WMOD00298

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

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supersaturation with respect to water, and the other, at water

saturation, is much slower (minutes). DeMott et al. (1983) had

previously distinguished these potential mechanisms as condensation to

aqueous embryos followed by freezing, and condensation to droplets

followed by freezing.
A method for generating more efficient condensation-freezing ice

nuclei (AgI-AgC1-xNaCl) has been described by Feng and Finnegan (1985;

1989) and has come into use in the operational weather modification

community over the past few years. Results of experiments in the

dynamic cloud chamber (DeMott, 1988) show that the activity of a

po1ydisperse aerosol of this composition is insensitive to large

changes in ambient droplet concentrations for equivalent injections

into expansion-formed clouds; i.e., the nuclei function predominately

by condensation-freezing or at least form cloudjhaze drops before

droplet-nuclei contact has time to occur. Below water saturation, Feng

et a1. (1989) found these aerosols to be several times to 10 times as

efficient in apparently nucleating ice by deposition, compared to AgI-

AgC1 aerosols.

Preliminary experiments in the CSU dynamic cloud chamber have

also suggested that nuclei aerosol injection prior to warm cloud

condensation influences both the mode and rate of formation of ice by

AgI-AgCl aerosols (DeMott et a1., 1984a). AgI-AgCl-4NaCl aerosols

showed no apparent sensitivity to their time history in cloudy air.

2.2 Descriptions of Ice Nuclei Behavior in Cloud Models

Nucleation mechanisms, rates and effectivities can directly and

indirectly influence the nature of numerical cloud model results. Lamb