Turbulent convection in an anelastic rotating sphere : a model for the circulation on the giant planets
Citable URI
https://hdl.handle.net/1912/2492DOI
10.1575/1912/2492Abstract
This thesis studies the dynamics of a rotating compressible gas sphere, driven by
internal convection, as a model for the dynamics on the giant planets. We develop
a new general circulation model for the Jovian atmosphere, based on the MITgcm
dynamical core augmenting the nonhydrostatic model. The grid extends deep into
the planet's interior allowing the model to compute the dynamics of a whole sphere
of gas rather than a spherical shell (including the strong variations in gravity and the
equation of state). Different from most previous 3D convection models, this model is
anelastic rather than Boussinesq and thereby incorporates the full density variation
of the planet.
We show that the density gradients caused by convection drive the system away
from an isentropic and therefore barotropic state as previously assumed, leading to
significant baroclinic shear. This shear is concentrated mainly in the upper levels
and associated with baroclinic compressibility effects. The interior flow organizes
in large cyclonically rotating columnar eddies parallel to the rotation axis, which
drive upgradient angular momentum eddy fluxes, generating the observed equatorial
superrotation. Heat fluxes align with the axis of rotation, contributing to the observed
flat meridional emission. We show the transition from weak convection cases with
symmetric spiraling columnar modes similar to those found in previous analytic linear
theory, to more turbulent cases which exhibit similar, though less regular and solely
cyclonic, convection columns which manifest on the surface in the form of waves
embedded within the superrotation. We develop a mechanical understanding of this
system and scaling laws by studying simpler configurations and the dependence on
physical properties such as the rotation period, bottom boundary location and forcing
structure.
These columnar cyclonic structures propagate eastward, driven by dynamics similar
to that of a Rossby wave except that the restoring planetary vorticity gradient
is in the opposite direction, due to the spherical geometry in the interior. We further study these interior dynamics using a simplified barotropic annulus model, which
shows that the planetary vorticity radial variation causes the eddy angular momentum
flux divergence, which drives the superrotating equatorial flow. In addition we
study the interaction of the interior dynamics with a stable exterior weather layer,
using a quasigeostrophic two layer channel model on a beta plane, where the columnar
interior is therefore represented by a negative beta effect. We find that baroclinic
instability of even a weak shear can drive strong, stable multiple zonal jets. For this
model we find an analytic nonlinear solution, truncated to one growing mode, that
exhibits a multiple jet meridional structure, driven by the nonlinear interaction between
the eddies. Finally, given the density field from our 3D convection model we
derive the high order gravitational spectra of Jupiter, which is a measurable quantity
for the upcoming JUNO mission to Jupiter.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2008
Suggested Citation
Thesis: Kaspi, Yohai, "Turbulent convection in an anelastic rotating sphere : a model for the circulation on the giant planets", 2008-06, DOI:10.1575/1912/2492, https://hdl.handle.net/1912/2492Related items
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