Numerical Simulations of Uranus' Berg Feature as a Vortex-Driven Cloud under Varying Background Vorticity Gradients

The Uranian Berg feature, first identified in the early 2000's, was a notable cloud feature in the southern hemisphere known for its visible similarity to an iceberg drifting off the Southern Polar Collar. This methane ice cloud remained in the region of 34° South until 2005 when it began a four-year equatorward drift spanning roughly 20° (de Pater et al., 2011). Similar behavior has been observed for some large-scale geophysical vortices (dark spots) on the Ice Giants (Uranus and Neptune). Most notably, the original Great Dark Spot (GDS-89), observed by Voyager II at Neptune, featured an equatorward drift of its own over at least 10 degrees (Sromovsky et al., 1993). The dark spots are so named for their darker contrast to the surrounding atmosphere and for several spots this contrast is further offset by bright orographic companion clouds that form above the vortex. While the Berg did not feature any such dark contrast, the persistent bright clouds and drifting behavior suggests that the Berg may have had an underlying vortex beneath the observed cloud that was too deep or insufficiently strong to develop visibly. This could account for the cloud's longevity as well as the drift in latitude. Smith and Ulrich (1993) postulated that the gradient of the background absolute vorticity played a key role in vortex dynamics. To better understand this phenomenon, the EPIC GCM (Dowling et al., 2006) was used to simulate the existence of a dark spot in the observed region for the Berg. Multiple simulations featuring various background vorticity gradients have been applied to investigate the relationship between vorticity gradient and spot drift rate. This relationship will then be utilized to determine an acceptable background vorticity gradient field to simulate drift features comparable to the Berg's observed drift rates.