WMOD00427

• Title: Design of Physical Cloud Seeding Experiment for the Arizona Atmospheric Modification Research Program - Final Report
• Date: 2/1/1991
• Weather Modification - Doc Type: Report

Some regions have serious airspace conflicts, for example, "victor" routes between major airports.
Such high traffic areas should be avoided in planning experiments because needed airspace blocks
will frequently be unavailable.

An even more serious consideration is to avoid mounltain barriers that preclude flight near the
surface. The usual restriction is that aircraft flying under IFR (instrument flight rules) must stay
at least 600 m above the highest terrain within 8 km of the flight path. Special waivers can be
obtained in some locations to allow flight within 300 m of the terrain, but nearby navigational aids
are usually required. Even if the target is at a high elevation, still higher peaks nearby may
preclude aircraft sampling within a kilometer or more above the target site. Yet a growing body
of evidence shows that most of the SL W is concentrated in the lowest kilometer over the windward
slope (e.g., Hobbs, 1975a; Holroyd and Super, 1984; Hill, 1986). Considerable ice particle growth
can occur in this zone, so a 1-km vertical "gap" between lowest aircraft observations and the surface
will cause considerable uncertainty concerning the growth, fallout, and targeting of ice particles
resulting from seeding. The SCPP experiments, for example, suffered from both airspace conflicts
and a rugged barrier making flight near the target site impractical. The latter condition was also
a problem in the early 1989 experiments in the Tushar Mountains. The ideal mountain barrier for
experimentation would allow surface sampling on the crestline, would have no higher peaks near
the target site, would not have an abrupt crestline which could create serious downwind turbulence,
and would have nearby navigational aids such as a VORTAC station. Fortunately, portions of the
Mogollon Rim approximate the ideal except for a lack of nearby navigational aids.

1.3.3 Direct Detection of Seeding Agent. - Another requirement for physical experiments is
that either the seeding material be detectable, or a tracer material be simultaneously released.
Silver iodide can be tracked with an acoustical ice nucleus counter but dry ice is not traceable.
Natural variations in IPC can easily mask the ice partides caused by seeding unless the seeding
material itself, or a tracer material such as SF6 gas, is independently measured to distinguish the
seeded volume from natural cloud. In other words, attempting to specify the seeded volume by
monitoring ice particles alone will lead to uncertain interpretation of seeding effects (Deshler and
Reynolds, 1990).

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A note of caution is in order in tracking either AgI or SF 6 with airborne detectors. Existing
detection systems require a skilled operator who can recognize system malfunctions and correct
them. A number of past attempts at airborne tracing failed because of improperly functioning
equipment and/or insufficiently knowledgeable operators, These problems are rarely mentioned
in project reports or publications and are only learned about through experience or personal
communication. However, with properly maintained equipment, either AgI or SF6 can be tracked
for tens of kilometers under stable or neutral atmospheric lapse rates with reasonable release rates.
For unstable conditions, required SF6 release rates may become excessive, but AgI can readily be
detected for long distances with an acoustical ice nudeus counter due to the vast quantity of
potential ice nuclei produced. For example, Super et a!. (1975) reported tracking an AgI plume
from a single generator as far as 190 km downwind.

1.3.4 Radar Detection. - Radar has sometimes bee:n used in attempts to follow the effects of
seeding between lowest aircraft levels and surface instruments. However, radar evidence of winter
orographic seeding effects is normally inconclusive unless the natural IPC is very low. That is
because the radar reflectivity factor is not only directly proportional to particle concentration, but
also proportional to the sum of the sixth powers of particle diameters. Thus, the returned signal
from a few large natural snowflakes can completely mask that from thousands of smaller crystals

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