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Subject: C5h) Why don't we try to destroy tropical cyclones by
seeding them with hygroscopic particles ?
Contributed by Neal Dorst
Hygroscopic refers to substances which tend to bind
preferentially with water vapor molecules. Anyone who has used a salt shaker
in humid conditions knows about this, as the hygroscopic salt absorbs water
vapor from the air and clumps near the top of the shaker, clogging the holes.
Some people have proposed seeding the inflow layer of a hurricane with
granules of some hygroscopic substance. The hope is that these granules
will help form tiny cloud droplets, many more than would form naturally.
This would tend to 'lock up' the moisture in small droplets, rather than
allowing the formation of large drops, which tend to fall out as rainfall.
This would cause a weight burden on the inflow, and reduce the hurricane's
winds.
There are several assumptions made in this chain of logic. The first is
that there are too few cloud condensation nuclei (CCN) available naturally.
If there aren't, then adding more wouldn't change things. The next assumption
is that more numerous but smaller cloud drops wouldn't coalesce into larger
drops, even in the turbulent updraft of a hurricane eyewall. And lastly, it
assumes that the increased burden on the updraft outweighs the increase in
latent heat released when more liquid water reaches the freezing level. If
less water is precipitating out, then more will be freezing.
That's a lot of assumptions, and it would have to be proven in computer
models first, then in field tests, that they are valid. Otherwise, you
would expend a great deal of money and effort, but not change a hurricane
sufficiently.
References
Woodcock, A.H., D.C. Blanchard, C.G.H. Rooth, 1963: Salt-Induced Convection
and Clouds, J. of Atmos. Sci., 20, 2, pp. 159-169.
Blanchard, D.C., A.H. Woodcock, 1980: The Production, Concentration, and
Vertical Distribution of the Sea-salt Aerosol, Ann. NY Acad. Sci.,
338, 1, p. 330-347.
Last modified 11/6/2007
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