Spatiotemporal Variations Of Saturn's Zonal Winds Based On Cassini Long-term (2004-2017) Multi-Instrument Observations

Abstract

The observations of Saturn by the Cassini mission spanned thirteen years (2004-2017) and provided scientists data and images that will expand our knowledge for decades to come. This dissertation is divided into four tasks: 1) Develop and validate a more general form of thermal wind equation (TWE), which does not apply the assumptions used in the classical TWE; 2) Apply the new TWE to study the spatiotemporal patterns of the atmospheric winds above the visible cloud layer, ~1 to 500 mbar; 3) Develop a global profile of the atmospheric winds below the visible cloud layer, ~1,000 to 3,000 mbar; and 4) Utilize atmospheric winds to investigate Saturn’s 2010 Great White Storm with particular attention to its asymmetric development. The results from Tasks 1, 2 and 3 provides a relatively complete picture of the seasonal variations of Saturn’s winds. Some important characteristics of Saturn’s winds are revealed: (1) The global profile of zonal winds in the deep troposphere is generated for the first time; (2) In the polar region, the 2,000-mbar winds undergo temporal variation; (3) Within the visible cloud layer, the zonal winds did not significantly change between 2009 and 2015; and (4) The stratospheric equatorial zonal jets weakened from ~500 m s-1 in 2009 to 0 m s-1 in 2015. This is the first systematic analysis of the seasonal changes of Saturn’s zonal winds across both the troposphere and stratosphere, expanding our understanding of spatiotemporal variations of Saturn’s atmosphere. In particular, the vertical structure and its changes over time yield new insight on the atmospheric dynamics (e.g., stability), which in turn aid in the development of new theories and models of the atmospheric systems of the giant planets. In Task 4, the wind field and the associated vorticity field are used to investigate 2010 Great White Storm. Specifically, during the mature phase of this event, the associated bright clouds expanded significantly equatorward, but its poleward growth was limited. The analysis of the wind and vorticity fields suggest that large meridional gradients of quasi-geostrophic potential vorticity, acting as a barrier to cloud mixing, was a factor in the asymmetric expansion.