WMOD00567

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

Bulletin American Meteorological Society

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FIG. 8. Seedline positions and width from 10 to 30 min after seeding assuming a 23.5 m . S'l windspeed and 1 m. S-l dispersion rate. Note
the very small volume of air actually treated. Letter identifiers denote locations of SCPP precipitation-gauge sites.

of ice or water saturation a high concentration of small ice
particles are induced by glaciogenic seeding. Results show only
about 10 percent of the crystals within the plume have preferred
growth towards precipitation-size particles. Therefore, ice-par-
ticle concentration changes attributed to seeding are an order
of magnitude less measured at the ground than measured by
aircraft shortly after seeding. Growth rates of seeded crystals
tend to be lower than laboratory-based growth estimates but
can be accounted for through appropriate theoretical consid-
erations. Growth rates for artificial crystals appear to be rather
consistent over a broad temperature range as welL These results
are significant in that they provide a portion of the necessary
information (time-dependent particle fall speeds) required to
effectively target the effects of seeding to a desired location.
Also necessary is adequate knowledge of the horizontal and
vertical wind speed over the mountains.
Figure 9 compares in situ observations of seeded-crystal-
growth rates as observed in three separate field studies to the
laboratory results of Ryan et aL (1976) for crystal growth during
the first few minutes after nucleation. Fukuta and Wang (1984)
confirm that Ryan's results are valid up to 20 min after nu-

c1eation. The seeded crystal growth rates average 0.4 to 0.5
pm S-I for temperatures from - I to - 140C. No distinct in-
creases occur at - 60C or approaching - 150C as expected.
Presumably this is because of substantially lower liquid water
contents than the 3 g m-3 Ryan used in the laboratory. Growth
rates observed in the field include diffusional, accretional, and
aggretational growth mechanisms.
Heymsfield (1982) has described an explicit particle growth
model that has abeen applied by Prasad et aL (1988) to the
SCPP region. Using a liquid-water profile typical of those ob-
served (figure 10) particles were allowed to grow and fall in
this regime using winds developed from the Parish model for
the Sierra Nevada (Parish 1982). Particle diameter as a function
of time is shown in figure 11. Note that all particles between
-50 and -120C reach only 600 pm after 40 min., similar to
observed particle sizes induced by seeding. Also note that the
more compact crystals like columns and plates (those most often
observed from seeding) (Stewart and Marwitz 1982; Hobbs
1975b; Super and Boe 1988; Super and Heimbach 1988) have
very efficient riming growth and become lump graupel (snow
pellets) prior to fallout. These model-generated predictions are