Numerical models of Saturn's long-lived anticyclones

New measurements of the dynamical properties of the long-lived Saturn's anticyclonic vortex known as "Brown Spot" (BS), discovered during the Voyager 1 and 2 flybys in 1980-1981 at latitude 43.1 degrees N, and model simulations using the EPIC code, have allowed us to constrain the vertical wind shear and static stability in Saturn's atmosphere (vertically from pressure levels from 10 mbar to 10 bars) at this latitude. BS dynamical parameters from Voyager images include its size as derived from cloud albedo gradient (6100 km East-West times 4300 km North-South), mean tangential velocity (45 +/- 11 ms(-1) at 2400 km from center) and mean vorticity (4.0 +/- 1.5 x 10(-5) s(-1)), lifetime > 1 year, drift velocity (5.3 +/- 0.1 ms(-1)) relative to Voyager's System III rotation rate, mean meridional atmospheric wind profile at cloud level at its latitude and interactions with nearby vortices (pair orbiting and merging). An extensive set of numerical experiments have been performed to try to reproduce this single vortex properties and its observed mergers with smaller anticyclones by varying the vertical structure of the zonal wind and adjusting the static stability of the lower stratosphere and upper troposphere. Within the context of the EPIC model atmosphere, our simulations indicate that BS's drift velocity, longevity and merging behavior are very sensitive to these two atmospheric properties. The best results at the BS latitude occur for static stability conditions that use a Brunt-Vaisala frequency constant in the upper troposphere (from 0.5 to 10 bar) above 3.2 x 10(-3) s(-1) and suggest that the wind speed slightly decays below the visible cloud deck from similar to 0.5 to 10 bar at a rate partial derivative u/partial derivative z similar to 2-6 ms(-1) per scale height. Changing the vortex latitude within the band domain introduces latitude oscillations in the vortex but not a significant meridional migration. Simulated mergers always showed orbiting movements with a typical merging time of about three days, very close to the time-span observed in the interaction of real vortices. Although these results are not unique in view of the unknowns of Saturn's deep atmosphere, they serve to constrain realistically its structure for ongoing Cassini observations. (C) 2007 Elsevier Inc. All rights reserved.