WMOD00312 ‎(2)‎

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

Because of their wide areal coverage, average duration of 10 h, and frequency of occurrence,
they were considered for a separate experiment. A complete review of the results of this
experiment is given in Deshler et al. (1990). A brief review will be given here.

A total of 36 experiments was conducted on 29 separate days in clouds matching the
experimental design. To determine the aircraft seeding location to assure seeding effects
would hit the intended target, a diagnostic targeting model was run in real time. This model
is reviewed by Rauber et al. (1988). The model uses upper-air wind and temperature data
collected near the foothills and at the crest of the Sierra Nevada to compute the winds
perpendicular and parallel to the mountain barrier. Knowing the slope of the barrier and the
perpendicular wind component, the vertical component of the wind can be derived. Using
both laboratory and field measurements of crystal growth processes as a function of
temperature and liquid water content, Heymsfield (1982), Ryan et al. (1976), the trajectory
of expected seeding induced ice crystals could be calculated. This calculation provided a
distance, direction, and height from the target for positioning the aircraft to increase seeding
effects within the target area. The model was not fully operational until midway through the
second to last season of SCPP. This delay had a definite impact on results obtained from
experiments conducted before and after model completion.

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Aircraft initial conditions were determined from measurements made within 5 km of the
initial seedline. These measurements included liquid water and ice crystal concentrations.
In general, aircraft seeding was conducted between -5 and -15 oC. Average liquid water
contents were rather low at 0.05 g m-3, attesting to the fact that the bulk of the liquid water
was below aircraft altitudes. Ice crystal concentrations varied considerably, but ranged from
about 5 to over 100 L-1. It should be emphasized that seeding was intended to produce an
additional 10 to 20 crystals per liter. Thus, if background particle concentrations were high
and quite variable, as was the case during many experiments, these small changes were
difficult to observe.

Seeding involved the use of three separate seeding materials: AgI 20-gmdroppable
pyrotechnic flares; combustion of an AgI, ammonium iodide, acetone solution; and CO2, For
the 36 experiments, a total of 238 seedlines was flown. Of these, 42 were placebos (no
seeding material released). For the 196 treated seedlines, the research aircraft penetrated
123, of which 43 had distinct seeding signatures (35 pct).

CO2 was used on 172 of the 196 seeded lines, 33 of which indicated seeding effects (19 pct).
For CO2 to be an effective seeding agent, it must be released in liquid cloud or in cloud
conditions above ice saturation to maintain particle growth. A combination oflow and highly
variable liquid water content along the seedline, highly variable background particle
concentrations, and poor dispensing of the CO2 pellets contributed to this low percentage.

AgI flares were used on 15 seedlines. All seedlines were penetrated by aircraft of which four
had distinct seeding signatures (26 pet). These low percentages were caused by the AgI flares
having poor ice nucleating capability (two orders of magnitude lower than AgI in acetone at
temperatures warmer than -10 OC) and a high failure rate. '

Seven seedlines were produced from the combustion of an AgI acetone solution. The research
aircraft penetrated all seven seedlines, of which six had distinct seeding signatures (86 pct).
This high success rate was partly the result of an ice nucleus counter operated onboard the
research aircraft. This counter consisted of a small cloud chamber operated at -20 oC. As

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