WMOD00298

• Title: Quantifying Ice Nucleation by Silver Iodide Aerosols
• Date: 5/1/1990
• State: CO
• Weather Modification - Doc Type: Report

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be expected, the fast mechanism seems to prevail. The slow and fast

nucleation processes in the Blumenstein analysis yielded large
differences in the amount and targeting of seeded snowfall in model
simulations.

2.3 Ice Nuclei Function in Real Clouds

There are few atmospheric seeding tests from which the rates and

mechanisms of nucleation can be deduced. One such study is that of

Davis (1974). He studied ice nucleation effectiveness and rates of ice

crystal formation for combustion-produced AgI aerosols in the CSU

isothermal chamber and in Elk Mountain (Wyoming) cap clouds. The

relatively simple clouds used and the ground access to sampling sites

at different cloud temperatures provided an effective outdoor

laboratory for testing the transfer of cloud chamber results to general

atmospheric clouds. Excellent agreement was found between the ice

nucleation yields (g-l AgI) in the laboratory studies and in the cap

clouds for aerosols produced from AgI-NH I-acetone-water solutions.
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This agreement was realized after assuming that contact-freezing

nucleation was the dominant nucleation mode in both ~he laboratory and

the field clouds, then adjusting the field data (based on the particle

sizes and the cloud droplet size distribution observed) to determine

the yield that would have been realized at any temperature if the

nuclei had resided at that temperature for infinite time. Davis

correctly inferred that contact-freezing was the dominant nucleation

mode for these AgI aerosols in the isothermal cloud chamber. However,

he did not conduct experiments to determine the potential contributions

of other mechanisms. Additionally, there are inherent limits to the