Sensitivity Analysis of Cloud, Precipitation, and Radiation Properties by Changing Aerosols in a WRF Case Study

The Thompson et al (2008) bulk microphysics scheme was recently updated to incorporate aerosols explicitly (Thompson and Eidhammer, 2014). The scheme explicitly nucleates water and ice from their dominant respective nuclei and fully tracks and predicts the number of available aerosols. Using the Weather Research and Forecasting (WRF) model, the scheme was tested in a high resolution (4-km spacing) simulation of a three-day winter storm event over the entire contiguous U.S. A control simulation was run with climatological aerosol conditions and then two sensitivity experiments with very clean and very polluted conditions were used to evaluate various impacts to cloud and precipitation development as well as radiation. As expected, increased aerosols led to larger cloud droplet number concentrations with overall smaller diameters that clearly produced the first aerosol indirect effect of increased cloud albedo (Twomey, 1974). Furthermore, overall warm-rain production decreased with increasing aerosols as predicted by the second indirect effect concerning precipitation formation. However, an increase in snow was found due to water transport and subsequent snow riming mechanisms. Therefore, due to potential snow accumulation differences from changing aerosols, there may be yet another aerosol indirect effect due to changed surface albedo.