WMOD00571

• Title: Static Mode Seeding of Convective Clouds - A Review
• Date: 5/23/1984
• Weather Modification - Doc Type: Report

additions, still based on the experimenter's judgment, involved indications
that the cloud was growing, that it would last at least 30 min., that it was
reasonably isolated, that it would not be affected by surrounding clouds,
and that it did not rain prior to selection (Bethwaite et al., 1966).

The University of Chicago Cloud Physics Project's experiments under the
Artificial Cloud Nucleation program (Braham et al., 1957) appears to be the
first time cloud physics data from an inspection pass with an aircraft was
used in determining cloud eligibility. A cloud was accepted if the inspec-
tion pass provided evidence that the cloud contained liquid water and the
lack of an echo on the calibrated nose radar indicated it did not contain
precipitation. With the advent of sophisticated airborne cloud physics
instrumentation, in situ microphysical data collected during a pretreatment
aircraft pass were incorporated into the cloud selection criteria (Bureau
of Reclamation, 1979 and 1983; Hobbs and Politovich, 1980; Marwitz and
Stewart, 1981; Engl i sh and Marwi tz, 1981; Isaac et al., 1982). Fi nal
selection of a cloud for treatment (either seeding or placebo) was made if
such characteristics as liquid water content, ice crystal concentration,
vertical velocity and cloud depth were within specific bounds. Rangno and
Hobbs (1983) have presented evidence that the passage of an aircraft
through supercooled clouds can produce high concentrations of ice crystals
which they called APIPs (Aircraft Produced Ice Crystals). If APIPs do, in
fact, occur as a result of a pretreatment aircraft pass or even the
treatment pass, its effects would have to be determined in the evaluation of
such seeding experiments.

Research on the relative effects of seeding clouds with different water-to-
ice ratios has been undertaken in an attempt to establish the limits of

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