Anisotropic macroturbulence and diffusion associated with a westward zonal jet: From laboratory to planetary atmospheres and oceans

Abstract

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.

Revealing the Intensity of Turbulent Energy Transfer in Planetary Atmospheres

“…In the data set of laboratory experiments, the energy transfer rate values for Π ϵ vary between (2–30) × 10 −8 W kg −1 using our fit of the energy spectra and between (0.6–40) × 10 −8 W kg −1 using the minimum value from spectral energy fluxes (see supporting information figures for additional spectra and energy fluxes). It has been established elsewhere that structure function analyses also confirm our estimate of the energy transfer rate Π ϵ and corroborate the robustness of our estimates of L β (Galperin et al, 2016, for laboratory jets, and Young & Read, 2017, for Jupiter). For all data sets, we compute L β using the energy transfer rates Π ϵ estimated from both the energy fluxes and our fit of the energy spectra.…”

“…Experiments differed by the strength and the direction of the forcing, investigating both eastward and westward jets (details are given in supporting information section S1). Experimental measurements of the surface velocities were acquired by recording the tracks of small floating particles from an overhead camera centered in the rotating frame and then by analyzing their paths using a Lagrangian tracking method (Galperin et al, 2016). The 2‐D surface velocity maps obtained for each configuration spanned 58 rotation periods (two minutes) at a frequency of 20 Hz (see field maps in Figure 1 and supporting information).…”

Eddy–wave duality in a rotating flow

“…Experiments were designed to excite a single westward jet in a barotropic rotating fluid using small-scale electromagnetic forcing. 17,18 Velocity measurements show that the experiments produce a meandering jet and westward propagating eddies, similar to the geophysical examples above. After time averaging, the mean flow appears as a system of alternating zonal jets: a strong westward jet with a weak eastward jet on either side.…”

“…The two necessary pre-requisites making an experimental facility suitable for this study are (a) the β-effect and (b) the possibility of creating an easily controllable westward zonal jet. The experimental apparatus at the University of Rome satisfies both requirements 18 (Fig. 1).…”

Some statistical properties of three-dimensional zonostrophic turbulence

“…Yarom and Sharon 2014). Previous laboratory studies (see for instance the recent studies by Di Nitto et al 2013, Smith et al 2014, Zhang and Afanasyev 2014, Read et al 2015, Galperin et al 2016 have reached weakly zonostrophic regimes (with R β < 2.5).…”