WMOD00507

• Title: The Persistence of Seeding Effects in a Winter Orographic Cloud Seeded with Silver Iodidde Burned in Acetone
• Date: 6/6/1990
• Weather Modification - Doc Type: Report

JUNE 1990

TERRY DESHLER AND DAVID W. REYNOLDS

483

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FIG. 3. Images from the 2D-C and 2D-P showing water drops and frozen drops with protruding needles.
The center row of images in each set is from the 2D-P.

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Images indicated that the particles in the seedline
were primarily graupel and plates, while outside the
seeded plume branched crystals and aggregates were
observed. Within the seedline the crystal habits are
consistent with nucleation and growth at temperatures
between -8 and -l2OC, where thick columns and
plates are expected (Magono and Lee 1966). These
. compact crystal-types rime efficiently and graupel could
easily result (Heymsfield 1986). In contrast, outside
the seedline branched crystals and aggregates were ob-
served, which are consistent with nucleation and
growth near the top of the cloud, -IS oc.
The cloud conditions and ice crystal types observed
during these final penetrations are not similar to cases
when ice multiplication was observed during the SCPP
(Deshler et al. 1990), and it is unlikely that an ice
multiplication mechanism would confine itself to the
seedline. The increases in ICC observed during the
seedline penetrations shown in Fig. 4 are also not be-
lieved to result from aircraft-produced ice particles
(APIPS). During the SCPP several specific experiments
were conducted to test for APIPS from the seeder air-
craft and no evidence was found (Gordon and Marwitz
1986), although in other experiments the research air-
craft was found to cause APIPS in 2 out of 37 pene-
trations (Marwitz et al. 1986). During the first II pen-
etrations of the seedline on this day the research aircraft
made repeated penetrations of a point drifting with the
wind. If the aircraft was producing ice particles they
should have been observed during these penetrations
since the aircraft repeatedly returned to the same point,
yet no unusual increases in ICC were observed until
the last 2 of these II penetrations when the aircraft
had climbed. However, definite increases in ICC at the
seedline were observed during the final five penetrations
(all shown in Fig. 4), and these were made at different
points along the line. The aircraft flew a "Z" pattern,

advecting with the seedline, but moving from the center
of the seedline to the north end. Thus these particles
could not have been nucleated by the research aircraft.
The seeding material was released in the upwind edge
of an orographic cloud with radar reflectivities of 15-
20 dBZ. Although there was evidence of ice generation
near 5 km, and subsequent enhancement of reflectivity
as the particles fell through the cloud, the echo region
was generally below 5 km. The seedline remained with
this same level of background echo throughout the 92
min of sampling and there were no changes in reflec-
tivity that could be attributed to seeding. This is typical
for this type of cloud. Deshler et al. (1990) found in
similar experiments that only 4% of the clouds had a
background echo low enough for the radar to be sen-
sitive to seeding effects.
A schematic of the cloud and barrier in two dimen-
sions is shown in Fig. 5. The seeding location, the X-
Zlocation of the research aircraft during seedline pen-
etrations, and the expected envelope of AgI IN, based
on the detection ofIN at 64 min and assuming a vertical
dispersion of 0.1 m S-I, are also shown. According to
this calculation the research aircraft was generally below
the IN for the first hour. Note that the ascent of the
seeding material between 61 and 92 min, 0.2 m S-I,
parallels the slope of the barrier. Also shown in Fig. 5
are predictions for the trajectories of ice crystals nu-
cleated after delays of 5, IS, 30 and 45 min, from a
targeting model used by the SCPP (Rauber et al. 1988).
Although for this case the direction of the model winds
and the aircraft measured winds are in good agreement,
the model winds are slower by 4 m S-I. This causes
the disagreement between the predicted seedline po-
sition and the aircraft determined seedline position at,
for example, 60 min.
The model predictions of fallout give an average ad-
vection of seeding material to the surface of 2330 at