Theoretical Investigations of the Water Cycle on Earth and Other Planets

Abstract

From exoplanets to Solar System bodies to modern Earth, the multi-scale and interconnected processes of the water cycle are fundamental drivers of planetary climate, evolution, and habitability. This thesis confronts problems stemming from the water cycle’s complexity across diverse planetary environments from a theoretical perspective. I construct simplified representations of more complicated systems within planetary water cycles to infer observable consequences of system behavior, to elucidate fundamental controls on system behavior, and to parameterize system behavior. I begin by using the coupling between the water cycle and the sulfur cycle to propose two new observational diagnostics for the absence of an exoplanet ocean—a challenging but highly desirable observable for constraining the prevalence of Earth-like worlds. Next, from a generalized planetary perspective, I use the simplicity of how raindrops fall and evaporate to place constraints on aspects of cloud evolution independent of the complex processes governing raindrop growth. I demonstrate across broad planetary conditions that raindrop size is the predominant determiner of raindrop ability to vertically transport condensed mass (i.e., precipitate) and that a new non-dimensional number can capture the fundamental behavior of falling raindrops. Finally, from a modern-Earth perspective, I consider the initiation of rain via liquid drop coagulation (i.e., collision and subsequent coalescence). I document the parameterizations of coagulation in global climate models participating in the most recent phase of the Coupled Model Intercomparison Project (CMIP6)—representing the world’s most comprehensive attempts to model modern-Earth climate. These coagulation parameterizations share five conceptual assumptions that I demonstrate lead to too rapid rain initiation in a manner consistent with a widespread and longstanding global model bias predicting too frequent precipitation relative to observations. To address the deficient conceptual assumptions underlying the CMIP6 coagulation parameterizations, I design and implement three approaches (two novel) for parameterizing coagulation that show improved rain initiation timing relative to the CMIP6-based approaches in an idealized test. Overall, the work of this thesis highlights the productivity of a comparative planetology approach for studying the water cycle.