WMOD00312 ‎(2)‎

• Title: Cloud Seeding Feasibility for the Shasta-Trinity Watershed
• Date: 12/1/1993
• Weather Modification - Doc Type: Report

AgI was encountered, the small AgI aerosols collided with small water droplets in the
chamber, forming ice crystals. These crystals would grow and fall to the bottom of the
chamber, where an acoustic counter would count them. This information provided
indisputable evidence that the seedline was being sampled.

Results indicated that the AgI acetone mixture produced much higher ice crystal
concentrations than AgI flares at temperatures warmer than -6 oc. AgI would continue to
produce new particles because it was forced to rise to higher elevations (colder temperatures)
as it crossed the barrier. Deshler and Reynolds (1990) describe one case in which seeding
effects are tracked for 90 min and some 90 km downwind from the release point. Super and
Boe (1988) also noted good success with aircraft seeding using AgI acetone burners in seeding
trials conducted over the Grand Mesa of Colorado.

Successful seeding must result in additional precipitation at the ground as an end product.
Measurements were made in an attempt to quantify seeding effects at the ground in the
target area. Kingvale, located at the center of the fixed target area, was instrumented with
various remote and in situ devices for sampling seeding effects and providing background
meteorological information. An aspirated two-dimensional particle probe was- used to count
and size snowflakes. A dual channel radiometer was used to determine if liquid water was
being affected by seeding. A vertically pointing K-band radar allowed measurements of small
snowflakes passing overhead such that seeded parcels of air might be detected. Snow
samples were collected for later analysis of AgI or other tracers used during seeding. An
upper-air station was used to measure the winds, temperature and humidity through the
depth of the cloud. Of the 15 experiments in which the targeting model predicted seeded
precipitation would fall at Kingvale, 5 indicated possible seeding effects. Two of these cases,
the last two seeding trials conducted in SCPP, provided convincing evidence of seeding effects
at both aircraft and ground elevations. Both these cases were conducted after the operational
targeting model was fully functional. Results from these two cases showed seeding produced
small rimed particles (<1 mm in size) at the ground 45 min after seeding, consistent with
model expectations. Each seedline was still distinct after 45 min and produced effects at the
surface of only 10 min in duration. Precipitation increases attributable to seeding were at
most 0.1 to 1 mm h-1. Combustion of an Ay-I-acetone solution produced an increase in ice
crystal concentration of greater than 100 L- at -6 oc.

These aircraft seeding trials in shallow orographic clouds have pointed out some limitations
of aerial seeding. Most pertinent is the limited volume of cloud which can be treated. Figure
3.5 shows the separation ofthree 37-km-Iong seedlines 10 to 30 min after seeding with wind
conditions as noted. The seedlines could be shortened to treat more cloud volume along the
direction of movement, but this procedure would sacrifice treatment of the larger cloud
volume outside this region.

Combining the SCPP results with seeding results from other field experiments and
microphysical model calculations performed using realistic liquid water distributions, the
following suppositions are made:

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