Randomly Forced, 2-D Planetary Turbulence of the Jovian Atmosphere

J. Y-K. Cho, A. P. Ingersoll (Caltech), L. M. Polvani (Columbia)

Recently, it has been demonstrated that the banded appearance and the zonal winds in the atmospheres of the giant planets can be well-modeled by idealizing the atmosphere as a shallow layer of turbulent fluid [Cho & Polvani, Science (1996)]. In that work, robust and steady jets form from random initial conditions and no forcing, in good qualitative agreement with the observed winds on all four of the giant planets.

In this work, we re-address the problem of jet morphology with a simple extension of the above model: random forcing with variable scale and correlation time. Such forcing can serve as a simple model of convection or baroclinic instability. Our objective here is to determine clearly the role of this forcing, which is independent of the flow, on the jet structures. To this end, we reduce the model to the non-divergent barotropic vorticity equation in spherical geometry. Similar systems were previously studied [see Rhines, Chaos (1994) and Cho & Polvani, Phys. Fluids (1996), and references therein]. Here, we extend the previous works by performing long-time simulations in full spherical geometry with a much broader exploration of the parameter space.

In our study, we find no region of parameter space that produces steady, robust jets as in the observations. Furthermore, when the forcing is not spatially symmetric, the banding becomes weak. These results strongly indicate that the simple random forcing used here and in many earlier works overwhelms the flow, and a more physical forcing, one that is affected in turn by the generated flow, is needed.