Tracer Transport in Three Dimensions: Dispersion of Methane on Mars, Coupled Chemistry and Dynamics on Exoplanets, and Submesoscale Mixing in the Ocean

Citation

Luo, Yangcheng (2023) Tracer Transport in Three Dimensions: Dispersion of Methane on Mars, Coupled Chemistry and Dynamics on Exoplanets, and Submesoscale Mixing in the Ocean. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/91p7-gg59. https://resolver.caltech.edu/CaltechTHESIS:01302023-185542422

Abstract

One-dimensional (1D) modeling from a horizontally averaged perspective can oftentimes greatly simplify problems in atmospheric and oceanic sciences and thus capture leading-order physics. Meanwhile, 1D numerical models have great advantages such as numerical stability and time efficiency, hence they are widely used to gain insights into complex problems. However, oversimplification by 1D models may cause failures in finding solutions, revealing novel phenomena, and discovering scaling laws in the three-dimensional (3D) real world, and those are when 3D thinking proves its value. Also, the rise in computational power has allowed investigations using 3D numerical models. This thesis discusses three examples of how 3D modeling transcends the limitations of 1D modeling and reveals new solutions, phenomena, and scalings in planetary atmospheres and Earth’s ocean.

Chapter 2 is focused on the dispersion of methane plumes on Mars and how it can reconcile the discrepancy between observations. In the face of ostensibly inconsistent observational results of methane on Mars, we adopt a novel approach—inverse Lagrangian modeling in 3D space—to find the scenarios in which the inconsistency in the observations can be reconciled and locate the methane source. We find that the inconsistency between the results of the near-surface in situ methane measurements and the satellite remote sensing measurements can be reconciled if and only if an active methane emission hot spot is located in the immediate vicinity of the Curiosity rover in northwestern Gale crater, or unknown physical or chemical processes are rapidly removing methane.

Chapter 3 presents a novel phenomenon that could exist on exoplanets—self-sustained photochemical oscillations, which is only produced by 3D atmospheric models. We use a 3D, fully coupled, chemistry-radiation-dynamics model to simulate the ozone-NOx-HOx photochemistry in the atmosphere of a tidally locked Earth-like exoplanet in the circumstellar habitable zone, and calculate the transmission spectra during transits. We find that under certain conditions, biological nitrogen fixation like the one on the Earth can drive large-magnitude, self-sustained photochemical oscillations in the atmospheres of terrestrial exoplanets. The resulting large temporal variability in ozone abundance on exoplanets, if observed, may suggest a strong surface NOx emission source, which could signal extrasolar life participating in the nitrogen cycle on exoplanets. Fully coupled, three-dimensional atmospheric chemistry-radiation-dynamics models can reveal new phenomena that may not exist in one-dimensional models, and hence they are powerful tools for future planetary atmospheric research.

Chapter 4 uses a 3D fluid dynamics model to study the vertical exchange in the upper part of Earth’s ocean that potentially has great implications for the marine ecosystem. We develop scaling laws for the exchange rate between the surface ocean and the ocean interior which is critical to the rate of nutrient supply to phytoplankton near the ocean surface. These scaling laws could substitute the crude 1D parameterizations that are currently widely used in ocean models. We find that submesoscale turbulence energized by baroclinic instability in the ocean mixed layer can induce tracer exchange between the surface ocean and the ocean interior. Various environmental physical parameters affect the exchange rate. The exchange is stronger where the ocean mixed layer is thicker, the Richardson number (defined as the ratio of the squared buoyancy frequency to the squared vertical shear of the horizontal flow) of the thermocline is smaller, and the Richardson number of ocean mixed layer is larger. The associated nutrient supply from the ocean interior to the surface ocean is also expected to be stronger under these conditions.

Thesis Files