WMOD00298

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

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also too ambiguous in explaining the difference between condensation-

freezing and immersion-freezing. For this study, condensation-freezing
is distinguished from immersion-freezing by the requirement that ice

crystal formation occurs instantly in response to a given water

supersaturation and temperature, and the lack of requirement for the

activation of a macroscopic cloud droplet. In fact, nucleation that

follows from predicted cloud droplet activation is termed a form of

immersion-freezing. Nucleation due to water supersaturation in excess

of the immersion-freezing fraction is termed condensation-freezing.

These working definitions can be further clarified by considering the

history of a particle in a condensing environment, as is done next.

As the schematic in Fig. 4.1 shows, the four nucleation modes

actually present five different pathways by which ice can form when

heterogeneous ice nuclei are present or are introduced into ice

supersaturated or cloudy air below ooC. Deposition nucleation requires

only that the saturation ratio with respect to ice exceed 1. Then, at

any temperature (Tl) and time (tl), some fraction of the aerosol (F1)

will act to form ice (lowest path in Figure 4.1). At a constant

temperature and ice supersaturation (Si1)' the fraction Fl may vary

with time, expressing the kinetic rate of ice crystal formation.

When saturation with respect to water (Sw1) is approached and

exceeded, some fraction (F2) of the ice nucleating aerosol may hydrate,

form a liquid haze particle, or activate as a solution droplet (middle

pathway in Figure 4.1). This will depend primarily on the chemical

hygroscopic characteristics of the aerosol, and the peak Swl achieved,

At temperatures Tl below OOC, the instantaneous freezing of the

condensing liquid phase is referred to as condensation-freezing