Predicting the Impacts of Tiny Ice Particles on Climate
Our knowledge of cloud and precipitation formation processes remains incomplete, particularly precipitation by ice containing clouds. In clouds warmer than -36 °C, ice must first form on tiny particles termed ice nuclei. Combining observations from field studies over a 14-year period from a variety of locations around the globe, DOE-funded scientists have shown that the concentrations of ice nuclei in mixed-phase clouds, i.e., clouds that contain both water and ice, can be related to temperature and the number of particles larger than 0.5 micron in diameter. This new understanding reduces the unexplained variability in ice nuclei concentrations at a given temperature from ~103 to less than a factor of 10. The remaining variability is apparently due to variations in aerosol chemical composition or other factors. When this updated formulation was tested in a global climate model, it strongly altered cloud water and icy water distributions compared to the currently used formulation that depends solely on cloud temperature. The updated formulation also indicates that each order of magnitude increase in ice nuclei concentration results in an increase of ~1 W/m2 in the global net cloud radiative forcing (heating of about 0.01º C/day). This result demonstrates the strong sensitivity of climate projections to assumptions regarding the initiation of cloud glaciation.
“Predicting global atmospheric ice nuclei distributions and their impacts on climate”, P. J. DeMott, A. J. Prenni, X. Liu, S. M. Kreidenweis, M. D. Petters, C. H. Twohy, M. S. Richardson, T. Eidhammer, and D. C. Rogers; PNAS doi:10.1073/pnas.0910818107.