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D. P. Hinson (Stanford Univ.), R. J. Wilson (GFDL/NOAA)
We report new results on the structure and dynamics of the tropical atmosphere of Mars derived from a combination of radio occultation measurements by Mars Global Surveyor and simulations by a Mars general circulation model (MGCM). Radio occultation experiments sounded the equatorial atmosphere at latitudes of 36 N to 30 S during midsummer of the Northern Hemisphere (Ls = 134-162), sampling the predawn thermal structure at a local time of ~0412. A subset of these observations is distinguished by the presence of conspicuous elevated temperature inversions. Their properties and spatial distribution are organized across the tropics on planetary scales. Inversions are strongest and occur most frequently above elevated terrain, and their altitude generally increases toward the south. In the most prominent inversions, which appear above Tharsis, the temperature increases from <165 K near 100 Pa to >190 K near 50 Pa. The MGCM accurately simulates the main features of the observations, providing compelling evidence that the temperature inversions arise from zonally modulated thermal tides. The best simulation includes an interactive hydrologic cycle, which results in strong coupling between the thermal tides and radiatively active water ice clouds. Prominent clouds form in response to dynamical cooling in a pattern closely correlated with the spatial and temporal structure of the tides. The tides in turn are intensified by radiative forcing from the clouds. Finally, we compared these inversions with the one observed by Mars Pathfinder in the preceding Martian year. The location and characteristics of the Pathfinder inversion are inconsistent with our observations and simulations.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.