HR: 12:05h
AN: P42A-08    [Abstracts]
TI: Maintenance of Equatorial Superrotation in Titan's Atmosphere
AU: * Zhu, X
EM: xun.zhu@jhuapl.edu
AF: The Johns Hopkins Univ. Applied Phys. Lab., 11100 Johns Hopkins Road, Laurel, MD 20723/6099 United States
AB: The long-standing problem of the existence of equatorial superrotation in a rotating planetary atmosphere has been solved. Various momentum force terms that could drive the equatorial superrotation are examined. In general, it is found that: (i) the pumping by thermal tides is the only dominant momentum source for the equatorial superrotation; (ii) a small planetary rotation rate is not crucial for the existence of an equatorial superrotation; (iii) 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; (iv) 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. Each of ten terms in the averaged zonal momentum equation is evaluated analytically. The analytic forms of the approach explicitly show how an equatorial superrotation of a planetary atmosphere is dependent on various external and internal parameters, including: (i) Sun-planet distance, (ii) radius of planet, (iii) rotation rate, (iv) inclination of the equatorial plane, (v) gravity, (vi) atmospheric scale height, (vii) atmospheric buoyancy frequency, (viii) frictional drag, (ix) albedo, and (x) the pressure level at which the deposition of the solar radiation occurs. When the general solution is applied to Titan's atmosphere, the results are: (i) no solution is found for Titan's stratospheric equatorial superrotation at 1-mb level; (ii) however, if the main absorption layer of the solar radiation in Titan's atmosphere is lifted from 1 mb ($\sim$185 km) to above 0.15 mb ($\sim$270 km) level, a stable equatorial superrotation of $>$86 m/s can be maintained near the solstice. Two momentum sinks for Titan's superrotation are frictional drag force and meridional advection of wind shear by horizontal branches of the Hadley circulation.
DE: 5409 Atmospheres--structure and dynamics
DE: 5445 Meteorology (3346)
DE: 3334 Middle atmosphere dynamics (0341, 0342)
DE: 3384 Waves and tides
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting