The Design Document page provides a description of the algorithms, implementation and planned testing including unit, verification, validation and performance testing. Please read  Step 1.3 Performance Expectations that explains feature documentation requirements from the performance group point of view.

In the table below, 4.Equ means Equations and Algorithms, 5.Ver means Verification, 6.Perf - Performance, 7. Val - Validation,   - completed, - in progress, - not done

The objective of this task is to improve the compatibility between MAM aerosols (in the atmospheric component) and treatment of light-absorbing particles deposited to snow and sea ice (in land and sea ice components), to fix recently identified bugs in calculating snow grain sizes in the released CLM (in ALM as well), and to provide new input files for snow and ice optics are needed for running the model. 

1. modify the atmospheric aerosol deposition code to separately treat BC mixed internally within hydrometeors and externally to hydrometeors, and to partition dust into four size bins instead of two

2. modify the SNICAR code to take size-dependent BC optical properties (i.e., depending on snow grain size and BC particle size) and provide a new lookup table for the optical properties

3. modify the code in ALM for calculating snow grain size to fix an identified bug

4. Do the same to the sea ice component to take new size-dependent BC optical properties    

Flanner, M. G., Liu, X., Zhou, C., Penner, J. E., and Jiao, C.: Enhanced solar energy absorption by internally-mixed black carbon in snow grains, Atmos. Chem. Phys., 12, 4699-4721, doi:10.5194/acp-12-4699-2012, 2012.

Liu, X., Easter, R. C., Ghan, S. J., Zaveri, R., Rasch, P., Shi, X., Lamarque, J.-F., Gettelman, A., Morrison, H., Vitt, F., Conley, A., Park, S., Neale, R., Hannay, C., Ekman, A. M. L., Hess, P., Mahowald, N., Collins, W., Iacono, M. J., Bretherton, C. S., Flanner, M. G., and Mitchell, D.: Toward a minimal representation of aerosols in climate models: description and evaluation in the Community Atmosphere Model CAM5, Geosci. Model Dev., 5, 709-739, doi:10.5194/gmd-5-709-2012, 2012.

Follow an initial implementation of most code changes to a developer version of CESM1.2 by Mark Flanner. The implementation has already been completed, but not in the B4B style, which will be modified soon.

We will run tests to verify that the B4B with default model is maintained and the new implementation is consistent with that in the older version of CESM1.2. 

Results of the ACME model with this new implementation will be validated against those in Flanner et al. (2012). Along with Polar Mods and other modifications in Aerosol/Clouds, BC-in-snow concentration and forcing will be evaluated against site measurements in field campaigns, like we did in Qian et al. (2014).

Qian Y., H. Wang, R. Zhang, M.G. Flanner, and P.J. Rasch, 2014: A sensitivity study on modeling black carbon in snow and its radiative forcing over the Arctic and Northern China. Environmental Research Letters, 9:064001.DOI:10.1088/1748-9326/9/6/064001.

The new implementation is not expected to have significant impact on the computational cost/performance according to the implementation in an older version of CESM, but we will work with the performance team to get the new implementation in ACME fully tested.