On spectral scaling laws for averaged turbulence on the sphere

Abstract

Spectral analysis for a class of Lagrangian-averaged Navier–Stokes (LANS) equations on the sphere is carried out. The equations arise from the Navier–Stokes equations by applying a Helmholtz filter of width $\alpha$ to the advecting velocity $\beta$ times, extending previous results on the Navier–Stokes-$\alpha$ model and enabling a precise selection of the smallest length scale in the flow. Power laws for the energy spectrum are derived and indicate a $\beta$-dependent scaling at wave numbers $l$ with $\alpha l\gg 1$. The energy and enstrophy transfer rates distinctly depend on the averaging, allowing control over the energy flux and the enstrophy flux separately through the choice of averaging operator. A necessary condition on the averaging operator is derived for the existence of the inverse cascade in two-dimensional turbulence. Numerical experiments with a structure-preserving integrator based on Zeitlin’s self-consistent truncation for hydrodynamics confirm the expected energy spectrum scalings and the robustness of the double cascade under choices of the averaging operator. The derived results have potential applications in reduced-complexity numerical simulations of geophysical flows on spherical domains.