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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crystal data were plotted and cloud top identified
at or slightly above the point where the dew
point trace broke sharply to colder temperatures.
About fifty percent of the soundings exhibit this
ideal behavior and make cloud-top determination
easy. However, on other days when the soundings
did not show such ideal behavior and the dew
point trace slowly separated from the temperature
trace, the 80 percent relative humidity point was
used as an estimate of cloud top. In addition,
if slight irregularities in either the temperature
or dew point trace occurred at or near the 80
percent point, this level was identified as cloud
top. Errors of a factor of ten in ice nucleus
concentrations can result from a four or five
degree centigrade error in cloud-top temperature.

An average ice nucleus spectrum was
obtained by drawing a straight line of best fit
through the ice nucleus data obtained from project
ice nucleus counters on semilog graph paper. The
limited number of ice nucleus counts and the
great amount of scatter in the data allow for
large errors to be made. It is conceivable that
errors as great as a factor of ten in the con-
centration of ice nuclei are possible for certain
temperatures.

Assuming that the ice nucleus concen-
tration is uniform with height, one can obtain
an estimate of cloud-top ice crystal concentrations
by "picking off" the concentration corresponding
to cloud-top temperature. This final estimate

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of ice crystal concentration was derived from a
series of assumptions, but the results discussed
in the next section show surprisingly good agree-
ment.

4.

RESULTS

Tables 1 through 4 contain a sunnna.ry
of the analyses performed on the eleven storms
of 1972-73 and 1973-74. In Table 4 December 13,
1973 is entered three times because at various
periods in this long-lived storm, the cloud top
changed sufficiently that a single average concen-
tration was meaningless.

4.1

CHARACTERISTICS OF NATURAL STORMS

In general, natural storms have a
crystal size distribution which gives a mean size
of about 500 microns, although the average size
ranges from 376 microns to 809 microns. The mode
is typically 200 microns with bhe distribution
decreasing rapidly to zero toward larger sizes
and having very few crystals larger than 2000
microns. The crystal type is predominately irregular
although graupel were very frequent in the storms
of 1972-73 whereas dendrites w~re quite frequent
in 1973-74. Special care was taken to ensure that
the analysis was comparable from year to year
although the classification into crystal types
and degrees of rime was somewhat subjective. It
is possible that these two characteristics may
change slightly from year to year due to this

Seeded Or Percent Percent
Date Nonseeded plates Columns
2- 6-73 NS 0.9 0.4
C'"l 2-10-73 S 2.2 0.7
" 2-23-73 NS 0.7 0.6
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N 3- 5-73 S 1.4 0.6
"
'" 3-21-73 NS 2.1 1.4
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4-18-73 NS 1.4 0.8

Percent Percent Percent
Dendrites Irregulars Graupe1
2.2 53.8 42.7
7.7 40.2 49.2
3.3 50.5 45.0
5.8 77 .5 14.8
1.9 64.7 29.9
1.0 61.2 35.6

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12- 2-73 NS 13.9 2.0 20.8 63.3 0.0
"'" 12-13-73 NS 1.8 0.1 13.2 84.9 0.0
" 1-26-74 S 6.2 1.1 32.1 60.2 0.4
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C'"l 2-19-74 S 0.1 0.4 0.0 95.6
" 3.9
'" 3-10-74 NS 11.9 0.4 6.8 80.9 0.0
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Table 1. Sunnnary of Crystal Type Analysis
Average Standard
Seeded Or Diameter Deviation
Date Nonseeded (Microns) (Microns)
2- 6-73 NS 502 500
C'"l 2-10-73 S 517 500
" 2-23-73 NS 376 565
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N 3- 5-73 S 410 384
"
'" 3-21-73 NS 506 407
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4-18-73 NS 427 349

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12- 2-73 NS 809 481
"'" 12-13-73 NS 733 635
" 1-26-74 646
J S 895
C'"l 2-19-74 156
" s 202
'" 3-10-74 536
..-l NS 529
Table 2. Summary of Crystal Size Analysis
470

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