WMOD00540

• Project Name: Sierra Cooperative Pilot Project
• Title: A Review of the Sierra Cooperative Pilot Project
• Date: 5/5/1986
• Weather Modification - Doc Type: Report

522

S.O

Droplet collison and
ice-droplet collison
produce natural
precipitation

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Vol. 67, No.5, May 1986

Increased concentrat ion
of crystals. from seeding

Active precipitation region
natural and seeded

80
X (KM)

100

140

160

120

FIG. 10. Illustration of the cloud conditions giving rise to optimum seeding potential. Seeding strategy is to take advantage ofliquid water
through addition of more ice to lower crystal trajectories, sweep out the liquid, and minimize loss of both liquid and ice escaping the barrier.

surements. Data from other project instruments are used to
stratify the day's meteorological activity.
An experimental unit may be called between 5 A.M. and
10 P.M. Monday through Saturday when observations indi-
cate the proper meteorological conditions exist (see also Sec-
tion 7). It is possible that more than one mission can be called
on a given day, but not more than two. In' a normal year as
many as 10 to 12 randomized cases may be expected. Seven
randomized fixed-target cases were completed in 1984/85,
which was a year when less than half the normal number of
precipitation days occurred. It is not expected that statisti-
cally significant results will be obtainable during this three-
year randomized experiment due to the large number of
seed/no-seed samples required (>200 seed/no-seed cases re-
quired for the ground-measured response variables assuming
a 10 percent effect). However, case studies using detailed .'.
physical analyses should provide evidence of the effects seed- .
ing is having on the precipitation process.

8. C,urrent status of SCPP

From the results presented and detailed studies from the past
two field seasons a general description of a storm period hav-
ing potential for precipitation increases through glaciogenic
seeding can be given. This storm period is one in which a por-

tion of the available condensate is transported over the bar-
rier and generally lost to the precipitation process by evapo-
ration. Condensate is lost in the form of supercooled droplets
and of small ice crystals generated naturally on the upwind
side of the barrier which do not grow large enough to fall out.
Normally; about twice as much condensate is lost in the form
of ice swept over the barrier as is lost in the form of liquid
water. Thus, an appropriate seeding strategy is one which
causes the liquid water to convert to ice earlier and further
upwind, so that the crystals can grow larger and fall out on
. the upwind barrier. Calculations by Rodi et al. (1985) using
actual cloud conditions measured during SCPP seeding op-
erations show that most particles growing in the presence of
SL W will obtain a fall speed of from 1 to 1.5 m . sec -} after
about 40 minutes, allowing them to fall 2 km (normal depth
of cloud). Seeding is normally done between -l 0 and -150C.
Thus a nominal value of -l2OC was used to initiate these
growth calculations.' Figure 10 illustrates how these proc-
esses might take place over the American River Basin.
It is planned to continue the present series of intensive field
seasons through 1987. Following the current series of in ten-
sive field seasons a two-year analysis effort will be per-
formed. This will allow for a review of almost 10 years of
physical observations and a synthesis ofthe results from both
randomized seeding experiments. The projected benefits
from a full-scale operational program will be assessed and
recommendations for future applications will be provided.