Kinetic theory and quasilinear theories of jet dynamics

TL;DR

This paper reviews the development of a quasilinear theoretical framework for understanding jet formation and maintenance in planetary atmospheres, focusing on regimes with small velocity fluctuations around the jets.

Contribution

It discusses the theoretical basis, validity, limitations, and potential errors of the quasilinear approach for modeling statistically stationary jets in planetary atmospheres.

Findings

The quasilinear approximation is valid when velocity fluctuations are small.

The theory shares similarities with S3T and CE2 closure methods.

Limitations and expected errors of the approximation are analyzed.

Abstract

We review progress that has been made to constructa theory for the jet formation and maintenance in planetary atmospheres. The theory is built in the regime where velocityfluctuations around the base jet are very small compared to the zonaljet velocity itself. Such situations are frequent in many naturaljets, for instance in the atmosphere of outer planets, the most prominentexample being probably Jupiter's troposphere jets. As discussed inother chapters of this book, fluctuations close to Jupiter zonaljets are smaller than the zonal jets themselves. In such a regime, it is natural and often justified to treat the non-zonalpart of the dynamics with a quasi-linear approximation: at leadingorder the dynamics of the non-zonal flow is described by theequation 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 sucha quasilinear description of statistically stationary jets. The basicquestions are: when does such an approach is expected to be valid,why, what are the limitations and the expected errors done doing suchapproximations?