Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing

TL;DR

This paper investigates the energy spectra and transfer mechanisms in two-dimensional stably stratified turbulence under large-scale forcing, revealing different regimes and spectral scalings depending on stratification strength.

Contribution

It provides a comprehensive analysis of energy spectra, fluxes, and flow structures across varying stratification levels in 2D turbulence, highlighting the coexistence of flows and scaling laws.

Findings

Strong stratification leads to a $k^{-3}$ KE spectrum with zero KE flux.

Moderate stratification exhibits Bolgiano-Obukhov scaling with a transition from $k^{-11/5}$ to $k^{-3}$.

Weak stratification shows upscale KE transfer with a $k^{-2.5}$ spectrum.

Abstract

In this paper we characterize the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) co-exists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate $k^{-3}$ scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a $k^{-3}$ power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano-Obukhov scaling that transitions from a shallow $k^{-11/5}$ form at large scales, to a steeper approximate $k^{-3}$ scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterized by an approximate $k^{-1.64}$ scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a $k^{-2.5}$ scaling, while PE exhibits a forward transfer and its spectrum shows an approximate $k^{-1.6}$ power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indices are well explained by accounting for the scale dependent nature of the corresponding flux.