Improving Arctic Cloud Simulations by Representing Aerosol Impacts on Ice Particles
The impacts of aerosols on Arctic “mixed phase” clouds (clouds made of ice and liquid water) are complex and difficult to include in climate models, yet are critical for accurate simulation of cloud and climate changes in the Arctic. Aerosols capable of seeding ice particles (ice nuclei or IN) affect not only cloud formation, but the balance of water and ice content in the clouds and the fallout of ice particles. Models, including the Community Atmosphere Model (CAM5), typically make simple assumptions about aerosol effects on cold clouds and often assume that ice formation depends on temperature and water vapor, without capturing aerosol effects. This typically leads to an overestimation of ice particles. To improve parameterizations of ice nucleation processes, U.S. Department of Energy scientists at Lawrence Livermore National Laboratory and Pacific Northwest National Laboratory have worked together to implement a new physically based aerosol-ice cloud scheme that links the variation of IN number concentration to aerosol properties into the CAM5 and have examined its impacts on Arctic clouds and radiation. This new IN scheme leads to a significant reduction in simulated IN number concentrations at all latitudes, with the greatest effects on clouds at high latitudes and mid-latitude storm-tracks. In the Arctic, the new scheme increases mid-level clouds but decreases low-level clouds. With fewer IN, the cloud liquid amount increases while ice decreases. Since liquid clouds tend to be thicker, the Arctic clouds therefore lead to a net cooling at the top of the atmosphere. This suggests that the new IN scheme would produce less warming in simulated Arctic climate than the older scheme.
Xie, S., X. Liu, C. Zhao, and Y. Zhang. 2013. “Sensitivity of CAM5 Simulated Arctic Clouds and Radiation to Ice Nucleation Parameterization,” Journal of Climate 26, 5981–99. DOI: 10.1175/JCLI-D-12-00517.1