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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Percent Percent Percent Percent
Seeded Or No Light Moderate Heavy
Date Nonseeded Rime Rime Rime Rime
2- 6-73 NS 0.0 13.3 29.3 57.4
M 2-10-73 S 0.0 10.6 24.7 64.7
r-...
I 2-23-73 NS 0.0 8.3 33.5 58.1
N
r-... 3- 5-73 S 0.0 35.8 43.7 20.4
0..
..... 3-21-73 NS 0.0 15.4 33.0 51.5
4-18-73 NS 0.0 14.0 40.7 45.3
----------------------------------------------------------------------------
12- 2-73 NS 17.3 40.9 26.9 14.9
'"" 12-13-73 NS 30.5 44.6 19.6 5.3
r-...
I 1-26-7t. S 17.9 37.9 34.9 9.3
M
r-... 2-19-74 S 19.1 34.2 38.1 18.6
0..
..... 3-10-74 NS 23.7 40.2 25.5 10.3
Table 3. SUIlIlIIary of Crystal Rime Analysis
Average Ratio of
Cloud Top Concentration Concentration Crystals
Seeded Or Temp. Of Ice Nuclei Of Ice Crystals to
Date Nonseeded (DC) (No./Liter) (No. /Liter) Ice Nuclei
2- 6-73 NS -17.5 .45 3.14 6.98
M 2-10-73 S -17.3 .35 .58 1.65
r--
I 2-23-73 NS -22.0 .30 1.08 3.60
N
r-... 3- 5-73 S -22.4 - .20
0..
..... 3-21-73 NS -30.0 12.5 6.50 .52
4-18-73 NS -30.0 19.0 3.71 .20
-----------------------------------------------------------------------------

12- 2-73
12-13-73
~ 12-13-73
~ 12-13-73
~ 1-26-74
..... 2-19-74
3-10-74

NS
NS
NS
NS
S
S
NS

-32.0
-25.0
-16.0
-17.0
-20.0
-16.0
-20.0

18.0
2.50
.80
1.20
1.40
.10
.40

4.78
.75
1.50
.55
2.47
4.51
2.20

.27
.30
1.88
.46
1. 70
45.1
5.5

Table 4. SUIlIlIIary of Crystal Concentration Analysis

subjectivity. Recognizing this inherent difficulty
there still appears to be a major difference in the
degree of rime between the t~o years. Table 3
shows a basic difference in the degree of rime
between the years. In 1972-73 zero percent of
the crystals had no rime, but 'up to 30.5 percent
had no rime in 1973-74. Up to 49.2 percent of the
crystals were graupel in 1972-73 while a maximum
of 3.9 percent were graupel in 1973-74. This
corresponds to the apparent difference in rime
from year to year. The incidence of dendrites
reached a maximum of 20.8 percent for nonseeded
storms in 1973-74 while the maximum was only 3.3
percent in 1972-73. The tendency for larger
average crystal sizes in 1973-74 is consistent
with the greater incidence of dendrites. These
various differences may be due to different
sampling from the two years but could also be
indicative of a basic difference in the storm
structure. The general characteristics of a non-
seeded storm at Wolf Creek Pass then, are 50-85
percent irregular crystals with an average size
near 500 microns and fairly heavy rime, frequently
including graupel.

Turning to Table 4 it is evident that the
observed concentration of crystals for natural
storms is always within a factor of seven of the

expected concentration, calculated from cloud-top
temperature and ice nucleus concentrations. There
is a tendency for the observed concentration of
crystals to be less than the expected concentration
at cold cloud-top temperatures and greater than
expected at warm cloud-top temperatures. This
reasonably good agreement should allow for detection
of increased crystal concentrations due to seeding
when present.

4.2 CHARACTERISTICS OF SEEDED STORMS

None of the three seeded storms on
February 10, 1973; March 5, 1973; or January 26,
1974 exhibited any change in ice crystal character-
istics which could be attributed to seeding. As
discussed earlier there is no direct evidence from
other sources that the seeding material was affect-
ing the cloud over Wolf Creek Pass. On February
19, 1974, however, the change in crystal character-
istics is obvious, particularly in Tables 1, 2,
and 4. No natural storm analyzed to date has had
such a preponderance of irregular crystals, such
a low average crystal size, or high concentration
of crystals relative to the expected concentration.
The average size of 202 microns is nearly half of
that for any other storm studied and the observed
concentration is 45 times greater than expected.

471