Relative dispersion and eddy diffusivity in laboratory experiments of $β$-plane turbulence

We present the first experimental measures of relative dispersion and turbulent diffusion in rapidly-rotating turbulence in the zonostrophic regime, i.e., in the presence of instantaneous and dominant zonal jets. Synthetic Lagrangian trajectories are computed from time-resolved experimental velocity fields, from which we measure relative (two-particle) dispersion. Time-based and separation-based statistics are calculated, including the cumulative inverse separation time (CIST), for which analytical predictions exist in the inertial ranges (direct enstrophy cascade and inverse energy cascade) and in the diffusive regime. These statistics show evidence of a transition from a Richardson regime at scales larger than the energy-injection scale, to a diffusive regime, at scales larger than the transitional scale, the scale at which turbulence becomes anisotropic due to the interaction between turbulent eddies and Rossby waves. The analytical predictions for the CIST allow us to measure the turbulent energy dissipation rate in the Richardson regime, and the turbulent diffusivity in the diffusive regime. Our measurements of diffusivity are broadly consistent with predictions from mixing-length and zonostrophic theories but suggest a shallower dependence on the energy dissipation rate.