Absorption of waves by large scale winds in stratified turbulence

TL;DR

This paper investigates how large-scale horizontal winds in stratified turbulence influence internal gravity waves, revealing Doppler shifting and wave absorption phenomena not explained by existing theories.

Contribution

It provides a detailed numerical analysis of wave-flow interactions, highlighting wave absorption at wind speeds and Doppler shift effects in stratified turbulence.

Findings

Most energy concentrates along a Doppler-shifted dispersion relation.

Waves matching wind speed have negligible energy, indicating nonlocal energy transfer.

Current theories do not account for observed wave absorption phenomena.

Abstract

The atmosphere is a nonlinear stratified fluid in which internal gravity waves are present. These waves interact with the flow, resulting in wave turbulence that displays important differences with the turbulence observed in isotropic and homogeneous flows. We study numerically the role of these waves and their interaction with the large scale flow, consisting of vertically sheared horizontal winds. We calculate their space and time resolved energy spectrum (a four-dimensional spectrum), and show that most of the energy is concentrated along a dispersion relation that is Doppler shifted by the horizontal winds. We also observe that when uniform winds are let to develop in each horizontal layer of the flow, waves whose phase velocity is equal to the horizontal wind speed have negligible energy. This indicates a nonlocal transfer of their energy to the mean flow. Both phenomena, the Doppler shift and the absorption of waves traveling with the wind speed, are not accounted for in current theories of stratified wave turbulence.