WMOD00434

• Title: The Generation of Secondary Ice Particles in Clouds by Crystal-Crystal Collision
• Date: 11/11/1978
• Weather Modification - Doc Type: Report

NOVEMBER 1978

2175

LARRY VARDIMAN

Cover

'"'

could be detected with this instrument was estimated
to be 50 JLm. '
Since the resolution from the 16 mm film was too
coarse to allow detailed classification of crystal types
and degree of rime, a microscope with an attached
camera was used to determine crystal structure. Ice
crystals were collected in cooled hexane immediately
before and after a sequence of 16 mm film was taken.
This technique for collecting ice crystals in hexane
was suggested by Dr. Charles Knight (personal com-
munication). The hexane prevents the crystals from
m~tamorphosing for short periods of time and in-
creases the contrast in and around a crystal. Ice
crystals were collected and photographed a short time
later. The microscope was equipped with a cold stage
to allow greater investigation of a sample of crystals
before the heat from the microscope lamp changed
the crystals. Fig. 4 shows an example of each of the
five crystal types studied in this manner.
The data obtained during the winter of 1973-74
were collected with the camera system described. In
the previous winter a completely different instrument
was used as a preliminary study to this. Unrimed
plane dendrites were the predominate crystals studied
that first winter and since little data were obtained
for this crystal type the second winter, these data
were included in this study. The instrument shown in
Fig. 5 consisted of a humidified cold chamber with
a hole in the top and a tray of supercooled sugar
solution in the bottom. Mounted about 25 cm below
the upper hole and about 10 cm above the sugar
solution was a 1 mm mesh copper screen. Ice crystals
were allowed to fall through the upper hole, preferably
one at a time, and impinge on the screen. The frag-
ments produced in the collision would filter through
the screen and fall into the sugar solution, growing
. to detectable sizes for counting. The chamber was
humidified to prevent small fragments from sublimating
before reaching the sugar solution.
The data obtained from this instrument were
qualitative because no measure of the impact velocity
was made. Therefore, the camera and fixed plate
system was designed for the following winter. One
advantage the cold chamber had" however, was the
detection of-very small fragments. The qualitative
agreement between the two systems for similar crystals
leads me to believe that most of the fragments produced
by mechanical fracturing are fairly large-large enough
to be detected by the camera system. The photograph
of unrimed plane dendrites in Fig. 4 shows the screen
used in the cold chamber instrument.

b. Determination of the fragment generation function

Given that the change in momentum can be de-
termined for a number of collisions with a fixed plate
and that this change of momentum may be applied
to collisions in a cloud by proper mathematical treat-

l

Opening in Plexigla s
Lid to Permit Ice
Crystal Through

Humidi f ier

Insulated Brass
Wall with Cooling
Coils

Imm

FIG. 5. Diagram of the instrument used to determine the number
of fragments from unrimed plane dendrites.

ment, the number of fragments generated may be
determined as a function of the change in momentum.
Each crystal type has a different relationship between
the number of fragments generated and the change
in momentum due to different likelihoods of frag-
mentation. Fig. 6 shows a plot of the number of
fragments as a fraction of the change in momentum
for each of the five crystal types studied. A least-
squares curve fit was applied to determine the most
likely fragment generation function.
Some interesting facts are evident in Fig. 6.

. For plane dendrites, the greater the degree of
rime, the greater the fragmentation.
. Light-moderate rimed spatial crystals are the
most effective crystals studied for generating
fragments.
. Graupel are surprisingly ineffective in generating
fragments.

4. Numerical modeling

a. Model description

The general solution to Eq. (17) can only be found
if K(t) is known as a function of time. K(t) will vary,

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