23 January 2013

Climate Impacts of Ice Nucleation

Science

Researchers, including a DOE scientist working at Pacific Northwest National Laboratory, used the Community Atmospheric Model version 5 (CAM5) to understand the effect of dust aerosols on upper tropospheric cirrus clouds through heterogeneous ice nucleation. Although very scarce, heterogeneous ice nuclei could significantly impact the ice crystal number concentration compared to pure homogeneous freezing by initiating ice nucleation earlier, depleting available water vapor and ultimately hindering the occurrence of homogeneous freezing. Using two parameterizations that consider homogeneous and heterogeneous nucleation and the competition between the two mechanisms in cirrus clouds, the team found heterogeneous nucleation on dust aerosol reduces the occurrence frequency of homogeneous nucleation and thus the ice crystal number concentration in the northern hemisphere. Global and annual mean net cloud forcing ranges from −0.24 ± 0.28 W m-2 to -0.40±0.20 W m-2. When compared to in situ aircraft data obtained in the northern hemisphere, research suggests that homogeneous ice nucleation may play an important role in the ice nucleation at these regions with temperatures of 205-230 K.

Impact

These results highlight the importance of quantifying the number concentrations and properties of heterogeneous ice nuclei in the upper troposphere.

Summary

Researchers, including a DOE scientist working at Pacific Northwest National Laboratory, used the Community Atmospheric Model version 5 (CAM5) to understand the effect of dust aerosols on upper tropospheric cirrus clouds through heterogeneous ice nucleation. Although very scarce, heterogeneous ice nuclei could significantly impact the ice crystal number concentration compared to pure homogeneous freezing by initiating ice nucleation earlier, depleting available water vapor and ultimately hindering the occurrence of homogeneous freezing. Using two parameterizations that consider homogeneous and heterogeneous nucleation and the competition between the two mechanisms in cirrus clouds, the team found heterogeneous nucleation on dust aerosol reduces the occurrence frequency of homogeneous nucleation and thus the ice crystal number concentration in the northern hemisphere. Global and annual mean net cloud forcing ranges from −0.24 ± 0.28 W m-2 to -0.40±0.20 W m-2. When compared to in situ aircraft data obtained in the northern hemisphere, research suggests that homogeneous ice nucleation may play an important role in the ice nucleation at these regions with temperatures of 205-230 K. These results highlight the importance of quantifying the number concentrations and properties of heterogeneous ice nuclei in the upper troposphere.

Contact
Xiaohong Liu
Publications
Liu, X, X Shi, K Zhang, EJ Jensen, A Gettelman, D Barahona, A Nenes, and P Lawson.  2012.  "Sensitivity Studies of Dust Ice Nuclei Effect on Cirrus Clouds with the Community Atmosphere Model CAM5."  Atmospheric Chemistry and Physics 12061-12079, doi:10.5194/acp-12-12061-2012.
Acknowledgments

Support for X. Liu was provided from the US Department of Energy (DOE), Office of Science, Atmospheric System Research (ASR) and Decadal and Regional Climate Prediction using Earth System Models (EaSM) Programs. Support for Donifan Barahona was provided by the NASA Modeling, Analysis and Prediction Program.