In 1989, the Voyager II encounter with Neptune discovered the Great Dark Spot (GDS-89), one of the largest known long-lived vortices. Unlike Great Red Spot on Jupiter, which has persisted in a fixed latitude for hundreds of years, GDS-89 showed equatorward drift in the speed of 1.2 degrees/month and could not be found in ground-based and Hubble Space Telescope (HST) observations several years later. Also, GDS-89 exhibited continuous oscillations in both aspect ratio and angle of orientation with a period of about 8 days. Later, between 1994 and 1996, HST observations revealed two more GDS in the northern hemisphere of Neptune, NGDS-32 and NGDS-15, named after their latitudes. Since 1999, no GDS have been obser ved on Neptune.

In contrast to Neptune, no GDS-like feature had been observed on Uranus until Aug. 23 2006, when astronomers from the University of Wisconsin-Madison took the first definitive GDS images in their HST observations. On Aug. 24 2006, this GDS was again spotted. The observation of this GDS coincided with the seasonal change toward spring equinox in the northern hemisphere, which happens once every 84 years and will occur in 2007.

Understanding the dynamics of these large vortex features will be provide consid erable insight into the atmospheres of these two planets. The approach in this presentation is to use the latest version of the Explicit Planetary Isentropic-Coordinate (EPIC) General Circulation Model (GCM) (Dowling, et al., 2006) to conduct CFD simulations of GDS-like features on these planets. Comparing the simulation results in the both planets provides a chance to engage in comparative planetology, determining what aspects of the atmospheres and the vortices are critical for their origin, appearance, and sustainability. This prospect is especially interesting for Uranus and Neptune, since in many ways they are the two most similar planets in the solar system, yet Neptune has had a history of large vortex features while Uranus for nearly two decades lacked any such observed features. The results include investigating the influence of the zonal wind profiles and the vortex wind distribution and depth. We check the sustainability and the movement of GDS-like features on both planets in different situations in the attempt to determine conditions that will allow long-lived vortex features and that yield motions most similar to those observed (in particular compared to GDS-89).