The Spherical Harmonic Spectrum of a Function with Algebraic Singularities

Abstract

The asymptotic behaviour of the spectral coefficients of a function provides a useful diagnostic of its smoothness. On a spherical surface, we consider the coefficients a m l of fully normalised spherical harmonics of a function that is smooth except either at a point or on a line of colatitude, at which it has an algebraic singularity taking the form θ p or |θ − θ 0 | p respectively, where θ is the co-latitude and p > −1. It is proven that each type of singularity has a signature on the rotationally invariant energy spectrum, E(l) = m (a m l ) 2 where l and m are the spherical harmonic degree and order, of l −(p+3/2) or l −(p+1) respectively. This result is extended to any collection of finitely many point or (possibly intersecting) line singularities of arbitrary orientation: in such a case, it is shown that the overall behaviour of E(l) is controlled by the gravest singularity. Several numerical examples are presented to illustrate the results. We discuss the generalisation of singularities on lines of colatitude to those on any closed curve on a spherical surface.

Full sphere hydrodynamic and dynamo benchmarks

“…Hence there is the possibility of the need to resolve very fine shear layers in this region; for a recent discussion see Livermore & Hollerbach (2012). The presence of very fine structures that need to be resolved can have very deleterious effects on a numerical method, particularly a spectral method based on spherical harmonics (again, for a discussion see Livermore (2012)). Thus, the choice of a full sphere is likely to be advantageous in the limit in which the viscosity is dropped to insignificant levels.…”