# Validity of Sound-Proof Approaches in Rapidly-Rotating Compressible Convection: Marginal Stability vs. Turbulence

## TL;DR

This study evaluates the validity of sound-proof approximations, specifically the anelastic and pseudo-incompressible models, in rapidly-rotating compressible convection, highlighting their regimes of accuracy and limitations through linear and nonlinear simulations.

## Contribution

It clarifies the conditions under which the anelastic and pseudo-incompressible approximations remain valid in rapidly-rotating compressible convection regimes.

## Key findings

- Pseudo-incompressible approximation remains valid at marginal stability.
- Anelastic approximation converges to compressible results at high Rayleigh numbers.
- Both models show good agreement in highly supercritical turbulent regimes.

## Abstract

The validity of the anelastic approximation has recently been questioned in the regime of rapidly-rotating compressible convection in low Prandtl number fluids (Calkins et al. 2015). Given the broad usage and the high computational efficiency of sound-proof approaches in this astrophysically relevant regime, this paper clarifies the conditions for a safe application. The potential of the alternative pseudo-incompressible approximation is investigated, which in contrast to the anelastic approximation is shown to never break down for predicting the point of marginal stability. Its accuracy, however, decreases close to the parameters corresponding to the failure of the anelastic approach, which is shown to occur when the sound-crossing time of the domain exceeds a rotation time scale, i.e. for rotational Mach numbers greater than one. Concerning the supercritical case, which is naturally characterised by smaller rotational Mach numbers, we find that the anelastic approximation does not show unphysical behaviour. Growth rates computed with the linearised anelastic equations converge toward the corresponding fully compressible values as the Rayleigh number increases. Likewise, our fully nonlinear turbulent simulations, produced with our fully compressible and anelastic models and carried out in a highly supercritical, rotating, compressible, low Prandtl number regime show good agreement. However, this nonlinear test example is for only a moderately low convective Rossby number of 0.14.