LAUSR

Abstract The source-oriented Weather Research and Forecasting chemistry (SOWC) model is modified to improve mixed-phase cloud parameterization by including aerosol aging processes (external mixture) in cloud activation/nucleation. The improved SOWC model is applied to investigate how aerosol mixing state influences cloud and ice formation and atmospheric optical properties during a winter storm. SOWC tracks 6-dimensional chemical variables (X, Z, Y, Size Bins, Source Types, Species) through an explicit simulation of atmospheric chemistry and physics. A new module is implemented into the enhanced SOWC model to simulate microphysics processes with all source-oriented hydrometeors (cloud, ice, rain, snow and graupel) by using the modified Morrison two-moment microphysics scheme. In this study, all aerosol source types can activate to form cloud droplets based on the Kohler theory, but the dust source type is the only source of ice nuclei (IN). The Goddard shortwave and longwave radiation schemes are modified to use a new geometric optics method to estimate cloud optical properties by considering the chemistry components and the physical shape of each hydrometeor. The improved SOWC model is used to study a winter storm event that occurred on 6 March 2011, along the western coast of the United States. Compared to ground-based observations, SOWC with the modified Morrison microphysics scheme and modified Goddard radiation schemes predicts reasonable rainfall but the onset of precipitation is delayed by 6 h. Contact freezing is the main mechanism for ice nuclei formation over the ocean because of the number of collisions and large radius gaps between cloud droplets and ice nuclei. However, immersion freezing becomes the dominant nucleation process when the storm approached over land due to increased CCN concentrations and smaller radius gaps between cloud droplets and ice nuclei, especially for the internally mixed simulation, as well as the orographic lifting effect. Increasing CCN and IN in the internal mixing experiment results in 1.7% less rainfall and snowfall over the western coast of the United States.