The instability of counter-propagating kernel gravity waves in a constant shear flow

TL;DR

This paper reviews kernel gravity waves and demonstrates their role in the instability of stratified shear flows with constant shear, revealing a mechanism similar to counter-propagating Rossby waves and challenging classical shear instability criteria.

Contribution

It introduces a detailed analysis of kernel gravity wave interactions causing instability in stratified shear flows, expanding understanding of wave-driven turbulence in such environments.

Findings

Identifies a band of unstable wavenumbers in stratified shear flow.

Shows the instability mechanism parallels counter-propagating Rossby waves.

Demonstrates stratification can destabilize flows without Rayleigh criteria satisfaction.

Abstract

The mechanism describing the recently developed notion of kernel gravity waves (KGWs) is reviewed and such structures are employed to interpret the unstable dynamics of an example stratified plane parallel shear flow. This flow has constant vertical shear, is infinite in the vertical extent, and characterized by two density jumps of equal magnitude each decreasing successively with height, in which the jumps are located symmetrically away from the midplane of the system. We find that for a suitably defined bulk-Richardson number there exists a band of horizontal wavenumbers which exhibits normal-mode instability. The instability mechanism closely parallels the mechanism responsible for the instability seen in the problem of counter-propagating Rossby waves. In this problem the instability arises out of the interaction of counter-propagating gravity waves. We argue that the instability meets the Hayashi-Young criterion for wave instability. We also argue that the instability is the simplest one that can arise in a stratified atmosphere with constant shear flow. The counter propagating gravity waves mechanism detailed here explains why the Rayleigh criteria for shear flow instability in the unstratified case does not need to be satisfied in the stratified case. This illustrates how the Miles-Howard theorem may support destabilization through stratification. A normal mode analysis of a foamy layer consisting of two density jumps of unequal magnitude is also analyzed. The results are considered in terms of observations made of sea-hurricane interfaces.