WMOD00485

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

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ARLIN B. SUPER AND JAMES A. HEIMBACH, JR.

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SH, which indicated that overall, about 50% more snow
fell on the BR T A for the population of seeded exper-
imental days with Main Ridge temperature ~-90C. It
would seem likely that seeding was ineffective on many
of the days, as suggested by Super (1986), so that larger
than 50% increases must have occurred on some other
days. The latter analysis further indicates that a dou-
bling of the precipitation is reasonable during periods
when seeding is particulary effective [see, e.g., Fig. 3
of Super (1986)] .
Super (1986) also suggested that seeding was es-
pecially effective in some of the cases where the 70 kPa
wind had a strong westerly component and cloud tops
were warmer than about -250C. He indicated that
seeding was highly effective during a fraction of the
cases, but had little or no effect most ofthe time. Fur-
thermore, no decreases in precipitation due to seeding
were suggested by the statistical analysis, and none were
found in the January 1985 physical observations.
Of the three experiments with marked seeding sig-
natures, the afternoon mission of 15 Jan had the most
abundant SLW, and the highest IPC and estimated
snowfall rate in the seeded zone. The least amount of
SLW, and shallowest liquid layer, occurred on 10 Jan,
which also had the lowest IPC and estimated snowfall
due to seeding. The morning mission of 15 Jan had
somewhat less SLW, IPC and snowfall than the after-
noon flight, but was well above 10 Jan. Cloud tops on
all three days were relatively warm (no aircraft obser-
vation was made on the afternoon of 15 Jan, but sat-
ellite imagery suggests a top temperature near -200C).
This places all three missions in the most favorable
category suggested by Super (1986).
Section 6 shows strong westerly flow over the BR T A
on 15 Jan and considerably less on 10 Jan, which was
likely a major factor in SL W production, and, in turn,
was apparently related to the effectiveness of the seed-
ing. This occurred despite the fact that the 2.7 km tem-
perature was -lrC on 10 Jan and -1OoC on 15 Jan,
which should provide less potential AgI ice nuclei on
the latter day. The greater abundance of SL W appeared
to be more important to seeding effectiveness, which
is reasonable if, as expected, contact nucleation was
the primary process (DeMott et al. 1983).
The three missions without apparent seeding effects
all had temperatures cold enough for AgI nucleation.
Although moderate westerly flow existed, little or no
SL W was available for AgI nucleation over the BRT A.
In the case of 19 Jan, this was due to the arrival of a
dry arctic air mass in the lower layers. Cloud tops were
warm, and SL W existed in the upper levels, but not
where the shallow AgI plume passed. The two missions
of 28 Jan had abundant natural ice, probably because
of the relatively cold cloud tops (Cooper 1986). Any
SL W production over the target was rapidly depleted
by the natural snowfall.
The limited number of January 1985 aircraft mis-
sions is too few to be able to claim that the resulting

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