Sources and Microphysical Impacts of Aerosols Observed during an Atmospheric River Event over Northern California

Western states such as California rely on seasonal snowfall for their water resources. In California specifically, the majority of the alpine snowpack is produced by only a few storms per winter season, primarily Atmospheric River (AR) events. ARs are impactful storms that can produce widespread flooding in addition to snowfall, and are notable for their strong zonal and meridional vapor transport. The cloud microphysics and resulting precipitation formation mechanisms of mid-latitude storms such as ARs are strongly influenced by the aerosols that serve as cloud condensation nuclei and ice nucleating particles, but the sources and characteristics of aerosols within ARs have not been widely investigated and remain poorly understood. Using observations collected via the DOE G1 aircraft during the Calwater-2015 field campaign, this study examines the 3-dimensional distribution of aerosols within and around precipitating clouds associated with the passage of a strong AR through Northern California on February 5-8, 2015. In this study, we highlight the different roles of advected versus locally-sourced aerosols within the strongest sectors of the storm. Advected aerosols include marine organic oligomeric particles, sourced from ocean phytoplankton blooms and aged through chemical and cloud processes over the Central Pacific. Our findings illustrate how the vertical distribution of aerosols through the storm environment is strongly influenced by surrounding synoptic and mesoscale conditions, and demonstrate how the availability of advected or locally sourced aerosols within different storm sectors can influence the cloud microphysics and precipitation characteristics of individual storms.