EGU21-14857
https://doi.org/10.5194/egusphere-egu21-14857
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Turbulent kinetic energy spectra and cascades in the polar atmosphere of Saturn

Peter L. Read1, Arrate Antuñano2, Simon Cabanes3, Greg Colyer1, Teresa del Rio-Gaztelurrutia4, and Agustin Sánchez-Lavega4
Peter L. Read et al.
  • 1Atmospheric, Oceanic & Planetary Physics, University of Oxford, Oxford, UK
  • 2Department of Physics & Astronomy, University of Leicester, Leicester, UK
  • 3DICEA, Sapienza Universita di Roma, Italy
  • 4Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco, Bilbao, Spain

The regions of Saturn’s cloud-covered atmosphere polewards of 60o latitude are dominated in each hemisphere near the cloud tops by an intense, cyclonic polar vortex surrounded by a strong, high latitude eastward zonal jet. In the north, this high latitude jet takes the form of a remarkably regular zonal wavenumber m=6 hexagonal pattern that has been present at least since the Voyager spacecraft encounters with Saturn in 1980-81, and probably much longer. The origin of this feature, and the absence of a similar feature in the south, has remained poorly understood since its discovery. In this work, we present some new analyses of horizontal wind measurements at Saturn’s cloud tops polewards of 60 degrees in both the northern and southern hemispheres, previously published by Antuñano et al. (2015) using images from the Cassini mission, in which we compute kinetic energy spectra and the transfer rates of kinetic energy (KE) and enstrophy between different scales. 2D KE spectra are consistent with a zonostrophic regime, with a steep (~n-5) spectrum for the mean zonal flow (n is the total wavenumber) and a shallower Kolmogorov-like KE spectrum (~n-5/3) for the residual (eddy) flow, much as previously found for Jupiter’s atmosphere (Galperin et al. 2014; Young & Read 2017). Three different methods are used to compute the energy and enstrophy transfers, (a) as latitude-dependent zonal spectral fluxes, (b) as latitude-dependent structure functions and (c) as spatially filtered energy fluxes. The results of all three methods are largely in agreement in indicating a direct (forward) enstrophy cascade across most scales, averaged across the whole domain, an inverse kinetic energy cascade to large scales and a weak direct KE cascade at the smallest scales. The pattern of transfers has a more complex dependence on latitude, however. But it is clear that the m=6 North Polar Hexagon (NPH) wave was transferring KE into its zonal jet at 78o N (planetographic) at a rate of ∏E ≈ 1.8 x 10-4 W kg-1 at the time the Cassini images were acquired. This implies that the NPH was not maintained by a barotropic instability at this time, but may have been driven via a baroclinic instability or possibly from deep convection. Further implications of these results will be discussed.

 

References

Antuñano, A., T. del Río-Gaztelurrutia, A. Sánchez-Lavega, and R. Hueso (2015), Dynamics of Saturn’s polar regions, J. Geophys. Res. Planets, 120, 155–176, doi:10.1002/2014JE004709.

Galperin, B., R. M.B. Young, S. Sukoriansky, N. Dikovskaya, P. L. Read, A. J. Lancaster & D. Armstrong (2014) Cassini observations reveal a regime of zonostrophic macroturbulence on Jupiter, Icarus, 229, 295–320.doi: 10.1016/j.icarus.2013.08.030

Young, R. M. B. & Read, P. L. (2017) Forward and inverse kinetic energy cascades in Jupiter’s turbulent weather layer, Nature Phys., 13, 1135-1140. Doi:10.1038/NPHYS4227

 
 
 
 

How to cite: Read, P. L., Antuñano, A., Cabanes, S., Colyer, G., del Rio-Gaztelurrutia, T., and Sánchez-Lavega, A.: Turbulent kinetic energy spectra and cascades in the polar atmosphere of Saturn, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14857, https://doi.org/10.5194/egusphere-egu21-14857, 2021.

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