Weather Mod Critical Issues Report

• Project Type: General OWC
• Project Name: Weather Modification
• Title: Critical Issues in Weather Modification Research
• Date: 1/1/2004
• Water Conservation - Doc Type: Final Report

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3.3 Summer operational programs

The NRC report (p 68) touts the potential for hygroscopic seeding of warm season
convective clouds, and encourages further investigations in this area. While we agree
that considerable potential does exist for hygroscopic seeding, we do not agree with the
NRC finding regarding glaciogenic seeding that there" is recognition of the lack of
credible scientific evidence that applying these concepts will lead to predictable,
detectable, and verifiable results." There are many situations in which hygroscopic
seeding is not feasible, and we believe that glaciogenic seeding still has much to offer,
even though more complete evidence of cause and effect is desirable,

Progress has been made. For example, the initial objective of weather
modification research work in Texas focused on formulating a conceptual model for rain
enhancement. The High Plains Experiment (HIPLEX) sponsored by the V,S. Bureau of
Reclamation, and based in Big Spring from 1975-1980, led to the identification of
experimental units, seeding hypotheses, covariates, and response variables for subsequent
fieldwork conducted a decade later as part of the NOAA Atmospheric Modification
Program (AMP), discussed below, The Texas HIPLEX led to the conclusion that seeding
for dynamic effects may have substantial impact on convective cloud clusters, deemed to
be the most favorable candidates for the "experimental units" in subsequent exploratory
research (Riggio et aI., 1984). While precipitation is often initiated in west Texas clouds
through the warm rain process, the ice phase was observed to dominate during much of
the subsequent cloud development, with the rapid development of greater ice particle
concentrations being a consequence of an active ice multiplication process. With radar
observations of merging cloud echoes, particularly clusters, suggesting an interaction
between individual convective towers with the mesoscale systems, it was deduced that
additional cloud growth could be facilitated through the seeding of turret clusters.

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Additional field work, consisting of the collection of 34 experimental units over a
number of weeks during four summers in the latter half of the 1980s, led to refinement of
the seeding conceptual model. Randomization of the seeding allowed comparisons to be
made between the behavior of treated and unseeded convective systems using C-band
weather radar. Results of the analyses indicated seeding with silver iodide more than
doubled the amount of rain volume produced by the clouds (Rosenfeld and Woodley,
1989). Moreover, the seeded systems lived on average 36 percent longer than their
untreated counterparts, expanded to produce rainwater over an area 43 percent larger, and
tended to merge with adjacent convective cells nearly twice as often. Intriguingly, the
seeded clouds grew only marginally taller (about 7 percent) than the unseeded ones.
(Both rainfall and merger statistics were significant at better than the 5 percent
significance level.) These results confirm earlier results from the Dakotas (Dennis et aI.,
1975) that show broader and longer lasting echoes from the seeded cells in that region.
In addition, the extra growth in height in the seeded clouds was an average 600 m, or less
than 10% of the cloud depth, These last authors commented on the fact that both
dynamical and microphysical changes appeared to be important in producing the
increases in rainfall from the seeded, cells,

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