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Aerosol indirect effects in climate models strongly depend on therepresentation of the aerosol activation process. In this study, we assessthe process-level differences across activation parameterizations thatcontribute to droplet number uncertainty by using the adjoints of theAbdul-Razzak and Ghan (2000) and Fountoukis and Nenes (2005) droplet activation parameterizationsin the framework of the Community Atmospheric Model version 5.1 (CAM5.1). Theadjoint sensitivities of Nd to relevant input parameters are usedto (i) unravel the spatially resolved contribution of aerosol number, mass,and chemical composition to changes in Nd between present-day andpre-industrial simulations and (ii) identify the key variables responsible forthe differences in Nd fields and aerosol indirect effectestimates when different activation schemes are used within the same modelingframework. The sensitivities are computed online at minimal computationalcost. Changes in aerosol number and aerosol mass concentrations were found tocontribute to Nd differences much more strongly than chemicalcomposition effects. The main sources of discrepancy between the activationparameterizations considered were the treatment of the water uptake by coarsemode particles, and the sensitivity of the parameterized Ndaccumulation mode aerosol geometric mean diameter. These two factors explainthe different predictions of Nd over land and over oceans whenthese parameterizations are employed. Discrepancies in the sensitivity toaerosol size are responsible for an exaggerated response to aerosol volumechanges over heavily polluted regions. Because these regions are collocatedwith areas of deep clouds, their impact on shortwave cloud forcing isamplified through liquid water path changes. The same framework is alsoutilized to efficiently explore droplet number uncertainty attributable tohygroscopicity parameter of organic aerosol (primary and secondary).Comparisons between the parameterization-derived sensitivities of dropletnumber against predictions with detailed numerical simulations of theactivation process were performed to validate the physical consistency of theadjoint sensitivities

Understanding the contributions of aerosol properties and parameterization discrepancies to droplet number variability in a global climate model