Tropical cyclone size is strongly limited by the Rhines scale: experiments with a barotropic model

TL;DR

This study demonstrates that tropical cyclone size is fundamentally constrained by the vortex Rhines scale, with experiments showing the vortex shrinks to this scale, which balances wave effects and circulation.

Contribution

It introduces a vortex Rhines scale concept and demonstrates its role in limiting tropical cyclone size through barotropic model experiments.

Findings

Vortex size converges to the vortex Rhines scale.

Shrinking timescale matches the vortex Rhines timescale.

Larger initial vortices shrink faster to the equilibrium size.

Abstract

Recent work found evidence using aquaplanet experiments that tropical cyclone size on Earth is limited by the Rhines scale, which depends on the planetary vorticity gradient, $\beta$. This study aims to examine how the Rhines scale limits the size of an individual tropical cyclone. The traditional Rhines scale is first re-expressed as a vortex Rhines scale and Rhines speed to characterize how wave effects vary with radius in a vortex whose wind profile is known. Experiments are performed using a simple barotropic model on a $\beta$-plane initialized with a TC-like axisymmetric vortex defined using a recently-developed theoretical model for the tropical cyclone wind profile. $\beta$ and initial vortex size are each systematically varied to investigate the detailed responses of the TC-like vortex to $\beta$. Results show that the vortex shrinks towards an equilibrium size that closely follows the vortex Rhines scale. Physically, this scale divides the vortex into a vortex-dominant region at small radii, where the axisymmetric circulation is steady, and a wave-dominant region at larger radii, where the circulation stimulates Rossby waves and dissipates. A larger initial vortex relative to its vortex Rhines scale will shrink faster, and the shrinking timescale is well described by the vortex Rhines timescale, which is defined as the overturning timescale of the circulation at the vortex Rhines scale and is shown to be directly related to the Rossby wave group velocity. The relationship between our idealized results and the real Earth is discussed.