CLOUD SCAVENGING IS THE DOMINANT REMOVAL PROCESS FOR A WHOLE SUITE OF AEROSOLS BUT MODEL PARAMETERIZATIONS OF THIS PROCESS ARE HIGHLY UNCERTAIN SUBSTANTIALLY CONTRIBUTING TO LARGE UNCERTAINTIES IN THE SIMULATED LOADINGS AND RADIATIVE FORCING OF AEROSOLS. IPCC AR5 HAS IDENTIFIED THE IMPROVEMENT OF WET DEPOSITION AND SCAVENGING PARAMETERIZATIONS IN LARGE-SCALE MODELS AS A PRIORITY. THE CURRENT WET SCAVENGING PARAMETERIZATION IN THE NASA GEOS-5 MODEL IS NOT EXPLICITLY LINKED TO CLOUD DROPLET AND ICE CRYSTAL NUCLEATION. RECENT DEVELOPMENT OF TWO-MOMENT CLOUD MICROPHYSICS IN GEOS-5 ALLOWS TREATING CLOUD SCAVENGING OF AEROSOLS MORE PHYSICALLY WITH SELF- CONSISTENCY. WE PROPOSE A 4-YEAR PROJECT TO DEVELOP MORE PHYSICALLY-BASED PARAMETERIZATIONS OF WET SCAVENGING AND STUDY THE IMPACT OF THEIR UNCERTAINTIES ON AEROSOL DIRECT AND INDIRECT EFFECTS IN GEOS-5. OUR RESEARCH OBJECTIVES ARE: (1). DEVELOP A MORE PHYSICALLY DETAILED SIZE-DEPENDENT BELOW-CLOUD SCAVENGING PARAMETERIZATION BY RAIN AND SNOW IN THE GEOS-5 MODEL (2). DEVELOP MORE PHYSICALLY BASED SIZE-DEPENDENT PARAMETERIZATIONS OF IN-CLOUD (NUCLEATION AND IMPACTION) SCAVENGING OF AEROSOLS IN THE FRAMEWORK OF AEROSOL-CLOUD INTERACTIONS (GOCART BULK AEROSOL OR MODAL AEROSOL MODULE AEROSOL MICROPHYSICS COUPLED WITH TWO-MOMENT CLOUD MICROPHYSICS) IN GEOS-5. (3). ASSESS THE IMPACTS OF THE NEW AEROSOL SCAVENGING PARAMETERIZATIONS ON THE GEOS-5 SIMULATED AEROSOL MASS AND NUMBER CONCENTRATIONS SIZE DISTRIBUTION DEPOSITION FLUXES AND AEROSOL OPTICAL DEPTHS AND EVALUATE WITH SURFACE IN SITU AND SATELLITE OBSERVATIONS. QUANTIFY THE UNCERTAINTIES IN THOSE PARAMETERIZATIONS AND THEIR IMPACTS ON GEOS-5 AEROSOL SIMULATIONS. (4). ASSESS THE IMPACTS OF THE NEW AEROSOL SCAVENGING PARAMETERIZATIONS ON CLOUD PROPERTIES AND PRECIPITATION AND EVALUATE WITH SATELLITE AND SURFACE OBSERVATIONS. EXAMINE THE INFLUENCE OF UNCERTAINTIES IN THOSE PARAMETERIZATIONS ON AEROSOL DIRECT AND INDIRECT EFFECTS UPON THE CLIMATE SYSTEM (CLOUD AND PRECIPITATION). THIS PROJECT WILL RESULT IN MORE PHYSICALLY-BASED PARAMETERIZATIONS OF AEROSOL SCAVENGING BY RAIN AND SNOW IN GEOS-5 AND WILL IMPROVE CONFIDENCE IN OUR ESTIMATES OF AEROSOL BURDEN LIFETIME AND DIRECT AND INDIRECT EFFECTS. THIS PROPOSAL DIRECTLY RESPONDS TO THE RESEARCH THEME "CONSTITUENTS IN THE CLIMATE SYSTEM" IN THE SOLICITATION BY IMPROVING OUR UNDERSTANDING OF THE PROCESS OF CLOUD SCAVENGING OF AEROSOLS AND ITS REPRESENTATION IN THE NASA GEOS-5 MODEL. THIS RESEARCH ADDRESSES NASA'S STRATEGIC GOAL 3A1 TO "UNDERSTAND AND IMPROVE PREDICTIVE CAPABILITY FOR CHANGES IN THE OZONE LAYER CLIMATE FORCING AND AIR QUALITY ASSOCIATED WITH CHANGES IN ATMOSPHERIC COMPOSITION."