WMOD00443

• Title: An Investigation of Precipitating Ice Crystals from Natural and Seeded Winter Orographic Clouds
• Date: 11/18/1974
• Weather Modification - Doc Type: Report

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Figure 2. Typical Frame from the Photographic
Ice Crystal Replicator

The photographic replicator was
positioned at the highway maintenance area about
one mile east of the summit on Wolf Creek Pass.
The elevation of this location is 10.660 feet.
Care was taken to locate the instrument with
good exposure at the highway camp.

A total of eleven storms have been
analyzed for the winters of 1972-73 and 1973-74.
Data from at least twice this many storms were
collected but have not been included in the final
summary because of instrumental problems or
uncertainties about representativeness. Seven
of the eleven storms analyzed were natural storms
with no seeding and four storms were seeded. Of
the four seeded storms only one is unquestionably
a seeded case. Ice crystal data were available
only for a few hours at the beginning or ending
of seeding for two of the storms and no evidence
was obtained either from research aircraft or
ground ice nucleus counters that artificial ice
nuclei were present over Wolf Creek Pass for a
third storm. However. the storm on February 19.
1974 was obviously seeded and the effects were
clearly evident in the ice crystal characteristics.

3.

ANALYSIS PROCEDURES

The replicator films were analyzed by
projection onto a calibrated screen. Ice crystals
were classified into five categories of crystal
type - plates. columns. dendrites. irregulars.
and graupel. Four categories of rime were used
none. light. moderate. and heavy. In addition
to the crystal type and rime classifications.
each crystal was measured for size to the nearest
100 microns. The maximum dimension was used and
called the crystal diameter. Particles less than
50 microns were not counted as ice crystals
because it was impossible to distinguish a crystal
less than 50 microns from the nap on the black
velvet belt. The entire frame was not always
reduced due to occasional unequal lighting on the
film. The screen was gridded into equivalent
5-mm blocks on the film. A total of 36 blocks
almost filled a frame. and from 10 to 36 of these
blocks were reduced for each frame depending on
the quality of the data.

The following equation was used to
compute concentration in number per liter:

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C ..

n
\ B N S
L W ViK A
i=l

(1)

where:

C concentration of 3rystals (No./liter)
B constant (1000 cm /liter)
N number of crystal analyzed (1 crystal)
S speed of velvet belt(cm/sec)
W = slit width of settling area (em)
V = fall velocity of ice crystal (em/see)
Ki= number of blocks reduced ~blocks)
A area of block reduced (em /block)
n total number of crystals analyzed

The concentration for each frame was
obtained by summing the concentration of each
crystal in the frame. The fall velocity equations
for unrimed plates. columns. and dendrites were
obtained from Brown (1970). No suitable fall
velocity equation exists for irregular crystals
since they are such a mixture of different forms.
Most irregular crystals are fairly compact and
probably have a reasonably high fall velocity;
therefore. the same equation was used for irregulars
as used for capped columns. The graupel equation
comes from Zikmunda and Vali (1972) for their
March 10, 1971 case. This particular equation
was used because the size of graupel at Wolf
Creek Pass best matched this case. Only one
equation was used for graupel because it was
always classified into a heavy-rime category.
However, all other crystals could vary in the
degree of rime. Very little, if any. work has
been done to develop fall velocity equations which
vary with the degree of rime, although it is
evident that greater amounts of rime increase
the fall velocity. Consequently, a qualitative
assessment was made of the fall velocity by riming.
For plates, columns, and dendrites. the fall ~elocity
was increased over the no rime category 10 percent
for light rime. 50 percent for moderate rime. and
100 percent for heavy rime. The fall velocity
for irregulars was only increased 10 percent for
light rime, 30 percent for moderate rime, and
50 percent for heavy rime because these crystals
have fairly high fall velocities even without rime
and should remain below the fall velocity for
graupel which is the extreme. The error in
concentration due to an error in fall velocity is
not serious compared to other problems. If the
fall velocity is in error by 100 percent, the
concentration will be in error by a factor of two.
which is small compared to many other assumptions
and errors.

Data were obtained for all storms at
I-minute intervals. Large, short-term variations
were evident in the concentrations, so a running
mean over a 5-minute interval was used to smooth the
data. In addition to the concentration-with-time
calculations, an average concentration was obtained
for each storm. Since space is limited in this
paper only the average concentration per storm
will be presented.

For comparison of crystal concentrations
with expected cloud-top ice nucleus concentrations,
the cloud-top temperature and the natural ice
nucleus spectrum must be known. The following
method was used to determine cloud top and thus
cloud-top temperature, The special Pagosa Springs
and Durango soundings conducted for the Colorado
River Basin Pilot Project nearest in time to the

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