Aerosols such as mineral dust, sea spray and bacteria are ubiquitous. And they can promote ice nucleation and cloud condensation at significantly warmer temperatures. How do these aerosols interact with the hydrologic cycle? Can we measure which particles produce the most efficient rainout? Is there evidence of a “bioprecipitation” feedback?

Bacteria such as Pseudomonas syringae, sea spray as well as mineral dust from the deserts of Asia and Africa play an important role in California precipitation.

Atmospheric rivers are long, narrow bands of extremely high water vapor transport. These storms are by far the most powerful to reach the western US, and they present an ideal opportunity to examine aerosol-precipitation interactions along coastal-inland trajectories in California. Along with colleagues at Portland State, the Center for Western Weather and Water Extremes, Scripps and UC San Diego, I have been analyzing the stable isotope composition of precipitation and water vapor in atmospheric rivers since December 2014. Our most significant findings stem from our analysis of the March 5-7, 2016 atmospheric river in coastal California:

When are storms dominated by rainout vs post-condensation processes such as exchange and evaporation? We address this question by analyzing the temporal evolution of stable isotopes in precipitation during an atmospheric river event. Our results demonstrate, unsurprisingly, that post-condensation processes dominate on the shoulders of precipitation events, while the heaviest storm conditions coincide with peak rainout.

What are the abundance and composition of ice nucleating particles at coastal and inland sites? We compare and contrast results in Bodega Bay and Cazadero, CA and present evidence for a possible warm-INP feedback. Do heavy rains loft bacteria from leaves, further increasing nucleation and rainfall?

Mix, H.T., Reilly, S.P., Martin, A., Cornwell, G. (2019) Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor. Atmosphere, v. 10, p. 86. Download ▸

Martin, A.C., Cornwell, G., Beall, C.M., Cannon, F., Reilly, S.P., Schaap, B., Lucero, D., Creamean, J., Ralph, F.M., Mix, H.T., Prather, K. Contrasting local and long-range-transported warm ice-nucleating particles during an atmospheric river in coastal California, USA, Atmospheric Chemistry and Physics, v. 19, p. 4193-4210. Download ▸