Convective Parameterization in a Jovian Shallow Water Model
A. P. Ingersoll (Caltech)
Understanding what maintains the zonal jets on the giant planets is a
central problem in planetary meteorology. There is general agreement
that random forcing at small scales can lead to steady zonal flows in a
thin layer of fluid on a rapidly rotating sphere. And there is general
agreement that, on Jupiter, convection provides the small-scale forcing.
But published numerical experiments do not rely on theories of convection
to define the forcing. Instead they use uniformly distributed random
sources of vorticity or vertical velocity. In contrast, jovian
convection seems to be confined to cyclonic regions, and is presumably
associated with heat sources and condensation of water. Also, as
described by Cho et al at this meeting, the numerical experiments do not
give particularly realistic flows: In these experiments the zonal winds
are not larger than the eddy winds; multiple jets are not stable; and the
curvature of the zonal jet profile does not exceed beta. Our theory is
based on simple notions about the physics of moist convection. It
naturally accounts for the apparent convective activity of cyclonic
regions. In the shallow water equations for a single isentropic layer
meant to represent the cloud zone of Jupiter, the forcing appears as a
source term in the mass balance equation: Radiation removes mass
uniformly over large horizontal scales, and convection adds mass on small
scales when the layer gets too thin. The next step, which we are
actively pursuing, is to run some numerical experiments with this new,
physically-based forcing. [This research was supported by the NASA
Planetary Atmospheres Program and the Galileo Project.]