WMOD00486

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part III
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

1180

JOURNAL OF APPLIED METEOROLOGY

suIted from the seeding was indistinguishable from the
natural shower. However, in view of the aircraft ob-
servations, it seems unlikely that seeding had much
effect on surface snowfall.

g. Summary of airborne seeding experiments

Results of the six airborne seeding experiments,
along with various supporting information, are sum-
marized in Table 2. The mean nonseeded IPCs in the
5 km zone upwind of the Snow Lab were ~2.6 L -1 in
all cases, with a mean value of 1.5 L -1 for all six ex-
periments. Mean seeded IPCs ranged from 2.5 to 21.5
L -Ion the two days, with an overall mean near 9 L -1.
The majority of the IPC increases appeared as small
crystals classified as hexagonal and spherical by the
processing software, the latter most likely tiny stellar
or hexagonal crystals. Most crystals classified as hex-
agonal were also small, and most probably of the PIa,
PI b, and PIc habits (see Magono and Lee 1966). The
majority of crystals in the seeded cloud volumes that
were classified as irregular were fragments of dendrites
or rimed hexagonal crystals.
Five of the six experiments resulted in significant
increases in estimated precipitation rates at the 3.8 km
aircraft sampling level. The exception was experiment
6, in which the precipitation rate was essentially un-
changed. The mean estimated precipitation rate for
the 5 km interval immediately upwind of the Snow
Lab was 0.21 mm h -1 for the seeded clouds from all
six experiments, but only 0.05 mm h-I for the non-
seeded clouds, a factor of 4 difference. Caution is again
necessary in regard to quantitative interpretation of
the estimated precipitation rates (see Part I).
Very little snowfall was recorded at the Snow Lab
within 0.5 h preceding or following the passage of the
seeded zones aloft in the first three experiments. The
greatest surface snowfall rate was 1.6 mm h -I in ex-
periment 3, and in all three experiments the peak rate
exceeded 1.0 mm h -1 during passage of the seeded
zone overhead. Total integrated snowfall attributed to
seeding was 0.22, 0.10 and 0.18 mm for experiments
1,2 and 3, respectively. Mean precipitation rates ranged
from 0.33 to 0.43 mm h-I for these experiments. Super
et al. (1986) reported a median rate of 0.7 mm h -1 for
a two-winter period on the Grand Mesa. Thus, the rates
observed at the Snow Lab in the first three experiments
were similar to natural rates during less intense storms.
The failure of experiment 4 to produce snowfall at
the ground is thought to be due to the proximity of the
seeding arc to the Snow Lab. The neophyte AgI plume
was afforded only 10 minutes for nucleation, growth,
and fallout to the Snow Lab, some 600 m below the
seeding altitude. Even immediate nucleation and dif-
fusional crystal growth at a rate of 0.1 mm min-I
(Holroyd 1986) could not have produced crystals of
sufficient size to result in detectable surface snowfall
so quickly.

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