This report documents the primary outcomes and findings of the NSF-funded, “RAPID: The Mechanisms Driving Extreme Precipitation in Atmospheric Rivers with an Integrated Stable Isotope and Aerosol Chemistry Approach” grant. The work was led by PI Dr. Hari Mix (Santa Clara University) with collaborators Drs. Jessie Creamean and Andrew Martin (UC San Diego / Scripps). The primary goal of this work was to evaluate the relationships between the amount, type and source of ice nucleating particles (INPs; the pieces of dust which initially seed a cloud) on the rainout of atmospheric moisture (how much of the atmospheric moisture rains out vs remains in the atmosphere). In order to evaluate these relationships, this grant focused on landfalling atmospheric rivers—the most powerful storms reaching the American West Coast. This work made several considerable contributions in this area:

1) We collected and analyzed precipitation samples in order to evaluate the rainout (the fraction of water vapor that condenses and falls to the surface) and post-condensation processes (e.g., evaporation of falling raindrops) throughout a landfalling atmospheric river in 2016. The results demonstrated an extraordinary degree of variability throughout the evolution of a major storm: at peak atmospheric river conditions, the rainout signature dwarfs post-condensation processes, while other portions of the storm, for example during drizzling, evaporation of raindrops is significant. Further, we documented the contributions of multiple Pacific moisture sources, with the early and peak atmospheric river conditions being dominated by tropical a tropical moisture source and the later portion of the storm receiving contributions from a colder, drier northern Pacific moisture source. These results were documented in Mix et al., 2019 (Atmosphere).

2) We analyzed the sources and roles of warm INPs (primarily bacteria that become the surface on which ice begins to form, initiating the formation of a raindrop) during the same 2016 atmospheric river event. These warm INPs are particularly important for promoting efficient rainout (a large fraction of atmospheric moisture rains out of the cloud). We analyzed these warm ice nucleating particles at the coast itself (Bodega Bay, CA) and just inland (Cazadero, CA). Interesting, we found evidence for a bioprecipitation feedback at the inland site, in which intense rainfall lofts bacteria on leaves into the air, reinforcing the intensity and efficiency of precipitation. The bioprecipitation feedback has large implications for the global water cycle and the ability for humans to manage it. These findings were documented in Martin et al., 2019 (Atmospheric Chemistry & Physics).

3) The grant had a number of tangential impacts. The largest of these made a contribution to paleoclimate. Our precipitation samples were used to reconstruct past precipitation and validate an ancient climate record from a speleothem (stalactite or stalagmite) in Shasta Lake Cave. This speleothem paleoclimate record spanned the “8.2 Ka event,” a massive global cooling event 8200 years ago caused by a sudden, catastrophic flood of glacial water into the North Atlantic. This event, which had been recorded in several sites around the world, had never been observed in western North America until this study. Our findings indicate that the flood intensified the Pacific winter storm track, increasing precipitation in California. The findings have implications for the future of California in a warming climate, and were published in Nature Scientific Reports by Oster et al., 2017.

Beyond the peer-reviewed contributions of this grant, the funding contributed to several broader impacts supporting the NSF mission. First, eight Santa Clara University undergraduates contributed to the research. Multiple students went on to graduate study in the environmental sciences, including one Fulbright and Rhodes Scholar. Several graduate students at UC San Diego / Scripps Institution of Oceanography were supported by this grant as well. Finally, the Army Corps of Engineers were supported by the grant in the form of precipitation, stream and groundwater analysis. This work furthered the Army Corps’ mission of improving flood prediction and resilience in Northern California, particularly in vulnerable watersheds such as the Russian River.

Last Modified: 04/17/2022
Modified by: Hari Mix