WMOD00294

• Project Name: Project Skywater - Sierra Cooperative Pilot Project
• Title: The Design of SCPP-1 - A Randomized Precipitation Augmentation Experiment on Winter Cellular Convection in the Central Sierra Nevada
• Date: 11/1/1983
• State: NV
• Weather Modification - Doc Type: Report

than in an untreated cloud. The increased effi ci ency is brought about by
making use of cloud water to support the growth of additional crystals to
precipitation size particles that would have been lost to evaporation by
entrainment. This extension of the precipitation process in seeded versus
non seeded convective clouds gives rise to the additional precipitation
desired.

e

c. Treatment Effects

Treating these clouds with a single curtain of dry ice pellets (0.1 g/m)
results typically in the following sequence of events (Marwitz and Stewart,
1981; Stewart and Marwitz, 1982; English and Marwitz, 1981; Cooper, et al.,
1982; Rodi, 1982; Lawson et al., 1980; and Rhea et al., 1983):

1. 0-10 minutes. - An initial vertical curtain of ice crystals
develops in about 5 minutes to a concentration of about 20 to 40/L
(0 >50 ILm). The ice crystals are uniformly distributed by turbulent
diffusion (dw/dt ~ 2 mis, w = plume width) within the curtain above the
o oc level where liquia water is present. Individual particle diameters
after 10 minutes can range from 0.4 mm at -8 Oc to >1.2 mm at -15 Oc with
bulk densities that are very low (melted diameters would be about 0.2 mm).
No appreciable difference should be noted in liquid water content due to
treatment because entrainment is a more effici ent process than vapor
depletion by the ice. As particles aggregate (between -10 and-15 OC) _....
(Hobbs et al., 1980; Ludlam, 1955; Strapp et al., 1979; Holroyd and ..,
Jiusto, 1971) forming appreciable back scatterers, the first radar echo
will develop. Thus, time to first echo is shorter in treated than in
nontreated clouds.

2. 10-20 mi nutes. - At about 10 mi nutes after treatment, the vert ica 1
curtain of ice crystals ceases to be uniform because of the influence of
vertical circulation caused by convection and entrainment. Reduction of
the LWC within the ice crystal curtains is about the same as in nonseeded
clouds. This is due to the updraft entering the dissipation stage during
this time period. New convective cells may appear, however, causing
localized increases in liquid water. The particle spectrum becomes
exponential because of the continued presence of small ice particles and a
balance between diffusional and aggregational growth (Passarelli, 1981)
with light riming possible dependent on available liquid water in cloud.
Growth of the art ifi cally induced crystal s wi 11 have provided precipita-
tion-sized particles that will begin falling out during this period. The
maximum radar echo of about 25 to 35 dBZ develops between -10 to-15 Oc in
this period as a result of the particles which have managed to aggregate.
As particles begin to fall through warmer regions, this aggregation
process accelerates.

3. 20-30 minutes. - In the period from 20 to 30 minutes after treatment,
the microphysical characteristics of the treated clouds as observed by the
aircraft at -8 Oc become indistinguishable from untreated clouds as the
seedi ng effects descend below the ai rcraft observ i ng 1 eve 1. Treatment,
therefore, has formed precipitation earlier than in natural clouds and has
increased the efficiency in these clouds by removing a certain proportion ....
of cloud water that would have been lost due to entrainment.

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