global Reference Atmospheric Model (Mars-GRAM 2001) is an engineering-level Mars atmosphere model widely used for many Mars mission applications. From 0–80 km, it is based on NASA Ames Mars General Circulation Model (MGCM), while above 80 km it is based on University of Arizona Mars Thermospheric General Circulation Model. Mars-GRAM 2001 and MGCM use surface topography from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). Validation studies are described comparing Mars-GRAM with Mars Global Surveyor Radio Science (RS) and Thermal Emission Spectrometer (TES) data. RS data from 2480 profiles were used, covering latitudes 75°S–72°N, surface to ∼40 km, for seasons ranging from areocentric longitude of Sun ( L s) = 70–160° and 265–310°. RS data spanned a range of local times, mostly 0–9 and 18–24 h. For interests in aerocapture and precision landing, comparisons concentrated on atmospheric density. At a fixed height of 20 km, measured RS density varied by about a factor of 2.5 over the range of latitudes and L s values observed. Evaluated at matching positions and times, average RS/Mars-GRAM density ratios were generally 1 ± 0.05, except at heights above ∼25 km and latitudes above ∼50°N. Average standard deviation (SD) of RS/Mars-GRAM density ratio was 6%. TES data were used covering surface to ∼40 km, over more than a full Mars year (February, 1999 through June, 2001, just before start of a Mars global dust storm). Depending on season, TES data covered latitudes 85°S–85°N. Most TES data were concentrated near local times 2 and 14 h. Observed average TES/Mars-GRAM density ratios were generally 1 ± 0.05, except at high altitudes (15–30 km, depending on season) and high latitudes (>45°N), or at most altitudes in the southern hemisphere at L s ∼ 90° and 180°. Compared to TES averages for a given latitude and season, TES data had average density SD about the mean of ∼2.5% for all data, or ∼1–4%, depending on time of day and dust optical depth. Average SD of TES/Mars-GRAM density ratio was 8.9% for local time 2 h and 7.1% for local time 14 h. Thus SD of observed TES/Mars-GRAM density ratio, evaluated at matching positions and times, is about three times the SD of TES data about the TES mean value at a given position and season.

Nonmigrating tides in the thermosphere of Mars

The vertical propagation of nonmigrating (i.e., longitude‐dependent or non–Sun‐synchronous) solar diurnal and semidiurnal tides into the thermosphere of Mars is investigated through numerical simulation. The waves are generated in the NASA Ames Mars general circulation model (MGCM) through solar radiative, topographic, and nonlinear processes using a comprehensive physics package and including a diurnal cycle. At an altitude near 70 km, zonal wave number decompositions of the diurnal and semidiurnal tidal fields are performed, and each wave component is extended from 70 to 250 km using a linear steady state global scale wave model for Mars (Mars GSWM). Conditions representative of aerocentric longitudes Ls = 30 (near equinox) and Ls = 270 (Southern Hemisphere summer solstice) are considered. Modeled total relative density variations of order ±10–40% near 125 km are analyzed in terms of the zonal wave numbers (ks) seen from the Sun‐synchronous perspective of the Mars Global Surveyor (MGS) accelerometer experiment, and yield reasonable agreement in amplitude and phase with the density measurements. The model indicates the two most important waves responsible for ks = 3 to be the eastward‐propagating diurnal and semidiurnal oscillations with zonal wave numbers s = 2 (∼15–40%) and s = 1 (∼8%), respectively. The eastward‐propagating diurnal component with s = 1 (∼15%) and the semidiurnal standing (s = 0) oscillation (∼4–23%) are concluded to be the main contributors to the ks = 2 longitudinal density variation seen from the Mars Global Surveyor (MGS). The standing (s = 0) diurnal oscillation (∼4–5%) and the westward‐propagating semidiurnal component with s = 1 (∼5–8%) emerge as the most likely contributors to ks = 1. Other waves that may make important secondary contributions include the westward‐propagating semidiurnal oscillations with s = 3 (∼4–6%) and s = 4 (∼3–9%). In addition, above 100 km the wind and temperature fields associated with the above waves represent ∼15–30% perturbations on the Sun‐synchronous wind and temperature fields driven in situ by EUV and near‐IR solar radiation absorption. Nonmigrating tides primarily arise from zonal asymmetries in wave forcing associated with Mars' topography; our results show for the first time that the dynamical effects of Mars' topography extend throughout the atmospheric column to Mars' exobase (∼200–250 km).

Simulation of surface meteorology at the Pathfinder and VL1 sites using a Mars mesoscale model

The Fifth‐Generation Penn State/NCAR Mesoscale Model (MM5) has been adapted to study circulations in the Martian atmosphere. The NASA Ames Mars general circulation model (GCM) provides initial and boundary conditions. The meteorology of this Mars MM5 (the OSU MMM5) is compared with Pathfinder and Viking Lander 1 (VL1) data for late northern summer. The MMM5 uses an equator‐crossing semiglobal polar stereographic mother domain, significantly reducing the boundary reflections inherent in Martian mesoscale simulations. Using two‐way nests, simultaneous simulations of two regions are performed: (1) Chryse Planitia (Mars Pathfinder (MPF)/VL1) and (2) the central chasmas of Valles Marineris. Simulations are hydrostatic and dry. The MMM5 uses the same atmospheric radiation package as the GCM but a much enhanced near‐surface vertical resolution and a different Planetary Boundary Layer (PBL) scheme. Model topography, thermal inertia, and surface albedo maps have all been developed using the most recent data from the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) experiments. The diurnal cycles of surface air temperature, surface pressure, and surface wind all show improvement in comparison with the GCM. For certain regions the local surface pressure tidal amplitudes are strongly dependent on the resolution of the model and/or the topography; VL1 and MPF are in such a region. The diurnal cycles of wind are complex, and for many locations the near‐surface winds are dominated by slope flows of multiple scales. Comparison with data indicates that resolving these slope flows is very important for simulating the diurnal wind cycle. At specific locations these slope flows dramatically influence the diurnal surface pressure cycle.

Mars-GRAM 2000: A Mars atmospheric model for engineering applications

This paper presents Mars Global Reference Atmospheric Model 2000 Version (Mars-GRAM 2000) and its new features. All parameterizations for temperature, pressure, density, and winds versus height, latitude, longitude, time of day, and Ls have been replaced by input data tables from NASA Ames Mars General Circulation Model, for the surface through 80-km altitude, and the University of Arizona Mars Thermospheric General Circulation Model for 80 to 170 km. A modified Stewart thermospheric model is still used for higher altitudes and for dependence on solar activity. “Climate factors” to tune for agreement with general circulation model data are no longer needed. Adjustment of exospheric temperature is still an option. Consistent with observations from Mars Global Surveyor, a new longitude-dependent wave model is included, with user input to specify waves having one to three wavelengths around the planet. A simplified perturbation model has been substituted for the earlier one. An input switch allows users to select either East or West longitude positive. The paper includes instructions on obtaining Mars-GRAM source code, data files, and a users guide for running the program. The Mars-GRAM users guide provides sample input and output and gives an example for incorporating Mars-GRAM as an atmospheric subroutine in a trajectory code.

A comparison of MGS Phase 1 aerobraking radio occultation data and the NASA Ames Mars GCM

The NASA Ames Mars general circulation model (MGCM) is used to investigate how different phenomena in the Martian climate system combine to produce temperature profiles measured by the Radioscience Instrument (RS) aboard Mars Global Surveyor (MGS) in early 1998. When integrated with dust amounts consistent with Thermal Emission Spectrometer (TES) observations, and with a topography data set determined from the Mars Orbiting Laser Altimeter (MOLA), modeled temperature profiles are within 5–10 K of observations. Modeled and measured profiles in the winter subtropics are warmer than pure radiative‐convective considerations suggest, providing further evidence of strong dynamical warming associated with descent in the Hadley cell. South of 40°S, the best fit to the data is obtained by confining airborne dust to below 2 scale heights in this region. The differences in the strength of low‐level inversions found in the MGCM and in the RS data indicate the presence of either a low‐level radiative cooling mechanism such as isolated dust clouds or water‐ice clouds, or a dynamical mechanism such as low‐level gravity wave drag. When initialized with a mean surface pressure that gives the best fit to Viking lander measurements, the model matches RS surface pressure data to within 0.1 mbar. This consistency is a further validation of the MGCM and shows how GCMs can be used to interpret measurements of surface pressure taken at different locations using different methods.

An interpretation of Martian thermospheric waves based on analysis of a general circulation model

Planetary‐scale longitudinal variations in density observed by the Mars Global Surveyor accelerometer in the 125 km region can be qualitatively reproduced by the NASA Ames Mars general circulation model in the 80 km altitude region, but only when locations having specific local times are used in the analysis. If the model results are averaged over all local times, the high‐altitude longitudinal variations nearly disappear, leaving only a small stationary wave 1 pattern, consistent with theory and previous modeling studies. This analysis suggests that the observed wavelike structures are a result of sampling tidal modes at a limited range of local times, rather than by topographically forced Rossby waves as previously suggested.

Cloud formation under Mars Pathfinder conditions

Temperature data taken during the Mars Pathfinder entry are applied to a time dependent microphysical model of cloud formation. Our goal is to reproduce the characteristics of clouds observed at the Pathfinder site. We assume that the total water column abundance is 11 percipitable microns. We also assume that the atmospheric dust initially has a lognormal size distribution with a modal radius of 0.6 μm, reff =1.5 μm, and an optical depth of τ = 0.5. Three temperature profiles are used in the model calculations. The first is the Pathfinder entry observation. The second is this same profile but with the inversion at 10 km removed. The third simulation uses the temperature profiles calculated by the Ames Mars general circulation model for Mars Pathfinder conditions. In all cases clouds form at altitudes of 35–45 km. In addition lower thicker clouds form within the 10 km inversion of the unaltered profile, while peak cloud formation occurred near local noon at 20 km for the GCM calculated profiles. Given a combination of all three temperature scenarios, cloud optical depths, particle sizes, and colors are consistent with IMP observations.

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 Pathfinder's 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.

Low‐level jets in the NASA Ames Mars general circulation model

Previous simulations of the Martian atmosphere have shown how topography acts to confine the low‐level Hadley cell flow into intense jets on the eastern flanks of Tharsis and Syrtis Major. We now conduct detailed studies of these jets using the NASA Ames Mars general circulation model (MGCM). The structure of the flow is found to be sensitive to local topography as well as large‐scale diabatic heating patterns, consistent with terrestrial studies, and MGCM studies carried out with simplified topography. The summer subtropical zonal winds associated with the Hadley circulation also form spatially confined intense jet cores. Diurnal variations in heating affect jet structure in three distinct ways. Global tides interact with the jets, resulting in effects such as the two reinforcing each other at the summer subtropics near midday, leading to high winds and surface stresses at this time. Slope winds act to change the character of the jets during the course of a day, especially at Syrtis Major and the Hellas basin, where slopes are large. Vertical mixing acts to decrease low‐level winds during the late afternoon. The sensitivity of the results to atmospheric dust loading is examined. We finally show how a decrease in boundary layer height due to dust loading actually augments mid‐afternoon jet strength near the surface. The resulting increase in maximum surface stress indicates that this is a positive feedback to dust lifting.

Dust storm impacts on the Mars upper atmosphere

Coupling between the Mars lower (≤80 km) and upper atmospheres is a fundamental issue affecting dynamics, tracer distributions, and energy balance. Also, this coupling has significant bearing on future mission planning to the planet ( e.g. Mars Global Surveyor aerobraking). Two general circulation models are currently being run in tandem to investigate tides and their generation, propagation, and impact upon the Mars lower thermosphere. The NASA Ames Mars General Circulation Model (MGCM) is first run for Mars near-perihelion (Ls ∼ 270) conditions for which a 20-SOL dust-storm is initiated and allowed to self-consistently evolve. Subsequently, selected MGCM upper boundary fields are used to specify the height of the lower boundary and to drive semi-diurnal tides in the NCAR Mars Thermospheric General Circulation Model (MTGCM) for identical Mars parameters. MTGCM key fields are presented to illustrate the profound impact of Mars lower atmosphere heating and tides upon the Mars thermospheric structure and dynamics.

Seasonal variations of storm zones on Mars

Using the NASA Ames Mars general circulation model, an annual cycle simulation has been performed corresponding to a low atmospheric dust loading. Surface pressure time series near the Viking Lander 2 location have been analyzed using singular spectrum analysis (SSA) and autoregressive models to isolate statistically significant spectral power. Also, circulation statistics have been separated using band-pass time filtering and analyzed for coherent spatial patterns. During northern autumn, winter and spring seasons, localized ‘storm zones’ can occur which are particularly strong within the lowlands of Arcadia, Acidalia and Utopia planitia. During early northern spring, the transient eddy activity is stronger than that found during any other season. With weakening baroclinicity accompanying seasonal transition toward midspring, the eddies become less vigorous and the associated storm ‘belt’ shifts into higher latitudes. Since transient baroclinic eddies are active agents in the transport of heat, momentum and moisture in middle latitudes, variations in the storm zones during the seasonal cycle will have important implications for Mars' regional climate.