WMOD00571

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

ice nuclei into clouds and the scientific exploration of the weather modi-
fication possibilities envisoned earlier could begin. Field experiments on
artifically stimulating rain from convective clouds based on the Bergeron
process began almost immediately (e.g., Kraus and Squires, 1947; Leopold
and Halstead, 1948; Squires and Smith, 1949; Smith, 1949).

The static seeding hypothesis for convective clouds is based on obser-
vations that precipitation development in these clouds is frequently inef-
ficient and on the expectation that the natural precipitation process can
be made more efficient by the introduction of additional precipitation
embryos through glaciogenic seeding. The presence of supercooled water and
concomitant low concentrations of ice particles at temperatures wamler
than about -20 oC are generally taken as evidence of the inefficiency of
the precipitation process. The static seeding strategy is to provide an
optimum concentration of ice particles for the available liquid water and
initiate the precipitation process earlier and, perhaps, lower in altitude
in the developing cloud than would occur naturally. Attaining on the order
of 1 to 10 ice crystal s per 1 iter that eventually woul d become 0.1 to 1.0
graupel embryos per liter is the usual goal of the seeding operation. If
too many ice crystals for the available water are produced then none! may
grow large enough to fallout and the cloud is said to be "overseede!d"
(Bergeron, 1949).

Static seeding is intended to improve the precipitation efficiency of a
cloud by affecting its microphysical properties only. Changes in the dyna-
mical properties of the clouds may occur, but they are neither intended nor
part of the cause-and-effect relationship in the seeding hypothesis.

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