WMOD00299 ‎(2)‎

• Title: Scientific Weather Modification Experimentation in the United States
• Date: 8/25/1984
• Weather Modification - Doc Type: Report

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the lifetime of HIPLEX-1 clouds and that, in turn, limits precipita-
tion development in natural and seeded clouds is consistent with the
findings from other experiments on similar clouds (Braham, 1960;
Schemenauer and Isaac, 1984). The dil emma for cloud seedi ng app1 i ca-
tions is to be able to predict which of the clouds in a population
will have liquid water that lasts long enough for seeding to be effec-
tive.
2. Precipitation development in most of the seeded clouds did not
proceed via the graupe1 process as hypothesized. Primarily because of
the high ice concentrations produced by the seeding, a combination of
aggregation and low-density accretion onto the loose aggregates was
the dominant precipitation process. Only small raindrops were pro-
duced in this way, which were insignificant in comparison to those
produced by the natural accretiona1 growth process. In this respect,
the seeding operation failed to produce the target concentrations of
10 per liter without penalty. The formation of aggregates following
seeding convective clouds apears to be a common occurrence (see, e.g.,
Dye et al., 1974; Strapp et al., 1979; English and Marwitz, 1981;
Rodi, 1984) despite the fact that graupe1 development is usually
postulated and expected. A better understanding of the formation and
growth of aggregates is needed to determine if and when aggregation
can lead to precipitation in the time available in convective clouds.
If aggregationa1 growth is relatively slow it would mean that the
seeding process is driving the cloud into a less efficient precipita-
tion process than its natural one with counterproductive consequences
and the formation of aggregates that result from high ice con-
centrations produced by seeding can be regarded as another definition
of "overseeding."

3. Precipitation development proceeded as hypothesized in those
clouds with sustained updrafts such that the main precipitation growth
occurred at temperatures colder than -10 oC (above the seeding level).
These results are supported by calculations (Cooper and Lawson, 1984;
Cooper, 1984) which show that accretional growth is more rapid and
more efficient at about -12 and -20 oC. In this respect, the seeding
hypothesis which emphasized the warm temperature region of the cloud
was in error and the choice of seeding level was, perhaps, too low
since it failed to take advantage of the region of rapid development
of graupel from ice crystals in most cases. The conclusion that
seeding would be more effective at a temperature level of -12 oC and
colder supports the notion that there is a warm-side cloud top tem-
perature limit for seedability of convective clouds (Grant and
Elliott, 1974; Gagin and Neumann, 1981).

3. FACE-2

The Florida Area Cumulus Experiment (FACE) was a long-term program to determine
the potential of dynamic seeding for increasing convective rainfall over a fixed
target area. The physical concept underlaying FACE was that massive seeding
would increase areal rainfall by promoting growth, mesoscale organization and
merger of cumulus in south Florida through dynamic invigoration of cloud towers
with adjacent cloud systems. Both an exploratory experiment (FACE-I) and a con-

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