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Richard W. Zurek

Abstract

Classical atmospheric tidal theory has been used to compute the bilinear tidal zonal-mean forcing per unit mass of the zonal-mean meridional and zonal winds, together with the tidal zonal-mean heating per unit mass for the dusty Martian atmosphere. The convergences of the tidal Eliassen-Palm (EP) flux have been computed for both clear and dusty atmospheric conditions, including the special case of a “dusty corridor” in the summer southern subtropics that is meant to simulate the early stages of a planetary-scale Martian dust storm. The calculation of the tidal EP zonal forcing differs from Hamilton in that more realistic thermotidal forcings and basic state temperatures are used. The zonal-mean convergences of the tidal fluxes of heat and momentum are large during a Martian great dust storm and should alter significantly the zonal-mean circulation and its residual component driven by the zonal-mean heating. In particular, the tidal forcing of the meridional wind, which is an order of magnitude greater than its zonal counterpart, is likely to give rise to a complex pattern of significantly ageostrophic zonal-mean flow in the Martian tropics.

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Richard W. Zurek

Abstract

Modern atmospheric tidal theory has shown that the dominance of the terrestrial semidiurnal surface pressure oscillation, relative to its diurnal counterpart, is the result of the elevated heating source generated by solar heating of stratospheric ozone. Observations of the daily surface pressure variation at the Viking Lander 1 site on Mars reveal a similar predominance of the semidiurnal surface pressure oscilliation only during the onset of Martian great dust storm. Application of a classical, analytic tidal model to the Viking Leader 1 data indicates that elevating the effective heat source due to solar heating of airborne dust by a few kilometers during the onset of a Martian great dust storm can account for the observed semidiurnal surface pressure variation.

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Richard W. Zurek

Abstract

Classical atmospheric tidal theory is extended to include the effects of large-amplitude planetary-scale variations of the terrain height. Utilizing simple models of the thermotidal forcing, the resulting technique is used to compute the diurnal tide in both the dust-free and the dust-laden Martian atmosphere. The main effect of the Martian variable terrain is to drive topographic tidal modes which can propagate vertically and to excite the possibly resonant diurnal Kelvin mode. The resulting surface wind can exceed 20 m s−1 and may determine the preferred location for the initiation of global dust storms. In the middle Martian atmosphere (30–80 km) static and shear instabilities embedded within the tidal fields will generate extensive, though variable, regions of turbulence. Vertical mixing by this turbulence and transport by the tide itself may help to stabilize the middle Martian atmosphere against photolysis.

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Richard W. Zurek and Robert M. Haberle

Abstract

We have computed a steady state, zonally symmetric response of an atmospheric circulation to the combined efffects of the very large zonal-mean diabatic heating and thermotidal forcing thought to exist in the dusty Martian atmosphere during one of its episodic global dust storms. The zonal-mean components of the tidal flux-convergences of momentum and heat are computed using an existing classical atmospheric tidal model constrained by the surface pressure observations at the two Viking Lander sites on Mars. The zonally symmetric response to the computed tidal flux-convergences and to the zonal-mean heating of the airborne dust is then computed by a nearly inviscid two dimensional (2-D) model previously used to study the Mars Hadley circulation. Tidally induced easterly forces at southern summer solstice can extend the core of the descending branch of the zonally symmetric circulation from 50° to 55°N, which is consistent with Viking Lander-2 surface pressure observations but not far enough poleward to produce an atmospheric warming like the one observed in 1977 by the Viking Orbiters. At low latitudes, the cross-equatorial (diabatic) circulation which would be driven by the zonal-mean diabatic beating alone is partially broken up, when viewed in the Eulerian framework, into several small circulation cells, stacked one above the other over the southern tropics. These cells are much less prominent in the residual-mean circulation, whose structure also differs from the diabatic circulation due in part to the ageostrophic tidal flux-convergence of meridional momentum, as well as to local thermotidal forcing and dissipation. For sufficiently large visible opacities (τ>2), the calculations suggest that the tidal flux-convergences will affect the zonal-mean advection of airborne dust into the Northern Hemisphere on Mars following the onset of a planetary-scale dust storm.

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Diane V. Michelangeli, Richard W. Zurek, and Lee S. Elson

Abstract

We have used a linearized nondivergent barotropic vorticity model on a sphere to intercompare the fastest growing, barotropically unstable wave modes computed for zonal jets at high latitudes in the middle atmospheres of Venus, Earth, and Mars. Such zonal jets have been observed in the wintertime stratosphere on Earth and have been inferred from remotely sensed temperatures in the Venus middle atmosphere and in the wintertime Martian atmosphere. The comparison was done by extending the results of Hartmann for his simple analytic profile of a latitudinally varying terrestrial zonal wind to zonal wind profiles characterized by the larger Rossby numbers (Ro) appropriate to Mars and Venus. As Hartmann's results suggested, the fastest growing barotropic waves continue to grow more quickly as Ro increases. Eventually, the fastest growing mode shifts from a zonal wavenumber k = 1 to a k = 2 mode, both located on the poleward flank of the high-latitude jet. However, for somewhat higher Rossby numbers, the k = 2 mode on the equatorward side of the zonal jet becomes the fastest growing planetary-scale barotropic mode, and this transition is marked by a discontinuous shift to longer wave periods. The Venus high-latitude zonal jet appears remarkably close to this transition Ro. For each of the three planets, satellite-borne instruments have detected wave patterns in the thermal radiance field in the vicinity of the high-latitude zonal jets. As reported earlier for the terrestrial wintertime stratosphere by Hartmann and for Venus by Elson, these observed waves have characteristics similar to those computed for the fastest growing barotropic modes. For Mars, we find that such modes would have zonal wavenumbers 1 or 2, with e-folding times of 2-3 days and periods of 0.75–2.5 days; the longer period (k = 2) equatorward mode would dominate for the faster and narrower zonal jets. A poleward mode with k = 1 and a period of 1.2 days is the barotropic mode most likely to be consistent with the Mariner-9 IRIS observations of thermal waves above the 1 mb (˜20 km) level in the Martian atmosphere.

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