The effect of asymmetric large-scale dissipation on energy and potential enstrophy injection in two-layer quasi-geostrophic turbulence

TL;DR

This paper investigates how asymmetric large-scale dissipation influences the energy and potential enstrophy injection in two-layer quasi-geostrophic turbulence, explaining the observed spectral transition near the Rossby deformation wavenumber.

Contribution

It demonstrates that the asymmetric Ekman term affects the injection rates, aligning the transition wavenumber with the Rossby deformation wavenumber, and extends the Tung-Orlando theory to this context.

Findings

The transition wavenumber $k_t$ is near $k_R$ due to injection rates.

Asymmetric Ekman damping suppresses potential enstrophy more than energy.

The ratio of injected potential enstrophy to energy decreases, reducing $k_t$.

Abstract

In the Nastrom-Gage spectrum of atmospheric turbulence we observe a $k^{-3}$ energy spectrum that transitions into a $k^{-5/3}$ spectrum, with increasing wavenumber $k$. The transition occurs near a transition wavenumber $k_t$, located near the Rossby deformation wavenumber $k_R$. The Tung-Orlando theory interprets this spectrum as a double downscale cascade of potential enstrophy and energy, from large scales to small scales, in which the downscale potential enstrophy cascade coexists with the downscale energy cascade over the same length-scale range. We show that, in a temperature forced two-layer quasi-geostrophic model, the rates with which potential enstrophy and energy are injected place the transition wavenumber $k_t$ near $k_R$. We also show that if the potential energy dominates the kinetic energy in the forcing range, then the Ekman term suppresses the upscale cascading potential enstrophy more than it suppresses the upscale cascading energy, a behavior contrary to what occurs in two-dimensional turbulence. As a result, the ratio $\gn/\gee$ of injected potential enstrophy over injected energy, in the downscale direction, decreases, thereby tending to decrease the transition wavenumber $k_t$ further. Using a random Gaussian forcing model, we reach the same conclusion, under the modeling assumption that the asymmetric Ekman term predominantly suppresses the bottom layer forcing, thereby disregarding a possible entanglement between the Ekman term and the nonlinear interlayer interaction. Based on these results, we argue that the Tung-Orlando theory can account for the approximate coincidence between $k_t$ and $k_R$. We also identify certain open questions that require further investigation via numerical simulations.