Simulating aerosols entrained into fair weather cumulus during CHAPS

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Description

The Cumulus Humilis Aerosol Processing Study (CHAPS) was conducted during June 2007 and was designed to investigate cloud-aerosol interactions in the vicinity of Oklahoma City, Oklahoma. The primary instrument platform was the U.S. Department of Energy’s Gulfstream-1 (G-1) aircraft, which was configured to make in situ measurements of the chemical and optical properties of aerosols, cloud microphysics, trace gas concentrations, and meteorological variables. The High-Spectral Resolution Lidar (HSRL) was deployed on the NASA King Air B-200 to obtain coincident profiles of aerosol optical properties along the G-1 flight paths. Previous analyses of CHAPS data have indicated that optical and CCN properties associated with anthropogenic emissions entrained into shallow cumulus were different than those sampled outside of the Oklahoma City plume. In this study, we investigate whether simulated cloud-aerosol interactions associated with emissions from a mid-sized city can be adequately represented by state-of-the-science treatments of aerosol processes in the WRF-Chem model. An outer domain that encompasses most of the central United States with a grid spacing of 10 km and an inner domain that encompasses the G-1 flights in the vicinity of Oklahoma City with a grid spacing of 2 km are employed. Simulated aerosol mass, composition, and size are compared with G-1 aircraft data. The volatility basis set approach of representing secondary organic aerosols has been incorporated into WRF-Chem and will be evaluated using the Aerosol Mass Spectrometer measurements on the G-1 aircraft. Simulated vertical profiles of extinction that are affected by uptake of water on aerosols in the vicinity of clouds will be compared with observed profiles obtained from the HSRL. Similar to the previous studies on the CHAPS measurements, analyses of the simulated quantities are conducted for aerosols below cloud, within cloud, and above clouds. Sensitivity simulations are performed with and without cloud-aerosol interactions to determine the impact of simulated aerosols on clouds, cloud-processing on aerosols, and radiative effects over the entire region, and whether those changes are consistent with the aircraft measurements.