New Theories on Ice Formation in Mixed-Phase Clouds from Observations and Climate Model Simulations
DOE scientists, using continuous measurements of Arctic clouds, have developed theories about how ice crystals form in these clouds, a long-standing, unsolved scientific problem. Ice crystals can form in four different ways. It is important to accurately represent their formation in climate models since ice formation impacts the total amount of cloud water, which in turn affects cloud lifetime and the net radiative effect on the earth’s surface. Numerical simulations using traditional ice formation formulations generally “produce” too much ice, resulting in reduced cloud lifetimes. Analyses of cloud water and ice, along with air vertical motion in clouds led to the idea that a substantial fraction of ice formed may be due to “immersion freezing” or the freezing of cloud droplets. When immersion freezing is tested in a cloud model, the behavior and ice content of simulated clouds compared favorably with Atmospheric Radiation Measurement (ARM) data. These improved cloud model simulations will guide development of climate models to accurately represent mixed-phase clouds, improving estimates of future Arctic climate.
de Boer, G., T. Hashino, G.J. Tripoli (2010), Ice Nucleation Through Immersion Freezing in Mixed-Phase Stratiform Clouds: Theory and Numerical Simulations. Atmos. Res., doi: 10.1016/j.atmosres.2009.09.012