Laboratory Analogues of the Banded Circulation on Gas Giant Planets

Details Laboratory Analogues of the Banded Circulation on Gas Giant Planets Laboratory Analogues of the Banded Circulation on Gas Giant Planets

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.4111r&link_type=abstract

American Astronomical Society, DPS meeting #40, #41.11; Bulletin of the American Astronomical Society, Vol. 40, p.471

Astronomy and Astrophysics

Astronomy

Scientific paper

Recent work on determining horizontal velocities and momentum fluxes from visual cloud-tracking in Cassini ISS images suggests that nonlinear interactions of small-scale eddies sustain the observed zonal jets and related bands of clouds and aerosols. These upscale energy transfer processes in the outer gas giant planets are also beginning to be reproduced in numerical models, although the details are still far from clear. We present here results from large-scale laboratory experiments on forced-dissipative geostrophic turbulence in a large rotating tank, under conditions that approach those relevant to the tropospheres of gas giant planets. Small-scale convection is driven either by a steady flux of dense salty water onto the top surface of a rotating water tank, or by direct heating of the lower boundary. The whole tank rotates at constant angular velocity Ω and dynamical effects of the spherical planetary curvature are emulated by use of a radially-sloping bottom. The experiments were carried out on the 13 m diameter rotating Coriolis platform in Grenoble, France. Results demonstrate the formation of multiple, undulating, parallel, barotropic zonal jets, in which u'v’ fluxes maintain the jets against viscous (Ekman) dissipation. Use of different rotation rates of the tank confirms the dual role of planetary vorticity gradients (β) and bottom friction in governing the jet separation scale, approximated by the well-known Rhines scale (U/β)1/2. The jets formed are also frequently found robustly to violate the Rayleigh-Kuo barotropic stability criterion, leading to formation of larger-scale waves and eddies and influencing the kinetic energy spectrum. These experiments serve to elucidate mechanisms for jet formation and saturation that may apply to gas giant planet atmospheres (and, more controversially, the terrestrial oceans), and suggest a number of new diagnostics to be investigated in observations and models of Jovian and Saturnian circulations.

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