Impact of aerosols on precipitation associated with atmospheric rivers: An observational and model-based approach

Aerosols can impact cloud and precipitation processes through their ability to act as cloud condensation nuclei (CCN) and ice nuclei (IN). Although we have made significant progress on understanding the aerosol-cloud-precipitation processes, there still exists a great deal of uncertainty on quantifying the impact of aerosols on precipitation as it is very difficult to separate the individual contributions from the changing meteorology and aerosol conditions. In this study, we combine detailed measurements from the NOAA-led CalWater field campaign over Northern California from February-March 2011 with Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) simulations to quantify the impact of aerosols on precipitation. We focus on two atmospheric river (AR) cases that brought substantial precipitation to Northern California from February 18-19 and March 5-7, 2011. In situ measurements revealed the presence of pollution and dust in cloud and precipitation residues during both of the AR events, which suggests that aerosols may have played an important role in modifying precipitation. The results from our WRF-Chem simulations will help quantitatively understand and separate the role of the varying meteorological and aerosol conditions on the precipitation associated with the ARs.