Abstract
The Berg cloud feature in the atmosphere of Uranus was first identified as a persistent grouping of clouds located just off the bright South Polar Collar at a latitude of around −34 degrees. Ongoing observations of this feature through the 1990s and 2000s suggested that the feature was oscillating in location by a few degrees in latitude for several years, and then unexpectedly began to drift towards the equator, which continued over the final 4 years until the cloud dissipated. One possible explanation for such a persistent drifting cloud is that it is a cloud-vortex system, in which an unseen vortex drives the creation of the cloud and the motions of the vortex control the cloud location. To explore this possibility, a series of vortices are studied numerically using the Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM). The evolution of these test vortices are simulated to examine their drift rates and the potential for cloud formation. The results indicate that conditions on Uranus could result in an equatorward drifting vortex over a range of latitudes and that significant cloud formation could occur, potentially obscuring observations of the vortex.
Highlights
The Berg cloud feature is a distinctive collection of clouds in the southern hemisphere of Uranus that were observed over a 15 year period as described in [1]
While the observations were intermittent and the observed cloud structure somewhat variable, Ref. [1] makes the case for this feature existing continuously over that time period, first as a feature located in the vicinity of −34◦ (34◦ S) latitude just off the South Polar Collar (SPC) (Figure 1), over the final several years as a feature that drifted from this position towards the equator, with a possible final observation in 2009 at a latitude of −8◦
The Berg was observed at a latitude of −34.4◦ on 12 August 2004 and at a latitude of −33.5◦ on 15 August 2005 [1], corresponding to a drift rate of 0.003◦ /day
Summary
The Berg cloud feature is a distinctive collection of clouds in the southern hemisphere of Uranus that were observed over a 15 year period as described in [1]. GDS-89 had a persistent methane cloud companion that existed over an eight-month period of observation as described in [3]. Over these eight months, the GDS-89 system appeared to have drifted in latitude from −27◦ to −17◦ based first on bright companion cloud and later on direct vortex observations. More recently a second dark spot on Neptune, SDS-2015, appeared to have drifted a few degrees poleward over two years [4].
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