Comparing Model Simulations of Arctic Mixed-Phase Clouds: Importance of Ice Size Distribution

Summary

To improve understanding and model representation of processes in mixed-phase Arctic clouds, a team of researchers, led by U.S. Department of Energy scientists at Pacific Northwest National Laboratory, analyzed simulations of these clouds in 11 different high-resolution, large-eddy simulation (LES) models. Using simulations guided by observations from the Indirect and Semi-Direct Aerosol Campaign (ISDAC), they explored the processes that controlled cloud structure and evolution in the numerical simulations. In contrast to previous intercomparison studies, all 11 numerical models used the same ice particle properties and a common radiation parameterization. This constrained setup exposed the importance of ice particle size distributions (PSDs) in influencing cloud evolution in the simulations.

Numerical models use two different approaches (bin or bulk) to represent ice PSDs. In the more accurate, but computationally more expensive bin approach, the models predict how the number of particles within a given size range (or bin) changes as the cloud evolves. This approach results in an explicit size distribution that can be used to calculate variables such as ice water path, particle fall speeds, and cloud mass. In the computationally cheaper bulk approach, which is the method used in large-scale climate models, a fixed shape is assumed for PSD and the models predict higher-order moments of the distribution to calculate the necessary cloud variables.

In this study, researchers found a clear separation in liquid water path (LWP) and ice water path (IWP) predicted by models with bin and bulk microphysical treatments. This difference was attributed primarily to the assumed shape of the ice PSD used in bulk schemes. Compared to the bin schemes that explicitly predict PSD, bulk schemes assuming exponential ice PSD underestimate ice growth by vapor deposition and overestimate mass-weighted fall speed leading to an under-prediction of IWP by a factor of two in the considered case. Sensitivity tests indicated LWP and IWP are much closer to the bin model simulations when a modified shape factor, which is similar to that predicted by the bin model simulation, is used in the bulk scheme. These results demonstrate the importance of ice PSD representation in determining liquid and ice partitioning and the longevity of mixed-phase clouds. The authors suggest that future work to improve modeling of mixed-phase clouds in climate models should focus on methods for predicting the shape and width of ice PSD for use in bulk schemes.

References

Ovchinnikov, M., A. S. Ackerman, A. Avramov, A. Cheng, J. Fan, A. M. Fridlind, S. Ghan, J. Harrington, C. Hoose, A. Korolev, G. M. McFarquhar, H. Morrison, M. Paukert, J. Savre, B. J. Shipway, M. D. Shupe, A. Solomon, and K. Sulia. 2014. “Intercomparison of Large-Eddy Simulations of Arctic Mixed-Phase Clouds: Importance of Ice Size Distribution Assumptions,” Journal of Advances in Modeling Earth Systems 6(1), 223-48. DOI:10.1002/2013MS000282.