WMOD00401

• Contract/Permit #: 2-07-81-V0256
• Title: Precipitation Development in Seeded Clouds Over the Sierra Nevada
• Date: 8/1/1986
• Weather Modification - Doc Type: Report

6
speed and the change in shape and bulk densi ty of the, ice crystal.
Ventilation was obviously of minor importance in the early growth
period of an ice crystal but was daninant in the: growth rate of
large ice crystals and aggregates at -lSoC.
Hindman and Johnson (1972) developed a numerical approximation
for growth of ice crystals by diffusion under conditions of water
saturation and varying temperature. Results from calculations of
crystal growth under these conditions illustrated the evolution of
columns to capped columns, and dendrites to spatial dendrites. The
model was also designed to simulate growth by simultaneous vapor
deposi tion and droplet accretion. The model allowed for
variability in the final mass of water accreted; if the rate of
growth by accretion was greater than that by deposition, the
crystal was assumerl to develop into a spherical graupel particle
after sufficient time. The crystal rimed moderately if the
converse existed. A drawback of this model was that all crystals
were treated alike.

Young (1974) developed a numerical model which extended
treatment of microphysical cloud processes to more than one level.
The model included processes such as warm cloud process, diffusion,
accretion and aggregation. A noteworthy aspect of the model is the
way aggregation was handled. For relatively large differences in
fall veloci ty between the particles, a collection kernal based on
the difference in fall velocity was used. For small differences
in fall velocities, a collision rate was based on a Gaussian

distribution of fall velocities (Basyo, 1971).

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