The Center for Atmospheric Sciences | » Seasonal Analysis of Zonal Winds and Vertical Wind Shear on Saturn from Cassini ISS Images

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  • Seasonal Analysis of Zonal Winds and Vertical Wind Shear on Saturn from Cassini ISS Images

    We are working on a project to calculate global mean zonal wind profiles for different altitudes in Saturn’s lower atmosphere using Cassini ISS images captured using the continuum band (CB) and methane absorption (MT) filters between 2004 and 2013. Specifically, I utilize the CB2 (750 nm), MT2 (727 nm), and MT3 (889 nm) filters for most tracking application. There is the option to also utilize the CB3 (939 nm) continuum filter and the IRP0/IRP90 infrared polarization filters (705 nm). Due to the presence of methane in the atmosphere, images captured using the MT filters sense features in the upper troposphere (up to 70 mbar ± 10 mbar) while the CB filters show thick tropospheric clouds (down to 350 mbar ± 150 mb). The IRP0/IRP90 filters are useful for synthesizing images that have similar properties as the CB filters to fill in gaps in the CB filter coverage (both spacially and temporally).

    By comparing the zonal wind profiles at the different altitudes, we will be able to determine the vertical wind shear between those levels. Ideally, we will be able to detect distinct differences between each of the three filters (CB2, MT2, MT3) and provide multiple shear calculations. Previous research suggests that we can expect that winds will appear around 20 m/s faster in the CB than in MT at the peaks of Saturn’s eastward jest, no difference in wind speeds in the westward jets, and differences of around 10 m/s at other latitudes not associated with jets. The vertical shear has been analyzed only in the southern hemisphere and equatorial region, so it will be interesting to see if the same kind of behavior holds true globally.


    Mean zonal wind profile for Saturn between 2004-2013 in the CB2 filter. White lines indicate our measurements while the red and blue lines are previously published zonal wind profiles.

    In addition to the vertical wind shear calculations, this project aims to create yearly, near-global records of Saturn’s zonal wind structure at the different altitude which we are sensing. Using our wind measurements, we will analyze the temporal evolution of the zonal wind. We specifically focus on changes in the wind profile after the 2009 equinox; we predict that changes in the insolation pattern caused by the shifting ring shadows affect the horizontal temperature gradient, and change the zonal mean wind through the thermal wind relationship. Additionally, we will extend the zonal wind analysis by Sayanagi et al (2013), who detected changes in the zonal wind related to the Great Storm of 2010-2011, to study the subsequent evolution of the region affected by the storm.


    Annual mean zonal wind profiles for Saturn 2004-2013


    Difference between annual zonal wind profiles and reference profile.

    Finally, Saturn’s northern hemisphere gradually emerged from the ring shadows from 2005 until the equinox in 2009. Today, the southern hemisphere is gradually pushed deeper into the ring shadows. These images offer an excellent opportunity to test the impact of the ring shadows on the atmospheric dynamics of Saturn. Of particular interest is the emergence of the northern mid-latitudes from the ring shadow and how the dynamics in that zone react to the insolation change. As has been hinted at above, the changes in both the vertical wind shear and zonal wind measurements are of great interests.