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C. E. Leovy, R. W. Zurek, and J. B. Pollack

Abstract

Characteristics of the Mars global dust storm are reviewed. At the Mariner 9 encounter, the dust consisted of highly absorbing particles distributed rather uniformly up to great height (∼50 km). These observations together with temperature distributions inferred from the Mariner 9 IRIS by Hanel and his collaborators are used to estimate global wind systems during the dust storm. The global distribution and direction of light surface streaks indicate that the axially symmetric circulation was a dominant part of flow during the dust storm. An energy balance argument is used to estimate the intensity of the equatorial part of such a wind system. Surface winds driven by the diurnal and semidiurnal components of solar heating are also estimated. The axially symmetric winds may become strong enough to raise dust over wide areas of Mars' tropics under unusual conditions: the incoming solar radiation must be near its seasonal maximum, the static stability must be low, and the atmosphere must be able to absorb and re-emit a sizeable fraction of the incoming radiation. Emission is aided by the formation of H20 ice clouds in the winter northern polar region. Absorption is enhanced by the presence of a small dust opacity in the atmosphere prior to the onset of the global dust storm. Strong winds around the periphery of the retreating south polar cap would be driven by the temperature gradient at the cap edge and by the mass outflow due to subliming C02. These polar winds could generate local dust storms, raising the general level of dustiness, and providing the conditions necessary for onset of a global dust storm. Observational evidence for this sequence of events is discussed. The proposed model is sensitive to the precise phase relationship between Mars' perihelion and southern summer solstice, and variations in this phasing may have caused a strongly episodic behavior of dust storms and of a number of related planetary processes.

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G. L. Manney, R. W. Zurek, L. Froidevaux, J. W. Waters, A. O'Neill, and R. Swinbank

Abstract

Trajectory calculations are used to examine ozone transport in the polar winter stratosphere during periods of the Upper Atmosphere Research Satellite (UARS) observations. The value of these calculations for determining mass transport was demonstrated previously using UARS observations of long-lived tracers. In the middle stratosphere, the overall ozone behavior observed by the Microwave Limb Sounder in the polar vortex is reproduced by this purely dynamical model. Calculations show the evolution of ozone in the lower stratosphere during early winter to be dominated by dynamics in December 1992 in the Arctic. Calculations for June 1992 in the Antarctic show evidence of chemical ozone destruction and indicate that ≈ 50% of the chemical destruction may be masked by dynamical effects, mainly diabatic descent, which bring higher ozone into the lower-stratospheric vortex. Estimating differences between calculated and observed fields suggests that dynamical changes masked ≈20%–35% of chemical ozone loss during late February and early March 1993 in the Arctic. In the Antarctic late winter, in late August and early September 1992, below ≈520 K, the evolution of vortex-averaged ozone is entirely dominated by chemical effects; above this level, however, chemical ozone depiction can be partially or completely masked by dynamical effects. Our calculations for 1992 showed that chemical loss was nearly completely compensated by increases due to diabatic descent at 655 K.

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G. L. Manney, R. W. Zurek, A. O'Neill, and R. Swinbank

Abstract

Trajectory calculations using horizontal winds from the U.K. Meteorological Office data assimilation system and vertical velocities from a radiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upper Atmosphere Research Satellite. Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratosphere. In the SH lower to middle stratosphere, strongest descent occurs near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, except when wave activity is particularly strong. Strong barriers to latitudinal mixing exist above about 420 K throughout the winter. Below this, the polar night jet is weak in early winter, so air descending below that level mixes between polar and middle latitudes. In late winter, parcels descend less and the polar night jet moves downward, so there is less latitudinal mixing. The degree of mixing in the lower stratosphere thus depends strongly on the position and evolution of the polar night jet and on the amount of descent experienced by the air parcels; these characteristics show considerable interannual variability in both hemispheres.

The computed trajectories provide a three-dimensional picture of air motion during the final warming. Large tongues of air are drawn off the vortex and stretched into increasingly long and narrow tongues extending into low latitudes. This vortex erosion process proceeds more rapidly in the NH than in the SH. In the lower stratosphere, the majority of air parcels remain confined within a lingering region of strong potential vorticity gradients into December in the SH and April in the NH, well after the vortex breaks up in the midstratosphere.

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G.L. Manney, R.W. Zurek, W.A. Lahoz, R.S. Harwood, J.C. Gille, J.B. Kumer, J.L. Mergenthaler, A.E. Roche, A. O'Neill, R. Swinbank, and J.W. Waters

Abstract

The transport of passive tracers observed by the Upper Atmosphere Research Satellite is simulated using computed three-dimensional trajectories of ≈ 100 000 air parcels initialized on a stratosphere grid, with horizontal winds provided by the United Kingdom Meteorological Office data assimilation system, and vertical (cross isentropic) velocities computed using a fast radiation code. The conservative evolution of trace constituent fields is estimated over 20–30-day periods by assigning to each parcel the observed mixing ratio of the long-lived trace gases N20 and CH4 observed by the Cryogenic Limb Army Etalon Spectrometer (CLAES) and H2O observed by the Microwave Limb Sounder (MLS) on the initialization date. Agreement between calculated and observed fields is best inside the polar vortex and is better in the Arctic than in the Antarctic. Although there is not always detailed agreement outside the vortex, the trajectory calculations still reproduce the average large-scale characteristics of passive tracer evolution in midlatitudes. In late winter, synoptic maps from trajectory calculations reproduce all major features of the observations, including large tongues or blobs of material drawn from low latitudes into the region of the anticyclone during February–March 1993. Comparison of lower-stratospheric observations of the CLAES tracers with the calculations suggests that discontinuities seen in CLAES data in the Antarctic late winter lower stratosphere are inconsistent with passive tracer behavior. In the Arctic, and in the Antarctic late winter, MLS H20 observations show behavior that is inconsistent with calculations and with that expected for passive tracers inside the polar vortex in the middle-to-upper stratosphere. Diabatic descent rates in the Arctic lower stratosphere deduced from data are consistent with those from the calculations. In the Antarctic lower stratosphere, the calculations appear to underestimate the diabatic descent. The agreement between large-scale features of calculated and observed tracer fields supports the utility of these calculations in diagnosing trace species transport in the winter polar vortex.

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