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Subject: C5a) Why don't we try to
destroy tropical cyclones by seeding them with silver iodide:
Contributed by Chris Landsea
Actually for a couple decades
NOAA and its predecessor tried to weaken hurricanes by dropping
silver iodide - a substance that serves as a effective ice nuclei -
into the rainbands of the storms. The
STORMFURY project , as it was called, proposed that the silver
iodide would enhance the thunderstorms of the rainband by causing
the supercooled water to freeze, thus liberating the latent heat of
fusion and helping the rainband to grow at the expense of the
eyewall. With a weakened convergence to the eyewall, the
strong inner core winds would also weaken quite a bit. Neat
idea, but it, in the end, had a fatal flaw: there just isn't
much supercooled water available in hurricane convection - the
buoyancy is fairly small and the updrafts correspondingly small
compared to the type one would observe in mid-latitude continental
super or multicells. The few times that they did seed and saw
a reduction in intensity was undoubtedly due to what is now
called "concentric eyewall cycles".
![concentric eyewalls](graphics/small_concen.jpg)
Willoughby et al.(1985)
Concentric eyewall cycles naturally occur
in intense tropical cyclones (wind > 50 m/s [100 kt, 115 mph]).
As tropical cyclones reach this threshold of intensity, they usually
- but not always - have an eyewall and radius of maximum winds that
contracts to a very small size, around 10 to 25 km [5 to 15 mi].
At this point, some of the outer rainbands may organize into an
outer ring of thunderstorms that slowly moves inward and robs the
inner eyewall of its needed moisture and momentum. During this
phase, the tropical cyclone is weakening (i.e. the maximum winds
die off a bit and the central pressure goes up). Eventually the
outer eyewall replaces the inner one completely and the storm
can be the same intensity as it was previously or, in some
cases, even stronger. A concentric eyewall cycle occurred
in Hurricane Andrew (1992) before landfall near Miami: a
strong intensity was reached, an outer eyewall formed, this
contracted in concert with a pronounced weakening of the storm,
and as the outer eyewall completely replaced the original one
the hurricane reintensified.
Thus nature accomplishes what NOAA had hoped to do
artificially. No wonder that the first few experiments were
thought to be successes. To learn about the STORMFURY project
read Willoughby et al. (1985).
To learn more about concentric eyewall cycles, read
Willoughby et al. (1982)
and Willoughby (1990).
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