  The Great Dark Spot (GDS-89) observed by Voyager 2 was the first of several
large-scale vortices observed on Neptune, the most recent of which was observed
in 2018 in the northern hemisphere (NDS-2018). Ongoing observations of these
features are constraining cloud formation, drift, shape oscillations, and other
dynamic properties. In order to effectively model these characteristics, an
explicit calculation of methane cloud microphysics is needed. Using an updated
version of the Explicit Planetary Isentropic Coordinate General Circulation
Model (EPIC GCM) and its active cloud microphysics module to account for the
condensation of methane, we investigate the evolution of large scale vortices
on Neptune. We model the effect of methane deep abundance and cloud formation
on vortex stability and dynamics. In our simulations, the vortex shows a sharp
contrast in methane vapor density inside compared to outside the vortex.
Methane vapor column density is analogous to optical depth and provides a more
consistent tracer to track the vortex, so we use that variable over potential
vorticity. We match the meridional drift rate of the GDS and gain an initial
insight into the evolution of vortices in the northern hemisphere, such as the
NDS-2018.
