Simulating cloud-aerosol interactions in shallow cumuli: Results from the 2007 CHAPS field study

The accurate representation of cloud-aerosol interactions in convective clouds has been a challenge for some time. The Cumulus Humilis Aerosol Processing Study (CHAPS), which was conducted during June 2007 near Oklahoma City, Oklahoma, is a unique dataset that can be used to evaluate simulations with a range of spatial scales. In this study, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was used to simulate cloud-aerosol interactions during CHAPS in both grid-resolved and parameterized convective clouds. The model reproduces the observed trends of higher nitrate volume fractions in cloud droplet residuals compared to volume fraction found in 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 are found to increase cloud droplet number concentration and decrease the droplet effective radius. In addition, as documented using an offline optical code, simulated aerosol optical properties are found to depend on several compensating effects including aerosol water content, size-resolved chemical composition and refractive index of various chemical species within the particles. The simulations clearly show an increase in 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 and can be used to evaluate the regional scale impacts associated with the cumulus transport of aerosol.