Date of Award
Doctor of Philosophy
Mechanical and Aerospace Engineering
Dr. William Liou
Dr. Tianshu Liu
Dr. Parviz Merati
Dr. Kunio Sayanagi
Polar vortices, giant planets, Jupiter, Saturn, Uranus, Neptune
My research investigates the polar atmospheric dynamics of the giant planets: Jupiter and Saturn (gas giants), and Uranus and Neptune (ice giants). I conduct my research modifying and applying the Explicit Planetary Isentropic Coordinate global circulation code to model the polar regions of the four giant planets.
The motivation behind my research is to uncover the reason why giant planet polar atmospheric dynamics differ. Jupiter features multiple circumpolar cyclones arranged in geometrical configurations, whereas Saturn features a single pole-centered cyclone. Uranus and Neptune also appear to have single pole-centered cyclones, albeit, larger than those on Saturn. It is widely accepted that moist-convective processes such as thunderstorms, are a leading candidate in generating small-scale turbulence, which self-organizes into larger structures, via a process called the inverse-cascade. In the polar regions, cyclonic vortices are the naturally preferred outcome of this self-organization. I model small-scale turbulence by continually adding or removing mass into the domain throughout the simulation at scales matching the size of thunderstorms. The continual injection of turbulence is known as a “forced-turbulence” model. The storms geostrophically balance into small cyclones (anticyclones) if mass is removed (added). Cyclones (anticyclones) drift poleward (equatorward) via the beta-drift mechanism, which leads to an accumulation of cyclonic vorticity at the pole. The resulting configurations, dynamics, and morphologies of polar cyclones are the subject of my numerical simulations.
In Chapter 4, I show that the Burger Number, Bu—the ratio of the Rossby deformation radius to the planet radius squared—controls the morphology and number of polar cyclones. If Bu is sufficiently small, as expected for Jupiter, multiple circumpolar cyclones emerge from the forced-turbulent simulations. If Bu is sufficiently large, as expected for Saturn and the ice giants, a single pole-centered cyclone emerges instead. Four dynamical regimes are found from my experiments, three of which match the Bu and configurations observed on the giant planets: the Jupiter (J)-Regime, the Saturn (S)-Regime, and the Ice Giant (I)-Regime. This first set of numerical simulations also tests the effect of the mass injection rate (a proxy for storm intensity), and tests the effect of the storm polarity fraction, i.e., fraction of cyclonic to anticyclonic storms, on the dynamics of polar cyclones.
Chapter 5, focuses on the effect of turbulent forcing for the S- and I-Regimes. Here, I test the effect of turbulent scale and turbulent intensity by varying the storm size and wind speed, which are not a part of my first set of experiments. I find that the intensity of the resulting single polar cyclone is affected by the storm size, storm wind speed, and storm polarity fraction. However, the radius of the polar cyclone is not affected by the size of the storms.
The results of my numerical experiments advance the state of knowledge of giant planet polar atmospheric dynamics by revealing a fundamental mechanism behind the differing configurations and morphologies of polar cyclones. Furthermore, my results provide insight crucial in developing spacecraft exploration missions to the ice giant planets, which will likely include atmosphere science objectives.
Brueshaber, Shawn R., "Accumulation of Polar Vorticity on Giant Planets: Towards a Three-Dimensional Theory" (2020). Dissertations. 3655.