Modeling aerosols and their interactions with shallow cumuli during the 2007 CHAPS field study

The Cumulus Humilis Aerosol Processing Study (CHAPS), which was conducted during June 2007 near Oklahoma City, Oklahoma, provides a useful data set for evaluating the ability of models to accurately simulate cloud-aerosol interactions in shallow cumuli. In this study, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was used to simulate aerosols, clouds, and their interactions during CHAPS with 2 km horizontal grid spacing. The model reproduces the observed trends of higher nitrate volume fractions in cloud droplet residuals compared to interstitial non-activated aerosols. Comparing simulations with cloud chemistry turned on and off, we show that nitric acid vapor uptake by cloud droplets explains the higher nitrate content of cloud droplet residuals. The model also reasonably represents the observations of the first aerosol indirect effect where pollutants in the vicinity of Oklahoma City increase cloud droplet number concentration and decrease the droplet effective radius. In addition, as documented using an offline optical code, simulated aerosol optical properties depend on several compensating effects including aerosol water content, size-resolved chemical composition and refractive index of various particle chemical species. The simulations clearly show an increase in simulated absorption and a decrease in single scattering albedo (SSA) within the Oklahoma City plume. This study highlights the ability of regional-scale models to represent some of the important aspects of cloud aerosol interactions associated with fields of short-lived shallow cumuli. Future work involves coupling aerosol chemistry modules including secondary organic aerosols (SOA) to our new shallow cumulus parameterizations and evaluating aerosol-cloud interactions in coarse grid model simulations.