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A.P. Showman, T.E. Dowling (U. Louisville)
Our goal is to understand the dynamics of 5-micron hot spots on Jupiter, which are local equatorial regions with unusually high 5-micron brightness temperatures resulting from low cloud abundance. The extremely low relative humidity measured by the Galileo probe inside a hot spot suggests that hot spots contain descending air. Three scenarios have been explored. First, several authors have suggested that hot spots contain convectively unstable downdrafts. However, this scenario likely requires the absence of a tropospheric stable layer that is thought to exist based on mesoscale wave observations and the stabilizing effect of clouds. Further, convective downdrafts produced in calculations to date are a factor of ~100 smaller than observed hot spots and do not account for their latitudinal confinement or wave-like spacing. Second, studies of equatorially trapped linear waves provide a connection between the fact that 8-10 hot spots encircle the globe and their apparent westward phase speed relative to the deep flow obtained by Doppler tracking of the probe signal. It is unclear, however, whether a linear wave study can explain the extreme dryness measured by the probe, which may require large amplitude (hence nonlinear) displacements. Third, Showman and coworkers have suggested that hot spots are nonlinear coherent structures --- propagating relative to the ambient flow --- that are embedded within Jupiter's stably stratified upper troposphere. In this model, air columns stretch downward as they enter the hot spot from the side and contract as they exit days later. We are testing the hypothesis that such nonlinear structures emerge naturally from initial states containing plausible wind and temperature profiles; our aim is to isolate the key processes responsible for hot spot formation and evolution. Using the EPIC atmospheric model, we proceed systematically by first investigating the behavior of low-amplitude equatorially trapped waves, followed by an exploration of the nonlinear regime.