WMOD00297

• Title: Hygroscopic Seeding in Oklahoma
• Date: 10/31/1971
• State: OK
• Weather Modification - Doc Type: Report

words, moderate turbulence probably gives adequate diffusion to cover
a normal convective cell and keep hydrometeor interference down to
unimportant levels. However, in several instances in Oklahoma, the
turbulence was Very light in the strong inflow area under the shelf cloud
of a major organized storm. The diffusion would be inadequate to spread
the plume through the region of importance. Even if the plume had been
released half an hour earlier, the diffusion would probably have been not
enough - - and pin-point forecasting of the area in which to dispense a
half hour ahead of time would be very difficult. Further study is required
, to see if in the absence of natural turbulence, the aircraft wake turbulence
and the gravity spread of particles is sufficient to spread the particles
to where they do not interfere with each others growth. If the answer is
no, the seeding in the smooth, strong updraft region should perhaps be
limited to the edges of the region where, up within the cloud, strong
turbulence can be expected. Such seeding would only contribute partially
to the cause of keeping water from being lost out the cloud top into the
cirrus anvil.

There is interest in even a larger scale of diffusion where the hygroscopic
material is "broadcast" into the general air mass at least an hour before
that air is involved in a cloud. ln convective conditions, with the associated
moderate turbulence, an hour of diffusion can give a spread of the order of
10 km. If one can dispense enough particles to yield the desired density
after complete mixing throughout the convective layer, the area coverage
in the convective layer is feasible. The flight trajectory could be parallel
legs 100 km long and 10 km apart, flown at least an hour before the develop-
ment of the cells from which rain is to be extracted.

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