WMOD00298

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

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type of field experiment he conducted. Primarily, the continuous

production of airborne ice crystals from blowing snow near a mountain

surface competes with seeding and is an uncontrollable factor (Rogers

and Vali, 1988). Finally, it is worth noting that Davis found that

AgI-NaI aerosols also continuously nucleated fresh ice crystals with

time in the cap clouds. This suggests that the fast condensation-

freezing mechanism measured by Blumenstein et al. (1987) does not occur

in these weakly dynamic (and probably very low water supersaturated)

clouds.

Recently, some careful observations of the evolution of ice in

seeded cumulus clouds have been conducted that are of direct relevance

to the studies reported in this dissertation. In these experiments, the

concentrations of sulfur hexaflouride tracer and ice crystals formed by

AgI-AgCl aerosols released simultaneously into cumulus clouds in North

Dakota have been measured as a function.of time after seeding (Stith et

al., 1990). This permitted estimates to be made of the active fractions

of ice nucleating aerosols versus time and temperature. The time

evolution of ice crystals formed (fractions of aerosols nucleating)

have been shown to crudely agree with predictions based on the results

of DeMott et al. (1983), and a predominance of the contact-freezing

nucleation mode (Stith et al., 1990). No attempt was made to make a

more exact comparison using an appropriate numerical model to account

for aerosol scavenging rates as a function of the observed cloud

droplet size distribution. Also, the possibility of the function of

other nucleation mechanisms was not considered.

Other field studies have produced results which indicate important

distinctions between the nucleating aerosols employed. For example,