WMOD00567

• Title: A Report on Winter Snowpack-Augmentation
• Date: 11/11/1988
• Weather Modification - Doc Type: Report

Bulletin American Meteorological Society

mean cloud depth of 2 km this value converts to an average
liquid-water content of less than 0.1 g . m ~3. Higher liquid-
water contents thus occur infrequently. At all three field sites
the SLW is concentrated in the lowest kilometer above the
barrier at temperatures of from 0 to - 100C (Heggli and Rauber
1988; Rogers et at 1986; Boe and Super 1986; Rauber et at
1986). All three programs report that SLW, when it is present,
is highly variable in space and time due to the variability of
orographic flow over complex terrain and the convective nature
of the cloud. However, SLW on the average can exist from 6
to longer than 48 hours. Values above 0.3 mm normally last
for only 6 to 10 hours. However, it is felt that even three hours
is sufficient time to ready seeding systems (ground or aerial)
and allow sufficient time for seeding within these SLW op-
portunities.
Developing techniques to predict SLW occurrence (oppor-
tunity recognition) is needed, especially if the best opportunities
occur during only 10 to 15 percent of the time SLW is observed.
Reynolds and Kuciauskas (1988) provide examples of the utility
of satellite and radar remote sensing in providing several hours
of lead time for cloud-seeding activities to be initiated during
these higher SLW periods. Satellite and radar observations are
useful because during the transformation of deep stable clouds
into more shallow orographic clouds, distinct differences in
cloud-top temperature can be seen in satellite infrared imagery
and the associated rainbands can be tracked in radar displays.
Diurnal and seasonal trends in SLW, as has been observed
in SCPP, also occur. Lee (1988) has shown that higher observed
SLW exists during the night than during daylight hours. His
results are based on radiometer, mountain top-icing data, and
rawinsonde data that infer a higher frequency of saturated layers
near the surface at night. Rangno (1986) alludes to this diurnal
trend based upon his observations of winter mountain clouds.
He suggests that cloud bases descend at night while cloud-top
heights increase, leading to higher frequencies of water-satu-
rated clouds near the mountain. The observations suggest a
higher potential for ground-based seeding at night than during
the day.
Regarding seasonal or monthly trends, one may now ask
what the potential seeding opportunities are in normal to below-
normal precipitation periods. One may consider as an index to
seedability, icing occurrence at mountain-top level as measured
by an automatic icing-rate meter (Henderson and Solak
1983), or by direct measurements of rime icing (Hindman 1986).
Figure 6 shows for the past two winters monthly precipitation
as a percentage of normal at Blue Canyon, California, a re-
cording gauge site at the 1500-m level in the central Sierra
Nevada. It also shows the number of "icing trips"3 logged at
Squaw Peak at the 2700-m level on the crest of the Sierra
Nevada.
Figure 6 shows an extreme example of a wet month and dry
month having similar icing. February 1986 precipitation for
that site was 411 percent of the normal precipitation; December
1986 precipitation was only 22 percent of the normal precip-
itation. In general, the figure shows similar icing events for

. 3 An icing trip is defined as one transmission from the sensor initiated
when a specified mass of ice collects on the sensing element. Each
transmission, or trip, is followed by a brief heating cycle which melts
the ice and prepares the instrument for further observations. A maxi-
mum of three trips can occur in a five minute period.

1293

average to below-normal months as well as excessively wet
months. This data implies that for certain dry months, storms
may be frequent but produce much less precipitation. As dis-
cussed earlier in this section, it is the weak or weakening
portions of storms that produce substantial periods of SLW.
The significance of this phenomenon is that seeding may well
be possible when it is most needed. A much longer record
period is required to confirm this, however. Super and Heim-
bach (1988) also show that considerable potential for increasing
snowpack is present in well below normal snowfall seasons.
This potential, however, may not be present in drought years,
such as 1976-1977 when almost no storms effected the western
U.S.
By way of review, the shallow orographic cloud, considered
the best candidate for winter snowpack augmentation, is de-
scribed in figure 7. The figure shows fairly low concentrations
of liquid water, within the lowest kilometer above the barrier,
at temperatures of from 00 to - 100e. There is some tendency
for a nighttime maximum and for possibly significant periods
of SLW to occur even during below-normal precipitation pe-
riods.

-C

100
90
(I)
Cl
III 80
-
c::
(I) 70
()
..
(I)
Q. 60
(I)
> 50
:;::
III
'5 40
E
:;,
0 30
20
10
0

85% < .2mm

... Utah Long (1 987)
. Colorado Super et al (1986)
. California Heggli & Rauber (1987)

o 10 20 30 40 50 60 70 80 90 100 110 120 130 140
10 20 30 40 50 60 70 '80 90 100 110 120 130 140 150
Integrated Liquid Water (mm)

FIG. 5. Cumulative distribution of integrated supercooled liquid
water as measured by a vertically pointing microwave radiometer at
various field site locations noted. Averaging interval varied from 15
min (Utah) to I h (Colorado and California).

120 D Precipitation
. Icing Trips 400
100 411%
1200
80 1000
60 800
600
40
400
20 200

FEB 86 JAN 86 FEB 87 MAR 87 JAN 87 DEe 86 NOV 86

FIG. 6. Monthly precipitation and percent of nonnal at Blue
Canyon, CA, and icing trips at Squaw Peak from ETI (1987).