Effects of Pre-Existing Ice Crystals on Cirrus Clouds in the Community Atmosphere Model


Cirrus clouds play an important role in regulating Earth’s radiative budget and water vapor distribution in the upper troposphere. Ice crystals in cirrus clouds may form by both homogeneous freezing of solution (aerosol) droplets and heterogeneous ice nucleation on insoluble aerosol particles, called ice nuclei. There are two processes that are currently missing in the ice nucleation parameterization used by the Community Atmosphere Model version 5 (CAM5). First, pre-existing ice particles may deplete available water vapor in the air and prohibit the ice nucleation process. Second, due to in-cloud variability of saturation ratio, the homogeneous nucleation can take place only in a small portion of the cloudy area. Motivated by these problems, a team of scientists, including a U.S. Department of Energy researcher at Pacific Northwest National Laboratory, implemented a new ice nucleation treatment in CAM5. The team found that the impact of considering pre-existing ice crystals and in-cloud variability of supersaturation is significant, and it increases the contribution of heterogeneous ice nucleation to ice crystal number production in cirrus clouds. Compared to observations, the work improved the new model in both the ice number concentrations and the probability distributions of ice number concentration simulated.


Shi, X., X. Liu, and K. Zhang. 2015. “Effects of Pre-Existing Ice Crystals on Cirrus Clouds and Comparison Between Different Ice Nucleation Parameterizations with the Community Atmosphere Model (CAM5),” Atmospheric Chemistry and Physics 15, 1503–20. DOI: 10.5194/acp-15-1503-2015.