Research in Engineering and Aviation

A Closer Examination of the Joint Behavior of Dark Spots and their Companion Clouds on the Ice Giants

October 2012

Author(s): R.P. LeBeau, S.W. Warning, Cs. Palotai, and X. Deng

A Closer Examination of the Joint Behavior of Dark Spots and their Companion Clouds on the Ice Giants. 44th Annual Meeting of the Division for Planetary Sciences, Reno, NV October  14-19.

Abstract

Starting with the Voyager observations of the Great Dark Spot-Bright Companion duo on Neptune, discrete cloud features have been linked to vortices on the Ice Giants. Building on these observations, numerical simulations of these features have begun to reveal a complex physical interaction in which the vortex can generate clouds which in turn influence the behavior of the vortex. Using numerical simulations, further details about these interactions can be revealed. For example, simulations in the EPIC general circulation model of a vortex similar to the Uranian Dark Spot can generate companion clouds from a cloud-free initial condition. These clouds are generated orographically, with a region of upwelling on the leading edge of vortex lifting methane from the warmer lower atmosphere to cooler conditions above the vortex. This increases the local humidity to the point where condensation can occur. The upwelling is perceptible in some simulations a scale height above the vortex with vertical velocities on the order of 0.01-0.1 m/s. The strength of this upwelling is dependent on the local humidity as well as the vortex characteristics; likewise, the meridional drift rate of these vortices is affected by the changes in methane distribution. While the described UDS simulation provides an illustration of the interactive physics underlying vortex-cloud phenomena, there are other, more perplexing observations that require further explanation, from the changing shape of the original bright companion cloud above and about the drifting, oscillating Great Dark Spot to the meridional drift and time-varying cloud structure of the “Berg” on Uranus. Ongoing numerical examination of these vortex-cloud pairings will provide further insight into these features and the overall atmospheric physics of the Ice Giants.