The Impact of a New Treatment of Parameterized Convection on Atmospheric Chemistry Associated with Measurements Made During the Two Column Aerosol Project (TCAP)

Adequately simulating the vertical distribution of aerosols and their effect on the radiation budget is important since it affects how the meteorological conditions respond to aerosol radiative effects.  In this study, we compare simulated profiles of aerosol optical properties obtained from the WRF-Chem model with those obtained from both in situ and remote sensing measurements during the Department of Energy’s (DOE) Two-Column Aerosol Project (TCAP).  TCAP was designed to investigate changes in aerosol mixing state, CCN concentration, aerosol radiative forcing, and cloud-aerosol interactions in two atmospheric columns: one over Cape Cod, Massachusetts and another located approximately 200 km to the east over the Atlantic Ocean.  Since a large fraction of aerosols in this region are transported over large distances, they are likely influenced by cloud processing.  Previous versions of WRF-Chem only accounted for cloud-aerosol interactions in resolved clouds, implying grid spacings of ~10 km or less were needed to adequately account for cloud-aerosol interactions.  We describe a new treatment of cloud effects on aerosol and trace gases within parameterized shallow and deep convection, and aerosol effects on cloud droplet number recently implemented in the WRF-Chem that can be used to better understand the aerosol lifecycle over regional to synoptic scales.  The modifications to WRF-Chem include the treatment of the cloud droplet number mixing ratio, key cloud microphysical and macrophysical parameters (including the updraft fractional area, updraft and downdraft mass fluxes, and entrainment) averaged over the population of shallow clouds or a single deep convective cloud, as well as vertical transport, activation/resuspension, aqueous chemistry, and wet removal of aerosol and trace gases in warm clouds.  The aircraft and surface measurements are used to evaluate its performance and assess the sensitivity to cloud processing, including the impact on aerosols associated with biogenic, biomass burning and anthropogenic sources.  We found that the parameterized subgrid scale convective clouds modulated the concentrations of aerosols aloft, but it did not significantly change the overall altitude and depth of the layers observed in the TCAP sampling domain.  However, we will also investigate the impact of parameterized clouds over other regions closer to sources of primary particulate and precursor trace gases emissions.