# Wave kinetic equation for inhomogeneous drift-wave turbulence beyond the quasilinear approximation

## TL;DR

This paper derives a wave kinetic equation for inhomogeneous drift-wave turbulence that includes wave-wave collisions, extending beyond the quasilinear approximation to better understand drift wave and zonal flow interactions.

## Contribution

It introduces a new wave kinetic model with a collision operator that accounts for wave interactions, surpassing the limitations of previous quasilinear theories.

## Key findings

- The model conserves enstrophy and energy.
- The collision operator breaks down at the Rayleigh--Kuo threshold.
- It highlights the limitations of homogeneous-turbulence theory.

## Abstract

The formation of zonal flows from inhomogeneous drift-wave (DW) turbulence is often described using statistical theories derived within the quasilinear approximation. However, this approximation neglects wave--wave collisions. Hence, some important effects such as the Batchelor--Kraichnan inverse-energy cascade are not captured within this approach. Here we derive a wave kinetic equation that includes a DW collision operator in the presence of zonal flows. Our derivation makes use of the Weyl calculus, the quasinormal statistical closure, and the geometrical-optics approximation. The obtained model conserves both the total enstrophy and energy of the system. The derived DW collision operator breaks down at the Rayleigh--Kuo threshold. This threshold is missed by homogeneous-turbulence theory but expected from a full-wave quasilinear analysis. In the future, this theory might help better understand the interactions between drift waves and zonal flows, including the validity domain of the quasilinear approximation that is commonly used in literature.