Towards a Better Representation of Cloud-Aerosol Interactions in the Community Earth System Model: With Applications to Heterogeneous Nucleation of Cirrus, and Aerosol-Cloud Intera

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Clouds and aerosols play a major role in the Earth's radiative budgets and in the hydrological cycle. It is a challenge problem to realistically represent/simulate clouds and cloud-aerosol interactions in global models of the Earth's climate system because they are very complex and usually much smaller than the typical tens of kilometer sized grid cells in current global models. In order to limit computing costs, the present Community Earth System Model (CESM), which is one of the nation's leading global models, uses a minimalist representation of clouds by computing only their mean particle size and their condensed water mass. These simplifications lead to a vast expansion of tuning parameters whose values are not well defined. As a result, the current CESM cloud model predicts liquid cloud particle sizes that differ greatly from those observed by satellite. The work replaces the CESM liquid cloud model with one that fully tracks the range of particles sizes in the clouds. With full knowledge of the particle sizes much more complete and realistic cloud physics will be implemented in the model, and the number of tunable, poorly defined parameters will be reduced. The new model will also be used to improve the parameterization of physical mechanisms. Another goal of the work is to link aerosols to the ice cloud model, and apply the ice cloud model to determine the role of new ice particle formation, particularly involving organic aerosols, in controlling cloud properties. By utilizing the size resolved liquid and ice models, the study will evaluate the aerosol-cloud climate forcing, one of the most uncertain factors in climate modeling.Cloud properties and aerosol-cloud interactions are among the greatest uncertainties in climate modeling. By improving the physics used to simulate clouds and their interactions with aerosols the uncertainties may be reduced. This project will improve representations of both ice and liquid clouds in CESM, test these changes against data mainly from satellites, and apply the models to well-defined, testable projects involving cloud-aerosol interactions. This project will integrate research and education by advancing discovery and understanding while promoting teaching, training, and learning; broaden participation of underrepresented groups in the Physical Sciences by involving female graduate students; enhance the national infrastructure for research and education by further developing the CESM model; and, provide benefits to society by reducing uncertainty in aerosol radiative climate forcing resulting from a better representation of interactions between clouds and aerosols as well as by improving national tools to address such issues.