Comparison of Quasi-Geostrophic, Hybrid and 3D models of planetary core convection

Abstract

We present investigations of rapidly-rotating convection in a thick spherical shell geometry relevant to planetary cores, comparing results from Quasi-Geostrophic, 3D and hybrid QG-3D models. The 170 reported calculations span Ekman numbers, $Ek$, between $10^{-4}$ and $10^{-10}$, Rayleigh numbers, $Ra$, between $2$ and $150$ times supercritical, and Prandtl numbers, $Pr$, between $10$ and $10^{-2}$. In general, we find convection is dominated by zonal jets at mid-depths in the shell, with thermal Rossby waves prominent close to the outer boundary when the driving is weaker. For the specific geometry studied here the hybrid method is best suited for studying convection at modest forcing, $Ra \leq 10 \, Ra_c$ when $Pr=1$, and departs from the 3D model results at higher $Ra$, displaying systematically lower heat transport. We find that the lack of equatorially anti-symmetric and $z$-correlations between temperature and velocity in the buoyancy force contributes to the weaker flows in the hybrid formulation. On the other hand, the QG models yield broadly similar results to the 3D models, for the specific range of parameters explored here. We cannot point to major disagreements between these two datasets at $Pr \geq 0.1$, although the QG model is effectively more strongly driven than the hybrid case. When $Pr$ is decreased, the range of agreement between the Hybrid and 3D models expands, indicating the hybrid method may be better suited to study convection in the regime $Pr \ll 1$. Previously proposed scaling laws for rapidly-rotating convection are retrieved: our simulations are overall well described by a triple balance between Coriolis, inertia and Archimedean forces with the length-scale of the convection following the diffusion-free Rhines-scaling.