The Voyager II encounter with Neptune in 1989 revealed arguably the most dynamic vortex feature in the outer solar system. The Great Dark Spot (GDS-89) exhibited repeated oscillations in shape over an eight-day period, unusual tail-like deviations from the base vortex ellipse, a bright cloud companion, and general equatorward drift on the order of one degree per month. Later Hubble Space Telescope observations revealed Northern Great Dark Spot 32 (NGDS-32) and 15 (NGDS-15). Both of these features faded and were no longer visible as dark spots in 1997. Thus, it appears that large vortices of a few to several years duration may be a common feature in the visible atmosphere of Neptune and that this atmosphere is an unusual opportunity to investigate through simulation a vortex-dominated flow.
The Great Dark Spot was a roughly elliptical region that was a darker blue (about a 10% change in albedo) compared to the surrounding region, the shape partially obscured by a bright methane cloud (the Bright Companion). The morphology of the Great Dark Spot (later to be designated GDS-89) based on the Voyager II observations is characterized by its drift rate and shape evolution. GDS-89 drifted northward in latitude between 27 and 17oS at a rate of 0.00170 degrees/hour or about 1.2 degrees/month. In longitude, the motion appeared to correlate well with simple advection by the zonal winds. The 8-day (193 hour) oscillation is defined by changes in ellipse orientation (with amplitude of about 14o about the horizontal) and aspect ratio (0.35 to 0.55) corresponding to a longitudinal extent of 30 to 45 degrees and 12 to 17 degrees in latitude in best-observed region. Other notable features of GDS-89 like the tadpole-like “tails” are not so easily characterize quantitatively, but are critical qualitative aspects of the morphology.
Based on Hubble Space Telescope (HST) observations in the fall of 1991, it appeared that the Great Dark Spot no longer existed at that time. The re-emergence of large vortex features on Neptune occurred with post-repair HST observations which revealed a possible GDS-like vortex in the northern hemisphere in the fall of 1994, later designated Northern Great Dark Spot NGDS-32. Later, two dark spots were observed, the aforementioned NGDS-32 (originally located at 32oN) and NGDS-15 (15oN). NGDS-32 appears likely to have existed throughout the 1994-1996 period while NGDS-15 appeared only in the 1996-1997 data. Later observations suggest that NGDS-32 as a dark albedo feature disappeared by July 1997, but that cloud patterns implied that a vortex feature may have persisted into the year 2000. NGDS-32 also did not exhibit any large latitudinal drift but might have oscillated slightly over a few tenths of a degree with a period on the order of several hundred days as inferred from changes in the longitudinal drift. NGDS-15 may have drifted equatorward by a degree or two between March 1996 and July 1997, but further definition is not possible due to the non-observability of the feature in November, 1995 and August, 1998.
At this time general circulation models of Neptune had not adequately explained the variation of motions seen in the vortex features given the assumed zonal wind and temperature structure. This paper will present simulations of the Great Dark Spots of Neptune using the Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM). This model has been used previously to simulate GDS-89 capturing at least qualitatively vortex shapes oscillation, latitudinal drifts, tail-like features, and Bright Companion-like cloud features. The Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM) has been evolving over the span of a decade. The basic equations solved are the three-dimensional Navier-Stokes equations with a hybrid vertical coordinate that blends a potential temperature and a pressure-based sigma coordinate.
The proposed paper will build on the recent simulations of the Great Dark Spot with the objective of explaining the different characteristics of the three vortices. Parameters considered include the zonal wind variation with latitude, the vertical structure of the atmosphere, initial spot characteristics such as aspect ratios, vorticity distribution and magnitude, and overall size. Better comprehension of this atmosphere could prove of practical import given the growing interest in the use of aerocapture on a future orbital mission to Neptune.