WMOD00298

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

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et al. (1981), developed a model to study dynamic seeding effects in

deep convection over Florida. This model predicted that glaciation

ultimately occurred due to natural secondary processes (ice

multiplication), but seeding initiated these mechanisms earlier. In

seeded cases the timing of latent heat release depended on the

mechanism of action of the nuclei (i.e., faster for deposition, slower

for contact freezing). Orville et al. (1984) found unexpected dynamic

effects in AgI seeding simulations of stratus clouds. This clearly

related to the presumed nucleation ativity and rates of ice crystal

formation (instantaneous). Orville et al. noted the potential

inadequacy of the nucleation scheme employed and its importance in the
quantitative results.

Numerical cloud models have been used to simulate the effects of

various seeding techniques in different types of clouds. The ice

nucleation schemes employed by the models vary widely. They are usually

based on a combination of theory and laboratory results that are not

necessarily compatible. Due to the lack of information, the schemes are

seldom nuclei-specific. Young (1974a) included both deposition and

contact nucleation (via Brownian and phoretic transport processes) in a

detailed microphysical cloud parcel model. Only contact-freezing
nucleation was used in the published seeding simulations (Young,
1974c), and the nucleus effectivity spectrum employed was not
representative of most that have been measured. Hsie et al. (1980)

simulated both contact-freezing and deposition nucleation in a study of

seeding effects in continental cumulus clouds using a 2-D time-

dependent model. The nucleation mechanisms were applied to the

laboratory-measured effectivity spectra of a nucleus for which the