05/12/2017

Improving Modeled Cloud and Aerosol Radiative Effects

Study shows that the width of the size distribution is crucial for simulation of cloud and aerosol radiative effects.

The Science

Accurate representation of cloud microphysics and aerosol-cloud interactions is essential to improving the ability of large scale models to simulate clouds, precipitation and aerosol indirect effects, and much remains to be done. Particularly understudied is the topic of the shape of the cloud droplet size distribution – how water is distributed among cloud droplets of different sizes – and its influence on large scale models. This study uses a global model to test a new representation of what happens when aerosols interact with cloud droplets and modify the droplet size distribution.

The Impact

The study shows that the new method for representing aerosol impacts on the shape of cloud droplet size distribution leads to better agreement of modeled shortwave cloud radiative effects with satellite observations. These results reinforce the importance of properly representing the shape of the cloud droplet size distribution in large-scale models.

Summary

Although the role of the spectral shape of cloud droplet size distribution in determining cloud and precipitation properties has been known, its representation in large-scale models is still in its infancy. This study explores this important topic by implementing into the Community Atmosphere Model Version 5 (CAM5) a set of parameterizations for the effective size of cloud droplets and the conversion of cloud droplets to rain drops that consider both the width and shape of the cloud droplet size distribution. Their results show that shortwave cloud radiative effect is much better simulated with the new cloud parameterizations when compared with the standard scheme in CAM5. Consideration of the width of the size distribution effect can significantly reduce the changes induced by aerosols in the cloud-top effective radius and the liquid water path, especially in the Northern Hemisphere. The influences on longwave cloud radiative forcing and surface precipitation are minimal, however, which may be related to the fact that cloud microphysics and aerosol-cloud interactions have not been considered in deep convection.

Principal Investigator(s)

Yangang Liu
Brookhaven National Laboratory
lyg@bnl.gov

Funding

The authors thank the two anonymous reviewers for valuable comments and suggestions. This work was jointly supported by National Key Research and Development Program of China (2016YFA0601904) and the National Natural Science Foundation of China (41690115, 41572150). He Zhang is supported by the National Natural Science Foundation of China (61432018). Yiran Peng is supported by 973 project 2014CB441302. Yangang Liu is supported by the US Department of Energy’s Atmospheric System Research (ASR) program.

References

X Xie, H Zhang, X Liu, Y Peng, and Y Liu. 2017. “Sensitivity study of cloud parameterizations with relative dispersion in CAM5.1: impacts on aerosol indirect effects.” Atmospheric Chemistry and Physics, 17(9), 5877-5892. [10.5194/acp-17-5877-2017].