Asymptotic scaling relations for rotating spherical convection with strong zonal flows

TL;DR

This study combines numerical simulations and asymptotic theory to analyze how strong zonal flows in rotating spherical convection depend on the Ekman and Rayleigh numbers, revealing non-negligible viscosity effects.

Contribution

It demonstrates the applicability of asymptotic theory to predict the scaling of zonal flows and flow speeds in spherical convection with strong zonal flows, supported by numerical simulations.

Findings

Asymptotic theory accurately predicts flow speed and length scales.

Zonal flow scaling with Ekman number is supported by simulations.

Viscous forces remain significant and do not diminish in the rapidly rotating regime.

Abstract

We analyse the results of direct numerical simulations of rotating convection in spherical shell geometries with stress-free boundary conditions, which develop strong zonal flows. Both the Ekman number and the Rayleigh number are varied. We find that the asymptotic theory for rapidly rotating convection can be used to predict the Ekman number dependence of each term in the governing equations, along with the convective flow speeds and the dominant length scales. Using a balance between the Reynolds stress and the viscous stress, together with the asymptotic scaling for the convective velocity, we derive an asymptotic prediction for the scaling behaviour of the zonal flow with respect to the Ekman number, which is supported by the numerical simulations. We do not find evidence of distinct asymptotic scalings for the buoyancy and viscous forces and, in agreement with previous results from asymptotic plane layer models, we find that the ratio of the viscous force to the buoyancy force increases with Rayleigh number. Thus, viscosity remains non-negligible and we do not observe a trend towards a diffusion-free scaling behaviour within the rapidly rotating regime.