WMOD00506

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

precipitation, 0.1 WE. h-:, and liquid water near 0.1 rom was then observed

until 0400 on 23 December 1986, with the liquid water persisting until 0700.

Based on research aircraft measurements during the experiment, 1700-2000,

the orographic cloud consisted of ice, 30- 100 L-1 on the 2D-C, 6-20 L-1 on the

2D-P, and almost no liquid water in the FSSP size range. FSSP concentrations,

when they existed, were < 40 cm-3, with droplets predominantly between 7 and

19 pm. Images from the 2D-C and 2D-P indicated that the ice was primarily

small graupe1 and needles, with many regions of large drops, 100-200 ~m. At

times water drops dominated the particle concentrations sensed by the 2D-C and

2D-P, still the FSSP saw at most low concem::rations of drops 7-19 ~m. .,
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Needles, or spikes, protruding out of frozen drops were often seen~in the

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regions of large drops.

b. Treatment

A deliberate seeding experiment was conducted as a test of the

effectivity of AgI ~HqI NH4CI04 at _60C. The seeding location was chosen

based on measurements by the research aircraft, Fig. 1, which showed a region

of liquid water of 0.05 g m-3 10 km west of SHR. This was nearly the only

region of liquid water observed during the ascent sounding and descent along

the barrier by the research aircraft. Considering the low droplet

concentrations and assuming t~e AgI nuclei to be active via contact

nucleation, the Ice expec~ed from seeding would be < 1.0 L-1.

From 1810 to 1818 the seeder aircraft released AgI NH4I NH4CI04 at 0.4 g

km-1 along one 40 ~~ long seedline. The seedline was 10 km west of SHR and

oriented nearly north-south, although average winds were from 2350. After

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