Emergence of helicity in rotating stratified turbulence

We perform numerical simulations of decaying rotating stratified turbulence and show, in the Boussinesq framework, that helicity (velocity-vorticity correlation), as observed in supercell storms and hurricanes, is spontaneously created due to an interplay between buoyancy and rotation common to large-scale atmospheric and oceanic flows. Helicity emerges from the joint action of eddies and of inertia-gravity waves (with inertia and gravity with respective associated frequencies f and N), and it occurs when the waves are sufficiently strong. For N/f < 3 the amount of helicity produced is correctly predicted by a quasilinear balance equation. Outside this regime, and up to the highest Reynolds number obtained in this study, namely Re approximate to 10 000, helicity production is found to be persistent for N/f as large as approximate to 17, and for ReFr2 and ReRo(2), respectively, as large as approximate to 100 and approximate to 24 000. DOI: 10.1103/PhysRevE.87.033016