WMOD00580

• Project Name: Sierra Cooperative Pilot Project
• Title: Supercooled Liquid Water and Ice Crystal Distributions Within Sierra Nevada Winter Storms
• Date: 11/11/1983
• Weather Modification - Doc Type: Report

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NOVEMBER 1983 . I HEGGLI

TABLE 2. Number of days each echo type and combination were
flown for the two-year period of st~dy (1978-79, 1979-80) and the
percentage frequency of occurrence lof the echo types from five winter
field seasons (1976-77 to 1981-8~2, not including 1980-81 when
there were no aircraft or radar op~rations),

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Area-ride Banded Cellular
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AW Q EB MB CT CI C2
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1978-79 3 6 5 3 I 10 6
1979-80 3 2 3 7 2 5 3
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Total 221 13 24
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Five-year climatological I
percentage frequency I
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of occurrence 11.2 q.8 7.5 11.0 4.2 25,2 11.5
Total 3215 15.2 36,7

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ability. A ratio of the two triableS is used because it
is a measure of the liquid Iwater available in a cloud
to each natural ice crystal present. The L WC/ICC ratio
is then the amount of liqJid water available to each
ice crystal if all the watet in a given volume were
equally distributed among I the ice crystals present. A
new ice crystal introduced into such an environment
by seeding would have approximately that amount of
water available for its groWth.
For example, a cloud ~hich exhibits a LWC/ICC
of 10 f.Lg per crystal has 1 p f.Lg of water available to
each ice crystal for its groWth, or 10 f.Lg available for
each new crystal that is in~roduced. The mass of ice
crystals has been related to !size by many investigators.
According to Nakaya . and Terrada (1935), a 10 f.Lg
plane dendrite has a diameter of about 1.5 mm and
a 10 f.Lg spatial dendrite has a diameter exceeding 1
mm. A 10 f.Lg needle could I exceed several millimeters
in length. Therefore, if 10 I f.Lg of water in such a hy-
pothetical cloud were avaihlble to each new ice crystal,
, they would grow to sufficie~t size to form precipitation.
If it were desired to significantly increase the ICC
in a cloud by seeding, the: liquid water available per
crystal would have to be di~tributed over the total ICC
which includes both the old and new crystals. For in-
stance, ifthe ICC were to bb doubled, a rough estimate
of the water available to each new crystal would be
half of the L WC/ICC ratio. Better estimates would
consider the sizes and habits of the crystal population
and stochastic processes. I
The LWC/ICC ratio could be the same in a cloud
that has a large LWC andi a large ICC or in a cloud
that has a small L WC and a small ICe. Such a measure
of seedability, then, is notl a measure of the amount
of precipitation that would be expected from a cloud,
but rather, a measure, of the susceptibility of such a
cloud to seeding. I
A few difficulties are encountered when attempting
to use the ratio of L wc tol ICe. When no ice crystals
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ET AL,

1879

are observed, the ratio is undefined. These instances
accountt';d for less than 3% of the total number of in
cloud observations. We have removed such cases from
our samples because of the mathematical intractability
of undefined quantities. In some of these cases the
cloud was highly seedable with abundant L WC and
no ice crystals. In others the cloud had little L WC,
but again no ice crystals. These two separate conditions
cannot be distinguished. A second problem in using
the ratio is the minimum observable values of L WC
and ICe. This problem would be no different in degree
from considering the variables separately. However,
the effect is distributed when a ratio is used rather than
being fixed at a minimum cut-off value as it would be
when considering a single variable.

5. Results of the climatological study of L WC and
ICC in the central Sierra Nevada

This section presents the analysis of 98 h of aircraft
data compiled over the 2-year period of study. The
98 h include 45 aircraft missions flown over 38 d. The
number of missions flown in each year was nearly
identical, with 23 missions included from the 1978-
79 season and 22 missions from the 1979-80 season.
The results are presented as relative frequency of oc-
currence which is defined as fin, or percentage fre-
quency as defined by 100fln, wherefis the frequency
of an event given n opportunities.
The data presented in the following subsections were
collected "in cloud". "In cloud" is defined by an ob-
servation of droplet concentration greater than 10 cm - 3
by the FSSP or an observation of ICC greater than 1
L -1 by the 2D-e.

a. Frequency in cloud

The in cloud percentages were greatest in area-wide
echo types and least in cellular echoes (Fig. 4). These
trends reflect the general environment in which each
PET existed. The clouds defining area-wide echoes fill
the sky with few breaks, though there may be cloud-
free layers. Clouds associated with banded echo types
often exist with clear sky ahead and behind the feature
producing a lower percent in cloud frequency. How-
ever, the band is often associated with vigorous con-
vection. Cellular clouds exhibit large areas of clear air
between cells and in the valley and foothill region,
thus providing the lowest percentage of being in cloud.

b. Distribution of L WC and ICC by temperature

This section presents the distribution ofLWC, ICC,
and the ratio of liquid water content over ice crystal
concentration (L WC/ICe). The following figures show
the percentage of time specific microphysical char-
acteristics are observed normalized to in cloud obser-
vations made every 2.50e.