Investigations of non-hydrostatic, stably stratified and rapidly rotating flows

TL;DR

This study investigates the dynamics of rapidly rotating, stratified turbulence across different stratification strengths using a reduced model, revealing distinct layering regimes and energy transfer mechanisms.

Contribution

It introduces a reduced model capturing wave-eddy interactions in stratified turbulence and explores how stratification influences layer formation and energy cascades.

Findings

Presence of persistent thin turbulent layers at low Froude numbers

Large-scale barotropic dipole observed in both regimes

Energy injected at small scales cascades to large scales without inverse transfer

Abstract

We present an investigation of rapidly rotating (small Rossby number $Ro\ll 1$) and stratified turbulence where the stratification strength is varied from weak (large Froude number $Fr\gg1$) to strong ($Fr\ll1$). The investigation is set in the context of a reduced model derived from the Boussinesq equations that efficiently retains anisotropic inertia-gravity waves with order-one frequencies and highlights a regime of wave-eddy interactions. Numerical simulations of the reduced model are performed where energy is injected by a stochastic forcing of vertical velocity, which forces wave modes only. The simulations reveal two regimes characterized by the presence of well-formed, persistent and thin turbulent layers of locally-weakened stratification at small Froude numbers, and by the absence of layers at large Froude numbers. Both regimes are characterized by a large-scale barotropic dipole enclosed by small-scale turbulence. When the Reynolds number is not too large a direct cascade of barotropic kinetic energy is observed, leading to total energy equilibration. We examine net energy exchanges that occur through vortex stretching and vertical buoyancy flux and diagnose the horizontal scales active in these exchanges. We find that the baroclinic motions inject energy directly to the largest scales of the barotropic mode, implying that the large-scale barotropic dipole is not the end result of an inverse cascade within the barotropic mode.