Relative Contributions of Aerosol Properties to Cloud Droplet Number: Adjoint Sensitivity Approach in a GCM

摘要

In this work we study the sensitivity of cloud droplet number concentration to aerosol characteristics in the framework of a Global Circulation Model (GCM) by using the newly developed adjoints of a number of physically based droplet activation parameterizations. The adjoint sensitivities of these activation parameterizations were implemented in the Community Atmospheric Model version 5, which includes interactive aerosols and detailed cloud microphysics. These simulations exhibit clear patterns of high-sensitivity areas to aerosol number concentration coinciding with relatively clean environments, as well as continental and relatively polluted areas showing saturation with respect to aerosol load changes. Furthermore, in order to identify process-level discrepancies between the activation schemes in the absence of feedbacks, aerosol input fields from the GlobalModeling Initiative (GMI) chemical transport model were used to drive off-line comparisons between the sensitivities of the droplet number predicted by these parameterizations against predictions with detailed numerical simulations of the activation process. Important differences were observed particularly in their response to number concentration and hygroscopicity of fine and coarse mode aerosols. Overall, the parameterizations were capable of capturing the response of droplet concentrations to perturbations in aerosol number concentration, and updraft velocity better than to chemical composition variations.