Kinetic theory and quasilinear theories of jet dynamics
Résumé
We review progress that has been made to construct
a theory for the jet formation and maintenance in planetary atmospheres. The theory is built in the regime where velocity
fluctuations around the base jet are very small compared to the zonal
jet velocity itself. Such situations are frequent in many natural
jets, for instance in the atmosphere of outer planets, the most prominent
example being probably Jupiter's troposphere jets. As discussed in
other chapters of this book, fluctuations close to Jupiter zonal
jets are smaller than the zonal jets themselves.
In such a regime, it is natural and often justified to treat the non-zonal
part of the dynamics with a quasi-linear approximation: at leading
order the dynamics of the non-zonal flow is described by the
equation linearized close to the quasi-stationary zonal jets.
The theory, based on a multi-scale method called stochastic averaging,
share similarities with Stochastic Structural Stability Theory (S3T) and with second order closure
(CE2), also discussed in other chapters of the book.
The aim of this contribution is to discuss the theoretical aspects of such
a quasilinear description of statistically stationary jets. The basic
questions are: when does such an approach is expected to be valid,
why, what are the limitations and the expected errors done doing such
approximations?
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