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X. Zhu (JHU/APL), D.F Strobel (JHU)
The atmospheric superrotation of a slowly rotating planet, if it exists, has often been viewed on the basis of thermal wind balance, which results from a combination of a gradient wind balance in meridional momentum equation and a hydrostatic balance in vertical momentum equation. A comprehensive two-dimensional (2D) model for Titan's stratosphere has been used to systematically explore the possible physical mechanisms that are responsible for the maintenance of thermal wind balance in Titan's stratosphere. The relationship among the thermal wind balance, the meridional circulation, and zonal wind are examined with numerical experiments of various parameter settings. It is found that the meridional circulation is insensitive to the key parameter of the planetary rotation rate. As a first approximation, the magnitude of mid-latitude jets is solely determined by the conservation of total angular momentum of the atmosphere, which in turn is mainly determined by the planetary rotation rate. On the other hand, it is an ill-conditioning problem to determine the meridional structure of temperature by the energy conservation law. For a moderately rotating planet when the thermal wind balance is well maintained, the meridional structure of temperature can be inferred from the zonal wind distribution, which contrasts the traditional view of zonal wind being determined by temperature distribution. Our numerical experiments show very low meridional temperature gradient in the Titan's stratosphere. By a specified redistribution of the angular momentum within the atmosphere the 2D model is able to produce an equatorial superrotation in Titan's stratosphere while the total angular momentum of the atmosphere remains constant with time. It is shown that the equatorial superrotation in Titan's stratosphere can be maintained without a thermal wind balance.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.