The aerosol indirect effect (AIE) remains one of the largest uncertainties in global climate models. The goal of this project is to systematically quantify the major uncertainties of aerosol indirect effect due to treatment of small-scale cloud dynamics (vertical velocity) which drives aerosol activation, cloud microphysics parameterizations, cloud macrophysics, weather-scale dynamics, and thermodynamic feedbacks in response to anthropogenic aerosol perturbations. The National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 5 (CAM5) will be used in this project, which includes advanced physics for aerosol treatment, aerosol-cloud interactions, cloud microphysics and macrophysics, and moist turbulence. Sensitivity tests of various parameterizations will be performed and inter-compared including:

1. Cloud condensation nuclei (CCN) activation kinetics and organic aerosol hygroscopicity
2. Unified parameterization for ice nucleation, including homogeneous nucleation (in cold cirrus) and heterogeneous nucleation on ice nuclei (IN) with different mechanisms in mixed-phase and cold cirrus clouds and their dependence on aerosol composition and size distribution
3. Treatment for subgrid scale vertical velocities for droplet activation and ice nucleation
4. Treatment of cloud radiation- turbulence interactions, including impacts of entrainment feedbacks on cloud nucleation and condensation.

Coupled model simulations (using Community Earth System Model version 1) with full interactions of atmosphere, ocean, land, and sea ice will be run on multidecadal timescales to investigate impacts of aerosol indirect effect on decadal/multidecadal climate variability through aerosol-cloud-precipitation-climate interactions.

The proposed research will provide important new insights into the impact of the aerosol indirect effect on global climate. This will result in reduced uncertainty in predictions of climate sensitivity to greenhouse gas levels.

The proposed project will enhance the nation's climate modeling infrastructure through improving parameterizations related to the aerosol indirect effect. The project will also contribute to human resource development through the participation of graduate students, including under-represented minorities.