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D.P. Hinson (Stanford University), R.J. Wilson (Geophysical Fluid Dynamics Laboratory/NOAA)
Radio occultation experiments with Mars Global Surveyor are providing profiles of pressure and temperature versus radius and geopotential between the surface and the 10-Pa (0.1-mb) pressure level. A series of experiments in the early mapping phase yielded 440 profiles at latitudes 36\degN-27\degS during northern summer (Ls=134-162\deg). Consecutive measurements were separated in longitude by 28.6\deg, but the local solar time (LT) remained within a narrow range (0407-0426). Some of these tropical profiles contain surprisingly strong inversions, with temperature increasing from ~165 K at 100 Pa to ~190 K at 50 Pa. The most dramatic and persistent inversions are found within ~5\deg of the equator at 210-270\degE, near Pavonis Mons, but they appear occasionally at other longitudes (e.g., 2\degN, 83\degE). Spatial coverage over Tharsis in the southern tropics was sporadic, but several strong inversions of this type also appear at 23-27\degS, 245-254\degE.
Simulations with a Mars general circulation model (GCM) exhibit 0400 LT temperature inversions that are similar to the measurements in magnitude, vertical structure, and spatial distribution. An examination of the GCM temperature field indicates that the simulated tropical inversions result from longitudinal modulation of the vertically-propagating, sun-synchronous thermal tide by the underlying topography. The resulting non-sun-synchronous tidal components produce a particularly strong response in the region above Tharsis. The influence of dynamical effects is indicated in both the measurements and the simulations by predawn temperatures at the top of the inversion that exceed the zonal average. We have also explored the possibility that radiative cooling by water ice clouds contributes to the overall magnitude of the inversion.
This work is funded by NASA through the Mars Global Surveyor Project and the Mars Data Analysis Program.
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