WMOD00298

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

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The seeding effect on ice crystal concentrations is expected to follow

the estimate of the nucleating activity of the seeding agent, which has

historically been determined for a variety of generation devices by

tests in an isothermal cloud chamber (Grant and Steele, 1966; Garvey,

1975). For most cloud seeding programs, this approach does not

adequately consider the differences between the conditions in the real

cloud and the environmental conditions in the cloud chamber where the

nucleant was characterized. This has been a necessary simplification,

since there is not enough known about the differences in nuclei

response to ambient conditions, transport through warm cloud, or

seeding method. A complete description of these effects is not
available for even one nucleant.

Cloud models can play an important role in answering questions

concerning the consequences of seeding a supercooled cloud with an ice

nucleating aerosol, but model descriotions of nucleation orocesses must

be derived from exoerimental evidence, because adequate descriptive

theories do not exist. An example of an experimental and model

interactive study was demonstrated by the incorporation into computer

simulations by Lamb et a1. (1981) of mathematical descriptions of
secondary ice formation processes derived from laboratory studies of
this phenomenon by Hallett and Mossop (1974). The characteristics of

clouds which form by simple lifting over short time periods (orographic

clouds, cumulus clouds and some stratiform clouds) can be closely

simulated with computer models. However, mathematical parameterizations

or simplifications based on analogy are often used in models in place

of physical laws, in order to circumvent limits of computer power and

cost or when theory and understanding are insufficient or incomplete.