Numerical Investigation of Orographic Cloud and Vortex Dynamics on Ice Giant Planets

Abstract

Farther out and smaller than Jupiter and Saturn, Neptune and Uranus form a pair of planets known as the ice giants. These ice giants share similar atmospheric conditions and chemistry, including strikingly bright methane cloud features. These features are typically the most visible phenomena in these atmospheres as observed from earth. Some of these clouds are orographic in nature, tracking the motions of large vortex features known as Dark Spots. In 1989, the Voyager II encounter with Neptune revealed such a pairing, with first Great Dark Spot (GDS-89), arguably the most dynamic large vortex feature in the outer solar system, matched with an orographic bright companion cloud. Later Hubble Space Telescope observations revealed two other Dark Spots on Neptune in the northern hemisphere, dubbed NGDS-32 and NGDS-15, the former of which appeared to have an orographic feature. All of the Neptune Dark Spots are transient, lasting months to a few years, and the most recent HST observations evidenced no GDS activity. In contrast, Uranus showed no vortex features during the Voyager II encounter in 1986, but in the summer of 2006 what is likely the first observed GDS on Uranus was spotted. There is some evidence that a bright, possibly orographic cloud feature tracked this vortex at certain times. This paper numerically investigates the source of these orographic features and their influence on the vortex dynamics through simulations with a general circulation model called EPIC. Understanding these features will provide increased insight into the planetary atmosphere environment on the ice giants, enabling comparative meteorology and enhancing our ability to design future missions to this planet.