WMOD00298

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

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mechanisms were not known. The relative contribution to ice crystal

formation by each mechanism was based on the theoretical work of Cooper

(1974) which has not been directly verified. More common and simplistic

schemes (e.g., Plooster and Fukuta, 1975) use the laboratory-measured

effectivity spectra to specify the number of ice crystals formed, with

no regard to mechanism or a consideration of the potential time

required for nucleation. None of the above mentioned models explicitly

include condensation-freezing or consider differences in nuclei

response with thermodynamic path.

Blumenstein et al.(1987) developed time and temperature dependent

empirical expressions from experimental data obtained in the CSU

isothermal cloud chamber and applied these in Rauber's (l98l) two

dimensional orographic cloud model. Her laboratory study demonstrated

slow and fast (rates) condensation-freezing nucleation processes by

AgI-NaI aerosols. Near water saturation, the freezing of droplets grown

on aerosols determined the rate at which ice crystals formed in the

isothermal chamber. This rate was expressed as a first order rate law

with a temperature dependent rate coefficient. The process was slow (up

to 100 minutes for completion), but the rate coefficient increased with

colder temperatures. In contrast, for water supersaturations, the

values of which were not defined, the nucleation of ice proceeded very

rapidly (within seconds), such that crystal diffusion growth and

crystal sedimentation rates determined the rate of collection of ice

crystals. Also, nucleation activity increased in the supersaturated

case. DeMott (1988) has shown that, at least for cloud parcels

undergoing moderate expansion rates (2.5 m s-l equivalent updraft) in

which sustained supersaturation with respect to water might reasonably