Strong sensitivity of aerosol concentrations to convective wet scavenging parameterizations in a global model

This study examines the influences of assumptions in convective wet scavenging pa-rameterizations on global climate model simulations of aerosol concentrations and wetdeposition. To facilitate this study, an explicit representation of the uptake of aerosolmass and number into convective cloud droplets and ice crystals by the processes of activation, collisions, freezing and evaporation is introduced into the ECHAM5-HAMmodel. This development replaces the prescribed aerosol cloud-droplet-borne/ice-crystal-borne fractions of the standard model. Relative to the standard model, themore consistent treatment between convective aerosol-cloud microphysical processesyields a reduction of aerosol wet removal in mixed liquid and ice phase convective clouds by at least a factor of two, and the global, annual mean aerosol burdens are in-creased by at least 20 %. Two limiting cases regarding the wet scavenging of entrainedaerosols are considered. In the first case, aerosols entering convective clouds at theirbases are the only aerosols that are scavenged into cloud droplets, and are suscepti-ble to removal by convective precipitation formation. In the second case, aerosols that are entrained into the cloud above the cloud base layer can activate, can collide withexisting cloud droplets and ice crystals, and can subsequently be removed by precip-itation formation. The limiting case that allows aerosols entrained above cloud baseto become cloud-droplet-borne and ice-crystal-borne reduces the annual and globalmean aerosol burdens by 30 % relative to the other limiting case, and yields the closest agreement with global aerosol optical depth retrievals, and black carbon vertical pro-files from aircraft campaigns (changes of about one order of magntiude in the uppertroposphere). Predicted convective cloud droplet number concentrations are doubledin the tropical middle troposphere when aerosols entrained above cloud base are al-lowed to activate. These results show that aerosol concentrations and wet deposition predicted in a global model are strongly sensitive to the assumptions made regardingthe wet scavenging of aerosols in convective clouds.ISSN:1680-7375ISSN:1680-736