Aerosol and Hydrometeor Concentrations during Rain-on-Snow Events of Atmospheric Rivers in Northern California

This study examines the role of aerosols and hydrometeors on rain-on-snow events during atmospheric rivers (AR). Atmospheric rivers are narrow filaments of enhanced water vapor transport from tropics associated with the development of extratropical cyclones, causing hazard flooding as well as serving as water resources, particularly on the mountain regions of the western United States. We define rain-on-snow events if precipitation increases about 0.03 inches per hour, while snow water equivalent (SWE) decreases by any amount. SWE and total precipitation data were collected from a network of observational sites organized by the California Department of Water Resources. Aerosol data was also collected from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network of observational sites.

The past 10 years (2006-2016) synoptic analyses shown by NASA Modern Era Retrospective analysis for Research and Applications 2 (MERRA2) and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-interim suggested that southeastward extension of low pressure combined with colder inland temperature of ARs could reduce rain-on-snow events. Preliminary results showed that rain-on-snow events tended to be associated with the increasing incident of the localized aerosols. It is essential to assess how aerosols are correlated with rain-on-snow of ARs and climate variability (e.g., el nino or la nina). We hypothesize that the rain-on-snow of ARs were modulated by the hydrometeor characteristics associated with warmer temperatures.