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X. Zhu (JHUAPL)
The long-standing problem of the existence of equatorial superrotation in a slowly rotating planetary atmosphere has been solved analytically. Each of ten terms in the averaged zonal momentum equation is analyzed systematically, first by scale analysis, then through analytic formulations and numerical evaluations, and finally by comparison with well-known planetary atmospheres. Key parameters of the analytic formulations are found to agree with observational constraints, resulting in a formulation consistent with fundamental physical concepts. The analytic forms of the approach explicitly show how an equatorial superrotation is dependent on various external and internal parameters.
The main results are: (i) Venus equatorial superrotation of 118 m/s results primarily from a balance between the momentum source of the pumping by thermal tides and the momentum sink of the meridional advection of wind shear by horizontal branches of the Hadley circulation; (ii) no solution is found for Titan's stratospheric equatorial superrotation at 1-mb level; (iii) however, if the main absorption layer of the solar radiation in Titan's stratosphere is lifted from 1 mb (~ 185 km) to 0.1 mb (~ 288 km), an equatorial superrotation of ~ 110 m/s can be maintained at the 0.1-mb level; (iv) a dust cloud in the Earth's lower stratosphere at the 100-mb level, such as one produced by an asteroid impact, can also produce an equatorial superrotation of ~ 120 m/s; (v) the postulated superrotation in the atmosphere of either Titan or Earth results mainly from a balance between the momentum source of the tidal pumping and the momentum sink of the frictional drag force; (vi) superrotation in Earth's lower stratosphere can enhance the meridional transport of dust clouds. This increased meridional transport can, for example, accelerate the globalization of unfavorable environments for the survival of the dinosaurs, following an asteroid impact.
Detailed analyses also lead to the following more general results on the equatorial superrotation: (i) planetary rotation rate is not crucial for the existence of an equatorial superrotation; (ii) it is unlikely that the eddy horizontal momentum transfer in a slowly rotating atmosphere such as Venus' is against its gradient, though this remains an open question; and (iii) upward momentum flux by Hadley circulation below the jet center of an equatorial superrotation is a momentum sink, which will impede rather than assist the formation of the equatorial superrotation.
This work was supported by NASA Grant NAG5-11962.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.