Saturn Mean Zonal Winds and Eddy Momentum Fluxes
The datasets here contain zonally averaged zonal winds and horizontal eddy momentum fluxes for Saturn's atmosphere based on an automated cloud tracking algorithm applied to Narrow and Wide Angle Camera images acquired by the Imaging Science Subsystem (ISS) instrument on the Cassini Saturn Orbiter spacecraft.
The winds and eddy fluxes are available at two wavelengths: A continuum wavelength (750 nm; filter CB2) that primarily views the tops of Saturn's ammonia clouds, but with some obscuration by its upper troposphere haze, and a methane band wavelength (727 nm; filter MT2) that is restricted to higher altitudes by methane absorption and senses motions within the upper troposphere haze. The images were acquired mostly in 2005 and 2007, during Saturn's late southern summer when the equatorial region was partly obscured by Saturn's rings and ring shadows. A small number of images from 2011 and 2012, during early northern spring, were included to provide additional equatorial coverage.
The winds and eddy fluxes are based on an automated tracking algorithm that uses a correlation technique to find the most likely displacements of features defined by 2° longitude × 1° latitude tracking boxes (see figure) in images separated by one Saturn rotation. The original technique is described in detail by Del Genio et al. (2007); the improved version used in the current dataset is described in Del Genio and Barbara (2012).
The figure shows Saturn's appearance at the CB2 and MT2 wavelengths for the region surrounding the 42°N eastward jet (indicated by the mean zonal wind curve shown at the left of each image). Less detail is apparent in the MT2 image because of the upper troposphere haze.
The haze is thinner in winter than summer, causing the ammonia cloud tops to be visible in more detail in the northern hemisphere than the southern hemisphere during 2005 and 2007 in CB2 images. With less detail, automated tracking becomes more difficult. Thus there are fewer CB2 wind vectors in the southern hemisphere than the northern hemisphere and fewer overall in the MT2 dataset. Also, because there tends to be less feature detail and more wavelike morphology in eastward jet regions, the tracking algorithm is less reliable there. Consequently some eastward jet latitudes are missing from the dataset. Otherwise the dataset has complete coverage between 60°N and 66.5°S.
Downloads
The datasets contain mean zonal winds and eddy momentum fluxes at 0.5° latitude intervals in planetocentric coordinates, along with the number of vectors included in the dataset at each latitude.
The individual wind vectors at each latitude and longitude are also available from the authors for collaborative projects.
References
The ISS instrument characteristics are described in detail in :
Porco, C.C., R.A. West, S. Squyres, A. McEwen, P. Thomas, C.D. Murray, A. Del Genio, A.P. Ingersoll, T.V. Johnson, G. Neukum, J. Veverka, L. Dones, A. Brahic, J.A. Burns, V. Haemmerle, B. Knowles, D. Dawson, T. Roatsch, K. Beurle, and W. Owen, 2004: Cassini imaging science: Instrument characteristics and capabilities and anticipated scientific investigations at Saturn. Space Sci. Rev., 115, 363-497, doi:10.1007/s11214-004-1456-7.
The original tracking algorithm and early southern hemisphere CB2 results can be found in:
Del Genio, A.D., J.M. Barbara, J. Ferrier, A.P. Ingersoll, R.A. West, A.R. Vasavada, J. Spitale, and C.C. Porco, 2007: Saturn eddy momentum fluxes and convection: First estimates from Cassini images. Icarus, 189, 479-492, doi:10.1016/j.icarus.2007.02.013.
The current version of the algorithm and most recent global CB2 and MT2 results obtained from the datasets available above can be found in:
Del Genio, A.D., and J.M. Barbara, 2012: Constraints on Saturn's tropospheric general circulation from Cassini ISS images. Icarus, 219, 689-700, doi:10.1016/j.icarus.2012.03.035.
An overall review of the dynamics of Saturn's atmosphere as revealed by Cassini can be found in:
Del Genio, A.D., R.K. Achterberg, K.H. Baines, F.M. Flasar, P.L. Read, A. Sánchez-Lavega, and A.P. Showman, 2009: Saturn atmospheric structure and dynamics. In Saturn from Cassini-Huygens. M.K. Dougherty, L.W. Esposito, and S.M. Krimigis, Eds. Springer-Verlag, pp. 113-159, doi:10.1007/978-1-4020-9217-6_6.
Contacts
This research was led by Dr. Anthony Del Genio. Please address any questions you may have about the data to Mr. John Barbara.
