WMOD00571

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

The finding that entrainment exerts a controlling influence on the lifetime
of HIPLEX-1 clouds and that, in turn, limits precipitation development in
natural and seeded clouds is consistent with the findings from other
experiments on similar clouds. For the HIPLEX-1 clouds the exponential
decay time constant of liquid water, defined as the time for the maximum
liquid water to reduce to lie of its initial value, was 14 min.
Marwitz (1984) reported that winter post-frontal cumulus congestus clouds
over the central Sierra Nevada mountains also had a decay constant of 14
min. Similarly, Hallett (1984) indicated a decay constant of 12 to 14
minutes for towering cumulus clouds over Florida. Kochtubajda and Rogers
(1984) calculated the decay constant of the liquid water in treatment plu-
mes (dry ice, silver iodide and placebo) in isolated towering cumulus over
Alberta. They found that the natural (placebo) clouds had a decay constant
of about 12 min and that the concentration of ice crystals produced by the
dry ice seeding depleted the liquid water faster than either the ice
crystal concentration produced by the silver iodide seeding or entrainment
(the natural clouds) for the first few minutes after seeding and,
thereafter, entrainment dominated. The decay constant in the dry ice and
silver iodide plumes was about 6 min and 13.5 min, respectively, with the
depletion rate being faster for the dry ice seeding presumably because of
the higher ice crystal concentration it produced.

Braham (1960) examined the duration of summer cumuli in the central United
States and Arizona which he defined as "the total life of the cloud after
its top reached the -5 aC level." He found that only 25 percent of the
clouds lasted longer than 15 min and of these about 60 percent developed

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