Jets and large-scale vortices in rotating Rayleigh-B\'enard convection

TL;DR

This study investigates how horizontal anisotropy influences the formation of large-scale vortices and unidirectional flows in rotating Rayleigh-Bénard convection, revealing that small anisotropy favors vortices while larger anisotropy leads to jets.

Contribution

Introduces horizontal anisotropy via unequal domain sizes in numerical models to explore coexistence of vortices and jets in rotating convection.

Findings

Small anisotropy induces vortices; larger anisotropy favors jets.

Multiple jets form quickly and merge over longer timescales.

Large vortices persist at the flanks of jets, showing robustness.

Abstract

One of the most prominent dynamical features of turbulent, rapidly-rotating convection is the formation of large-scale coherent structures, driven by Reynolds stresses resulting from the small-scale convective flows. In spherical geometry, such structures consist of intense zonal flows that are invariant along the rotation axis. In planar geometry, long-lived, depth-invariant structures also form at large scales, but, in the absence of horizontal anisotropy, they consist of vortices that grow to the domain size. In this work, through the introduction of horizontal anisotropy into a numerical model of planar rotating convection by the adoption of unequal horizontal box sizes (i.e. $L_x \le L_y$, where the $xy$-plane is horizontal), we investigate whether unidirectional flows and large-scale vortices can coexist. We find that only a small degree of anisotropy is required to bring about a transition from dynamics dominated by persistent large-scale vortices to dynamics dominated by persistent unidirectional flows parallel to the shortest horizontal direction. When the anisotropy is sufficiently large, the unidirectional flow consists of multiple jets, generated on a timescale smaller than a global viscous timescale, thus signifying that the upscale energy transfer does not spontaneously feed the largest available mode in the system. That said, the multiple jets merge on much longer timescales. Large-scale vortices of size comparable with $L_x$ systematically form in the flanks of the jets and can be persistent or intermittent. This indicates that large-scale vortices, either coexisting with jets or not, are a robust dynamical feature of planar rotating convection.