14 April 2015

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

Science

Cirrus clouds play an important role in regulating the 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 the in-cloud variability of saturation ratio, the homogeneous nucleation can take place only in a small portion of the cloudy area.

Approach

Motivated by these problems, a team of scientists including a 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 the pre-existing ice crystals and the in-cloud variability of supersaturation is significant, and it increases the contribution of heterogeneous ice nucleation to ice crystal number production in cirrus clouds.

Impact

Compared to observations, the work improved the new model in both the ice number concentrations and the probability distributions of ice number concentration simulated.

Summary

Cirrus clouds play an important role in regulating the 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 the 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 the pre-existing ice crystals and the 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.

Contact
Kai Zhang
Pacific Northwest National Laboratory
Acknowledgments

X. Liu and K. Zhang were supported by the Office of Science of US Department of Energy as part of the Earth System Modeling Program. X. Shi would like to acknowledge the support from the National Natural Science Foundation of China (grant no. 41205071). We would like to acknowledge the use of computational resources (ark:/85065/d7wd3xhc) at the NCAR-Wyoming Supercomputing Center provided by the National Science Foundation and the State of Wyoming, and supported by NCAR’s Computational and Information Systems Laboratory. PNNL is a multiprogram laboratory operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830.