Saturn's North Polar Vortex Structure Extracted From Cloud Images by the Optical Flow Method

• Published in: Journal of geophysical research. Planets 2019-11, Vol.124 (11), p.3041-3062
• Main Authors: Liu, Tianshu, Sayanagi, Kunio M., Brueshaber, Shawn R., Blalock, John J., Ingersoll, Andrew P., Dyudina, Ulyana A., Ewald, Shawn P.
• Format: Article
• Language: English
• Subjects: Cameras; Cassini mission; cloud tracking; Clouds; Correlation analysis; Cyclonic vortexes; Divergence; Flow pattern; flow structure; Fluid flow; Invariants; north polar vortex; North Pole; optical flow; Optical flow (image analysis); Polar environments; Polar regions; Polar vortex; Relative vorticity; Saturn; Spacecraft; Spatial resolution; Subsystems; Swirling; Tracking; Upwelling; Velocimetry; Velocity; Velocity distribution; Vortex structure; Vortices; Vorticity; Wind measurement; Wind shear; Wind speed
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2019JE005974

The paper presents velocity fields with ~3‐km spatial resolution of Saturn's north polar vortex (NPV) retrieved using the optical flow method from a sequence of polar‐projected cloud images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft. The fields of the velocity magnitude, velocity variation, relative vorticity, divergence, and second invariant are determined to characterize the flow structures of the inner core of the NPV. The mean zonal and mean meridional velocity profiles of the NPV are compared with previous measurements. We also describe the relevant details of application of the optical flow method to planetary cloud‐tracking wind measurements. The mean zonal velocity profile is consistent with the previous measurements using correlation image velocimetry methods. The small but significant meridional velocity corresponds to outwardly spiraling streams observed in the region near the north pole (NP). The concentrated vorticity and second invariant within 1° planetographic latitude of the NP indicate strong rotational motion of the fluid. An analysis is presented to explore a possible physical origin of the observed spiraling node at the NP. Plain Language Summary A swirling flow pattern with wind speeds peaking at about 100 m/s was revealed in Saturn's north polar vortex in high‐resolution images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft in November 2012. Using sequences of images that show clouds in the north polar vortex, the motions of these clouds were analyzed to measure the wind speeds in the north polar region. The high‐precision wind measurements presented in the current report are enabled by the optical flow cloud‐tracking method. The time‐averaged wind field shows a well‐defined counterclockwise (cyclonic) vortex at the pole. In particular, the observed flow structures and wind shear near the pole indicate strong rotational motion of the north polar atmosphere with upwelling at the center. Key Points The high‐resolution velocity fields of Saturn's north polar vortex (NPV) are extracted from cloud images by using the optical flow method The vorticity, divergence, and second invariant are obtained to characterize the flow structures of the NPV The strong rotational motion of the fluid with upwelling is found near the pole