An unstable mode of the stratified atmosphere under the non-traditional Coriolis acceleration

TL;DR

This paper reveals a previously unrecognized unstable mode in the stratified atmosphere caused by the non-traditional Coriolis acceleration, challenging the traditional approximation's validity in certain geophysical scenarios.

Contribution

The study demonstrates that including full Coriolis terms leads to an unstable atmospheric mode, which is absent under the traditional approximation, providing new insights into atmospheric stability analysis.

Findings

Full Coriolis terms induce exponential instability.

Traditional approximation remains stable under the same conditions.

The instability growth rate matches theoretical predictions.

Abstract

The traditional approximation neglects the cosine components of the Coriolis acceleration, and this approximation has been widely used in the study of geophysical phenomena. However, the justification of the traditional approximation is questionable under a few circumstances. In particular, dynamics with substantial vertical velocities or geophysical phenomena in the tropics have non-negligible cosine Coriolis terms. Such cases warrant investigations with the non-traditional setting, i.e., the full Coriolis acceleration. In this manuscript, we study the effect of the non-traditional setting on an isothermal, hydrostatic and compressible atmosphere assuming a meridionally homogeneous flow. Employing linear stability analysis, we show that, given appropriate boundary conditions, i.e., a bottom boundary condition that allows for a vertical energy flux and non-reflecting boundary at the top, the atmosphere at rest becomes prone to a novel unstable mode. The validity of assuming a meridionally homogeneous flow is investigated via scale analysis. Numerical experiments were conducted, and Rayleigh damping was used as a numerical approximation for the non-reflecting top boundary. Our three main results are as follows. 1) Experiments involving the full Coriolis terms exhibit an exponentially growing instability, yet experiments subjected to the traditional approximation remain stable. 2) The experimental instability growth rate is close to the theoretical value. 3) A perturbed version of the unstable mode arises even under sub-optimal bottom boundary conditions. Finally, we conclude our study by discussing the limitations, implications, and remaining open questions. Specifically, the influence on numerical deep-atmosphere models and possible physical interpretations of the unstable mode are discussed.