Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Jeffrey R. Barnes x
  • Refine by Access: All Content x
Clear All Modify Search
Jeffrey R. Barnes

Abstract

Viking Lander 2 meteorological data from the fall and winter seasons of a second Mars annual cycle have been subjected to time spectral and cross-spectral analysis, so as to determine important characteristics of the observed transient disturbances having periods longer than one Mars solar day (a Sol). Phase relationships between the highly coherent pressure, wind, and temperature oscillations are found to be very similar to those previously determined from first-year data, and thus consistent with those expected for eastward traveling, baroclinic waves centered to the north of the Lander 2 site. Wave-numbers (both zonal and meridional) and phase speeds also are estimated and are similar to those of the first year, though there is relatively less variance at low zonal wavenumbers (1.5–2.0) and the phase speeds are slightly larger. It is proposed that the reduced long wavelength disturbance amplitudes are a general consequence of a less dusty, second-year atmosphere characterized by lower static stabilities.

Composites of the waves for two highly regular subperiods were constructed. No sharp frontal structures are apparent in these composites, though the pressure and temperature waves are distinctly asymmetric. Wavenumbers and phase speeds were also estimated from the composited wind and pressure data and agree very well with those obtained from the spectral analysis.

Full access
Jeffrey R. Barnes

Abstract

The linear baroclinic instability of zonal-mean flows like those in the wintertime Martian atmosphere under both relatively nondusty and highly dusty conditions is examined using a spherical quasi-geostrophic model. The basic states are idealized, but based closely upon Mariner 9 and Viking observations. Zonal wavenumbers 3 and 4 are found to be most unstable and phase speeds are ∼10–20 m s−1 in middle latitudes generally consistent with observations as well as with previous results obtained with simpler models. Growth rates are not greatly reduced by the rapid Martian radiative relaxation, though the growth at higher wavenumbers is significantly inhibited by Ekman friction. Even with this dissipation minimum e-folding times of ∼2 days are obtained.

For a dust storm basic state characterized by enhanced static stability growth rates are substantially decreased, but the most unstable wavenumber is essentially not altered. This behavior differs from that found in simple models, but is consistent with that expected in a Charney-type model. The most unstable scale is shown to be sensitive to the vertical distribution of static stability, rather than the mean stability.

The structures of the spherical modes are similar to those for terrestrial zonal flows, if similar zonal wavelengths are compared. Wavenumber 2 exhibits considerable vertical penetration. The modes for the dust storm state are situated farther poleward than the others, significantly reducing the relative amplitudes in middle latitudes. Several aspects of the meridional and vertical structure of the modes are discussed in relation to Viking lander observations and Mariner 9 IRIS data.

Zonally symmetric topography like that in the northern hemisphere of Mars is found to decrease the linear growth rates substantially, without significantly changing the most unstable scale, and to increase the phase speeds.

Full access
Jeffrey R. Barnes

Abstract

A nearly analytic model of a gravity wave driven middle atmosphere circulation is formulated. The simplified model represents the one-dimensional interaction of a single gravity wave mode with a zonal-mean flow. Solutions of this model are shown to agree very well with numerical results from a more complete mean flow-gravity wave model (essentially the model developed by Holton). A single nondimensional parameter, the ratio of a mean flow radiative forcing parameter, and the gravity wave momentum flux incident into the middle atmosphere from below largely determines the solution of the model. For typical middle atmosphere parameter values, an approximate analytic solution can be obtained, and this solution permits the parameter dependence of the circulation to be characterized very simply. The gravity wave driven circulation extends downward from the level (the breaking level, approximately) where the momentum flux equals the mean flow radiative forcing. Stronger forcing implies higher breaking levels and stronger flows, while stronger flux (incident from below) yields lower breaking levels and weaker flows. Given the exponential growth of the flux in height, the breaking and jet maximum levels are relatively insensitive to the mean flow and gravity wave parameters; the zonal flow is “closed off” within the middle atmospheric region over a wide range of parameter values.

The nearly analytic model is briefly considered in relation to observations and modeling of Earth's middle atmosphere, and some of its limitations are discussed. It is also applied to the middle atmosphere of Mars.

Full access
Jeffrey R. Barnes

Abstract

The effects of thermal damping on the finite-amplitude behavior of a baroclinic wave are investigated using a quasi-geostrophic, β-plane model The model possesses high vertical resolution so that there are many vertical eddy modes, but both the wave field and the zonal-mean flow are truncated to a single horizontal mode; the wave-mean flow interaction is thus purely baroclinic. In addition to thermal dissipation, lower boundary Ekman friction is incorporated.

The regime characteristics of the model are qualitatively fairly similar to those of the two-layer models previously studied. For sufficiently strong thermal damping, steady wave flows are obtained, while for weaker damping, nonsteady vacillating behavior is found. In the absence of damping the vacillations are relatively irregular and the minimum eddy amplitude relatively large, as has been found to be the case in the two-layer model with multiple horizontal wave modes.

In the steady wave flows the mean flow is linearly neutral, and this neutrality is qualitatively suggested by Lindzen et al. Similar considerations apply in a time-mean sense to at least some of the nonsteady flows, but in the absence of all damping the mean flow is “overstabilized”—a substantial fraction of the mean available potential energy being removed by the wave.

A particularly interesting type of vacillation characterized by two widely separated time scales constituting a sort of long “Life-cycle” of eddy activity, occurs in the model nonsteady regime, and appears to be similar in some respects to behavior found in the two-layer model with only lower-layer dissipation. In the present model, the life-cycle behavior is closely linked to the strength of the Ekman dissipation and to the asymmetry in the forcing and dissipation time scales which is most pronounced for relatively weak thermal damping.

The regime behavior of the model examined here is considered to be suggestive, in spite of the model's simplicity, of the importance of the thermal damping time scale. The terrestrial and Martin atmosphere may represent examples of weakly and strongly damped behavior, respectively.

Full access
Jeffrey R. Barnes

Abstract

The nature of the synoptic period variations in the Viking 2 pressure, wind and temperature data is investigated, using time-spectral and cross-spectral analysis, for selected portions of the Mars fall, winter and spring seasons. Estimates of the phase relationships between the highly coherent pressure, wind and temperature oscillations are obtained, and are very similar to those expected for baroclinic waves, and to these obtained from terrestrial surface data. Phase speeds and zonal wavenumbers are inferred by interpreting the pressure and meridional wind variations in terms of eastward traveling, quasi-geostrophic waves. The calculated phase speeds are on the order of 5–15 m s−1, consistent with the baroclinic wave interpretation, while the wavenumbers of the two dominant fall and spring periodicities are approximately 2 and 4 (the smaller value corresponding to the longer period of 6–8 Mars solar days or sols, and the larger to a 3-sol wave). These wavelengths are in general agreement with those expected for baroclinic waves in the martian atmosphere, on the basis of linear baroclinic instability theory and the results of numerical experiments.

Only the pressure data from Lander 1 are examined: the synoptic period variations are reduced in amplitude and slightly frequency shifted relative to those at Lander 2. Additionally, some atmospheric opacity data obtained by the Lander 2 cameras is studied and seems to show evidence of synoptic period fluctuations, possibly due to advection of the north polar hood clouds over the Lander 2 site.

Possible connections between changes in the atmospheric thermal state and circulation, associated with the global-scale dust storms, and the wave characteristics are discussed, as well as the regularity of the waves.

Full access
Jeffrey R. Barnes and Richard E. Young

Abstract

A multilevel, global, primitive equation model is used to examine the nonlinear development of baroclinic waves (confined initially to a single zonal wavenumber) on a symmetric zonal flow. The focus is on the influence of highly simplified surface drag and thermal damping on the evolution of the flow beyond an initial eddy life cycle.

Without surface drag and thermal damping and with relatively weak internal diffusion, it is found that multiple life cycles occur in the evolution to an essentially wave-free state. With stronger diffusion, only a single life cycle characterized by baroclinic growth and barotropic decay is obtained. The life cycles that follow the initial cycle with weak diffusion are strongly barotropic, entailing barotropic growth and decay. The episodes of barotropic growth are associated with wave radiation, wave breaking, and overreflection on the equatorward flanks of the jet near a critical surface. With strong diffusion, the potential vorticity mixing associated with the breaking is reduced considerably, and significant barotropic growth (and overreflection) does not occur. In all the simulations without drag and damping, the “barotropic governor” identified by James and Gray is primarily responsible for the stabilization of the mean flow in the final wave-free state.

Repeated life cycles characterized largely by baroclinic growth and barotropic decay are obtained with the simplified surface drag and thermal damping. Significant barotropic growth occurs in the early stages of most of the life cycles, associated with wave breaking and overreflection on the equatorward side of the jet. Multiple life cycles of baroclinic growth and barotropic decay occur in the complete absence of thermal damping (though damping is certainly necessary for sustained life cycles), demonstrating the crucial role of surface drag in suppressing the barotropic governor. With thermal damping but without surface drag, the evolution is similar to that without both drag and damping in being highly barotropic.

The simulations with surface drag and thermal damping are compared with observations of medium-scale baroclinic waves in Southern Hemisphere summer and are found to be very similar.

Full access
Jeffrey R. Barnes and Robert M. Haberle

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 season the simulated Mars circulation is greatly weakened in comparison to that for solstice conditions, angular momentum is not approximately conserved by the mean meridional circulation, and potential vorticity increases relatively smoothly away from the equator.

Full access
Jeffery L. Hollingsworth and Jeffrey R. Barnes

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 extratropical eddies exhibit a definite wavenumber 2 pattern with comparable amplitudes for wavenumbers 1 and 2. Southern eddies, however, are very strongly dominated by wavenumber 1. Because of enhanced refractive properties of the dusty basic state, dusty responses are an order of magnitude larger than nondusty ones. Horizontal and meridional wave propagation is illuminated by diagnostics for the wave activity flux, for example, Eliassen–Palm and Plumb fluxes. As a result of the separation distance between major orographic features on Mars, together with a planetary waveguide that enhances zonal propagation, Rossby wave interferences between western and eastern hemispheric wave trains occur. This analysis is relevant to future global mapping missions to Mars (e.g., polar-orbiting space craft) that will return key atmospheric observations of planetary wave activity.

Full access