Graduate Thesis Or Dissertation
 

Tracer transport in the Martian atmosphere as simulated by a Mars GCM

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  • This paper investigates the atmospheric circulation and transport characteristics of the Martian atmosphere (as modeled by a Mars GCM) for three sets of conditions. The conditions are based on a combination of season and dust loading (as parameterized by the optical depth, τ). The first experiment is for the Northern Spring Equinox with no dust loading (τ=0). Experiment 2 is for Northern Hemisphere Winter Solstice with no dust loading. Experiment 3 is for Northern Hemisphere Winter Solstice under moderately dusty conditions (τ=1.0). These cases allow a comparison between seasons and a look at the effects of dust in the atmosphere on the circulation and transport processes. After presenting some of the theoretical and mathematical background pertinent to atmospheric transport and circulation the results of the study are given. These include analyses of the zonal-mean winds, the time-evolution of the mean tracer field, the mean meridional circulation, and the effective transport circulation [Plumb and Mahlman, 1987]. In addition we estimate the time scales for "stratospheric" overturning and calculate a set of eddy diffusion, coefficients (K[subscript yy] and K[subscript zz]) for each case. These coefficients are a means of parameterizing the strength of eddy mixing. Others [Conrath, 1971; Zurek, 1976; Kong and McElroy, 1977; Toon et al., 1977; Anderson and Leovy, 1987] have estimated, using various methods, values for the vertical diffusion coefficient K[subscript zz] of the order of 10³ m²/s. The results here show that there is no "typical" value of K[subscript zz] (or K[subscript yy]) which can be used to characterize the atmosphere globally, and K[subscript zz] seldom reaches 10³ m²/s except in isolated regions and/or under dusty conditions. Both K[subscript yy] and K[subscript zz] are dependent upon season, dust loading, and location in the atmosphere. In addition to identifying the regions of strong mixing, probable sources of the eddy activity which is responsible for the mixing are discussed. In all three cases the effective transport circulation (which includes both advection and diffusion) is structurally similar to the mean meridional circulation but somewhat more intense. The Martian equinox circulation is structurally similar to Earth's circulation; both are characterized by a dual Hadley cell system with rising branch over the equator, poleward flow aloft, and return flow at low levels. The mean zonal winds are westerly in both hemispheres with easterlies near the ground and at high altitudes over the equator. The jet stream in the northern hemisphere peaks at 45 m/s at equinox. Unlike the Earth, Mars' circulation changes dramatically with the seasons. For solstice conditions the mean meridional circulation is characterized by a large, intense cross-equatorial Hadley cell which dominates the circulation pattern. The mean zonal winds are now predominately westerly in the northern winter hemisphere and easterly in the southern hemisphere. The westerly jet reaches 95 m/s while the easterly jet reaches 30 m/s. There is a band of westerlies (up to 10 m/s) found in low southern latitudes near the ground. Dust in the atmosphere acts to intensify the strength of the circulation (while having little effect on the structure); there is a two- to three-fold increase in the strength of the mean winds between the two winter solstice experiments.
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