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The laboratory modelling of a rotating turbulent flow subjected to a beta-effect by means of laboratory experiments is considered. In particular the focus has been put on the emergence and the evolution of zonal jet-like structures due to the anisotropization of the upscale energy transfer that can be observed in geophysical flows. The experimental setup consists of a rotating tank in which a turbulent flow is reproduced by electromagnetically forcing a shallow layer of saline solution; this model then reproduces the dynamics in the polar regions simulating the so-called gamma-plane by the parabolic surface of the rotating fluid. Several experiments have been performed by changing the main external parameters in order to investigate if the setup is suitable for reproducing the basic dynamics associated with a banded configuration analogous to large scale atmospheric and oceanic circulations. Velocity measurements performed by image analysis have allowed characterization of the flow in terms of mean azimuthal velocity, degree of anisotropy, distribution of energy, and characteristic scales. As expected, zonal jets have been found to dominate the dynamics when the beta-effect is stronger. (C) 2013 AIP Publishing LLC

The emergence of a sequence of alternating intense and elongated eastward-westward bands i.e. zonal jets in the atmosphere of the giant planets and in Earth's oceans have been widely investigated. Nevertheless jets formation and role as material barriers remain still unclear. Jets are generated in a quasi-2D turbulent flow due to the latitudinal variation of the Coriolis parameter (the so-called β-effect) which modifies the inverse cascade process channeling energy towards zonal modes. In previous experiments we have investigated the impact of the variation of the rotation rate, of the domain geometry and of the initial spectra on jets organization in a decaying regime. In this work we investigate the formation of jets in a continuously forced flow, we characterize the observed regime and also we attempt to verify the existence of an universal regime corresponding to the so-called zonostrophic turbulence. The experimental set-up consists of a rotating tank where turbulence is generated by electromagnetically forcing a shallow layer of an electrolyte solution, and the variation of the Coriolis parameter has been simulated by the parabolic profile assumed by the free surface of the fluid under rotation. Flow measurements have been performed using image analysis.

Turbulence with inverse energy cascade and its transport properties are investigated experimentally in a flow associated with a westward propagating jet. Turbulence and the jet were produced by an electromagnetic force in a rotating tank filled with an electrolytic saline solution. The parabolic free surface emulated the topographic β effect which evoked the zonation. The spectral and transport flow characteristics were highly anisotropic. Turbulence is diagnosed by exploring the analogy between vertical and horizontal turbulent overturns in, respectively, stably stratified and quasigeostrophic flows which gives rise to a method of potential vorticity (PV) monotonizing. The anisotropization of transport properties of the flow is investigated using the finite scale Lyapunov exponent technique. After initial exponential particle separation, radial (meridional in geophysical and planetary applications) diffusion attains a short-ranged Richardson regime which transitions to the Taylor (scale-independent diffusivity) one. The azimuthal (zonal) diffusion exhibits a double-plateau structure which attains a superdiffusive regime on large scales. The transition to the Taylor regime for the radial diffusion takes place at a scale of turbulence anisotropization. The radial eddy diffusivity in both regimes as well as the transition scale are all determined by the rate of the inverse energy cascade, ε, that can be diagnosed by the PV monotonizing. Conversely, ε can be deduced from the scale of the Richardson-Taylor regime transition in the radial eddy diffusivity which, thus, provides an additional tool of diagnosing anisotropic macroturbulence with inverse energy cascade.

A detailed analysis of Lagrangian tracer dispersion is performed on datasets obtained from laboratory experiments that simulate rotating turbulence in the presence of a b effect. Compatible with the limitations of the experimental apparatus, a relatively wide range of the zonostrophy index R b , a parameter used to characterize flow regimes in b-plane turbulence, is explored. The considered range spans from values ;O(10 21 ), for which the flow is nearly isotropic, to values ;O(1), corresponding to the so-called transitional range in which the flow gradually leaves the friction-dominated regime to enter the full zonostrophic regime. The degree of anistropy and the characteristic scales of the flow have been estimated by means of a Lagrangian approach based on the reconstruction of tracer trajectories and on the measure of the finite-scale Lyapunov exponents (FSLE). The FSLE analysis allows one to identify the regimes of two-particle dispersion and to relate them to the physical parameters of the system. Moreover, a Lagrangian anisotropy index (LAI) is introduced and defined in terms of the FSLE zonal and radial components, in order to describe the onset of anisotropy and to check if it is consistent with the theoretical predictions. It is remarkable that the finite-scale dispersion rates are very sensitive to the degree of anistropy of turbulence, more so than other indicators defined in terms of Eulerian quantities. Furthermore, they offer an effective diagnostic tool of the degree of anisotropy that can be used even prior to attaining a fully developed regime of zonostrophic turbulence.