Comparative CFD Simulations of the Dark Spots of Uranus and Neptune
Xiaolong Deng, Raymond P. LeBeau, Jr., Dept. of Mechanical Engineering, University of Kentucky
Farther out and smaller than Jupiter and Saturn, Neptune and Uranus are less observed gas-giant planets in the outer solar system. However, like the better known Great Red Spot, both atmospheres have exhibited large vortex features, known as Great Dark Spots. The Great Dark Spots are roughly elliptical regions in the planetary atmosphere identi¯ed by their darker albedo. In 1989, the Voyager II encounter with Neptune revealed the ¯rst Great Dark Spot (GDS-89), which is arguably the most dynamic vortex feature in the outer solar system. GDS-89 showed repeated oscillations in shape with the period of 8 days, tail- like features, a bright cloud companion, and equatorward drift of 1.2 degrees/month. After that, in 1991 and 1994, Hubble Space Telescope (HST) failed to observe a southern dark spot feature. However, in 1994, HST observations found an apparent dark spot feature in the northern hemisphere. Later, in 1996, HST observations found another smaller dark spot. They are named NGDS(Northern Great Dark Spot)-32 and NGDS-15 respectively, referring to the latitude where the dark spot was discovered. Both of them were no longer visible as dark spots in 1997. Unlike Neptune, the Voyager II encounter with Uranus, did not reveal any vortex features. However recent HST observations in the summer of 2006 have shown what appears to be the ¯rst observed GDS on Uranus. The spot is located at 27± latitude in the northern hemisphere, where spring is coming after about two decades of near-total darkness. This paper will present General Circulation Model (GCM) simulations of the GDSs on Neptune and Uranus, with a comparison of the behavior of GDSs on these two gas-giant planets and a discussion of possible explanations for the previous lack of spot features on Uranus despite its similarities to Neptune.