WMOD00297

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

Diffusion

One way of looking at the requirements for diffusion of the hygroscopic
seeding material is to identify two diffusion regimes. First, the
material must diffuse enough so that particles do not interfere appreciably
with each other for condensing water or coalescing with cloud droplets --
i. e., the growth mechanism should not be inhibited. Secondly. the material
should cover the whole cloud regime of interest, so as to make full use of
seeding potential.

For the first diffusion requirement, we find that the material must spread
evenly throughout a volume about the size of the whole wake of the C-97
to keep the particles from interfering with each other in condensing water
vapor. If 1 kg of material is released per second, if 30 gms of water are
required per gram of hygroscopic material, and we limit ourselves to
extracting 0.3 gm of water per kg. of air, (reducing local RH to 980/0 from
1000/0 near cloud base), we need to mix into a volume about 90,000 m3/sec.
The vortex wake system from the C-97 has a volume rate of about 150,000 m3/sec.
As presently dispensed from the tail of the C-97 the particles are above the
main wake, but they are in the turbulent wake of the large, bluff body fuselage
and mingle somewhat with the top of the regular vortex-wake system. From
observing the initial spread of the hygroscopic plume in night, and from
observing contrails from tail-mounted engines which suggest how such
material diffuses, it is our judgement that, a) even in smooth air, in less
than 200 seconds the hygroscopic particles are diffused by the aircraft wake
to where they will grow without appreciable competition from each other, and
b) the particles will be at an average elevation of about 30 meters below the
release point, with respect to the surrounding air. Within the cloud where
the liquid water content is 1 gm/m3, if the hygroscopic material and the
condensed water is contained in a wake-sized volume, it will represent only
about 150/0 of the cloud water, with the cloud droplets being 85%. In other
words, at least initially the coalescence process can work without major
interference between growing particles. As coalescence proceeds, the larger
particles growing on hygroscopic embryos will fall more than their neighbors
into areas with no competition. We feel that this vertical redistribution will
keep the competition problem from being really severe throughout the early
stages of coalescence growth.

The large scale diffusion of the plume within a convective cloud arisps from
atmospheric turbulence, aided especially by the aforementioned gravity spread
of particles. Quantitative aspects are reviewed by Smith, Chien, and MacCready
(1968). To generalize for the purposes of this program, in moderate turbulence
(which we experienced below cloud base most of the time in Oklahoma) the
plume will spread to a scale of several kilometers in 10-15 minutes. ln other

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