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2.2 Atmospheric model simulations

Only recently has a general circulation model (GCM), originally developed for the simulation of the terrestrial weather and climate, begun to be successfully adapted to the study of atmospheric superrotation on Titan. Del Genio et al. (1993) have shown that the addition of an optically thick, statically stable upper cloud layer to a terrestrial GCM, also modified with a sixteen day planetary rotation period, results in the generation of an equilibrated zonal-mean flow of several tens of meters per second. Their diagnosis of the responsible flux transports indicate that this regime is supported by the horizontal mixing of quasi-barotropic eddies. Similar results have also been obtained with a Titan GCM under development by Hourdin et al. (1992). Despite their preliminary state of development, these GCM experiments lend some confidence to the indirect inference of superrotational winds from the observations.

Allison et al. (1994) have argued that the GCM experiments as well as the limited planetary observations are suggestive of the dynamical maintenance of these circulations by efficient "potential vorticity" mixing. In the implied "ZPV" (zero potential vorticity) limit for stable zonal flow, the latitudinal wind profile will be bounded by a maximum envelope of the form

where is the local Richardson number (the squared ratio of the Brunt--Vaisala frequency to the vertical wind shear), and is the zonal velocity at the equator. Except where is less than 2, this envelope implies an increase in the maximum possible velocity with latitude, and can therefore be expected to approximate the actual winds only between the equator and the latitudes of the "jet" maxima. The Titan zonal wind profile presented by Hubbard et al. (1993) is consistent with this prescribed envelope within 60 latitude, assuming a large value for the Richardson number, as appropriate for the statically stable stratosphere. The combination of the vertical wind shear measured by the Doppler Wind Experiment and the static stability inferred from the Huygens Atmospheric Structure Instrument (HASI) will provide an in situ determination for the vertical profile of , which could then be interpreted with the ZPV constraint for comparison with independent remote sensing observations from the Cassini Orbiter.



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