Simulating Aerosol Transport to Remote Regions with the Community Atmosphere Model
Global models are especially challenged to simulate pollution aerosols, including the dark aerosol black carbon (BC), in the remote Arctic, far from the BC source regions. Models typically greatly underestimate BC and fail to simulate the peak values observed in springtime, when aerosols deposited on snow enhance snow melting rates. To improve simulation of Arctic aerosols in the Community Atmosphere Model (CAM5), U.S. Department of Energy scientists at Pacific Northwest National Laboratory improved processes associated with aerosol chemical aging that affects their uptake by cloud water, wet removal, and transport by convective clouds, all key to determining the amount of aerosols reaching remote regions. The team created a new scheme that better synthesized aerosol transport and removal by convective clouds for CAM5. An explicit treatment of BC aging with slower aging assumptions produced a 30-fold increase in the Arctic winter BC burden. The new model was evaluated using surface and aircraft measurements. With the improvements, the Arctic BC burden has a 10-fold increase in the winter months and a 5-fold increase in the summer, resulting in a better simulation of the BC seasonal cycle. The modifications also produce much better aerosol optical depth when compared to multiyear surface-based retrievals of aerosol optical depths, both globally and regionally. The improved aerosol distributions also improved aspects of the CAM5 climate simulation, including global cloud water amount and cloud radiative forcing. Overall, the model aerosol process improvements make CAM5 a better tool to study the role of aerosols in Earth’s climate system.
Wang, H., R. C. Easter, P. J. Rasch, M. Wang, X. Liu, S. J. Ghan, Y. Qian, J.-H. Yoon, P. L. Ma, and V. Velu. 2013. “Sensitivity of Remote Aerosol Distributions to Representation of Cloud–Aerosol Interactions in a Global Climate Model,” Geoscientific Model Development 6, 765–82. DOI: 10.5194/gmd-6-765-2013.