Discrete resonant Rossby/drift wave triads: an explicit parameterisation and a fast direct numerical search algorithm

TL;DR

This paper introduces an explicit parameterisation of discrete Rossby-wave resonant triads and a fast numerical search algorithm, improving the efficiency of identifying resonances in geophysical fluid dynamics.

Contribution

It provides a new explicit parameterisation of wavevectors and a cubic complexity numerical method for finding all discrete resonant triads within large wavevector domains.

Findings

Explicit parameterisation restricts wavevector search space.

Numerical method efficiently finds all triads in large domains.

Discussion on dynamical implications of triad interactions.

Abstract

We report results on the explicit parameterisation of discrete Rossby-wave resonant triads of the Charney-Hasegawa-Mima equation in the small-scale limit (i.e. large Rossby deformation radius), following up from our previous solution in terms of elliptic curves (Bustamante and Hayat, 2013). We find an explicit parameterisation of the discrete resonant wavevectors in terms of two rational variables. We show that these new variables are restricted to a bounded region and find this region explicitly. We argue that this can be used to reduce the complexity of a direct numerical search for discrete triad resonances. Also, we introduce a new direct numerical method to search for discrete resonances. This numerical method has complexity ${\mathcal{O}}(N^3)$, where $N$ is the largest wavenumber in the search. We apply this new method to find all discrete irreducible resonant triads in the wavevector box of size $5000$, in a calculation that took about $10.5$ days on a $16$-core machine. Finally, based on our method of mapping to elliptic curves, we discuss some dynamical implications regarding the spread of quadratic invariants across scales via resonant triad interactions, in the form of sharp bounds on the size of the interacting wavevectors.