UC San Diego

Trans-Pacific dust is thought to increase precipitation from atmospheric rivers (ARs) by enhancing ice formation in mixed-phase clouds. However, constraints on measurements of dust have limited our understanding of the magnitude of this effect.

In this dissertation, we utilize satellite-retrievals, ground-based observations, and reanalysis to: 1. Develop global, daily, observation-based datasets of dust aerosol optical depth extending from 2001 through 2018, 2. Develop a dust score to characterize the dust-content of the environment surrounding ARs that made landfall along the U.S. west coast between 2001 and 2018 and characterize the year-to-year variability of the dust content of the surroundings of ARs, and 3. Analyze the meteorological conditions associated with the presence of dust in the vicinity of ARs. We show that dusty ARs occur primarily in March, at the confluence of the end of the AR season and the beginning of the trans-Pacific dust season. We show that dust is preferentially found within the warm sector of the extra-tropical cyclones associated with ARs making landfall along the U.S. but dust is also enhanced in the cold sector. Our results suggest that the year-to-year variability in the dust score is primarily the result of variability of the dust content of the eastern North Pacific and correlated variability of the frequency of ARs and the dust content of surrounding area. Finally, we investigate the meteorological conditions that lead to the presence of dust in the vicinity of an AR by analyzing the conditions over trans-Pacific dust source regions and over the North Pacific in the days prior to an AR. Dusty ARs are associated with enhanced upper- and mid-level westerly winds over Asia, an extended North Pacific Jet, and eastward migrating extra-tropical cyclones. In contrast, pristine ARs are associated with a persistent ridge over the North Pacific.

The research presented here addresses gaps in our understanding of how often and why dust is present in the vicinity of an AR, a critical step in constraining the influence of dust on precipitation from ARs.