FAQ: Tropical Cyclone Modification and Myths
Doesn't the low pressure in the tropical cyclone center
cause the storm surge?
No. Many people assume that the partial vacuum at the center of a tropical
cyclone allows the ocean to rise up in response, thus causing the destructive
storm surges as the cyclone makes landfall. However, this effect would be, for
example, with a 900 mb central pressure tropical cyclone, only 1 m (3 ft). The
total storm surge for a tropical cyclone of this intensity can be from 6 to 10
m (19 to 33 ft), or more. Most (>85%) of the storm surge is caused by winds
pushing the ocean surface ahead of the storm on the right side of the track
(left side of the track in the Southern Hemisphere).
Since the surface pressure gradient (from the tropical cyclone center to the
environmental conditions) determines the wind strength, the central pressure
indirectly does indicate the height of the storm surge, but not directly. Note
also that individual storm surges are dependent upon the coastal topography,
angle of incidence of landfall, speed of tropical cyclone motion as well as the
wind strength.
Doesn't the friction over land kill tropical cyclones?
(Parts of this section are written by Sim Aberson.)
No. During landfall, the increased friction over land acts - somewhat
contradictory - to both decrease the sustained winds and also to increase the
gusts felt at the surface
(Powell and Houston 1996). The sustained (1 min or longer
average) winds are reduced because of the dampening effect of larger roughness
over land (i.e. bushes, trees and houses over land versus a relatively smooth
ocean). The gusts are stronger because turbulence increases and acts to bring
faster winds down to the surface in short (a few seconds) bursts.
However, after just a few hours, a tropical cyclone over land will begin to
weaken rapidly - not because of friction - but because the storm lacks the the
moisture and heat sources that the ocean provided. This depletion of moisture
and heat hurts the tropical cyclone's ability to produce thunderstorms near the
storm center. Without this convection,the storm rapidly fills.
An early numerical simulation
(Tuleya and Kurihara 1978) had shown that a hurricane making
landfall over a very moist region (i.e. mainly swamp) so that surface
evaporation is unchanged, intensification may result. However, a more recent
study (Tuleya
1994) that has a more realistic treatment of surface conditions
found that even over a swampy area a hurricane would weaken because of limited
heat sources. Indeed, nature conducted this experiment during Andrew as the
hurricane traversed the very wet Everglades, Big Cypress and Corkscrew Swamp
areas of southwest Florida. Andrew weakened dramatically: peak winds decreased
about 33% and the sea level pressure in the eye filled 19 mb
(Powell and Houston 1996).
Aren't big tropical cyclones also intense tropical
cyclones?
No. There is very little association between intensity (either measured by
maximum sustained winds or by central pressure) and size (either
measured by radius of 15 m/s [gale force, 30 kts, 35 mph] winds or the radius
of the outer closed isobar)
(Weatherford and Gray 1988). Hurricane Andrew is a good example
of a very intense tropical cyclone (922 mb central pressure and 64 m/s (125 kt,
145 mph) sustained winds at landfall in Florida) that was also relatively small
(15 m/s winds extended out only about 150 km [90 mi] from the center).
Weatherford and Gray (1988) also showed that changes of both
intensity and size are essentially independent of one another.
Has there ever been an attempt or experiment to reduce
the strength of a hurricane?
Contributed by Chris Landsea
The U.S. Government once supported research into methods of hurricane
modification, known as
Project STORMFURY. 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. During the STORMFURY
years scientists seeded clouds in Hurricanes Esther (1961), Beulah (1963),
Debbie (1969), and Ginger (1971). The experiments took place over the open
Atlantic far from land. The STORMFURY seeding targeted convective clouds just
outside the hurricane's eyewall in an attempt to form a new ring of clouds
that, it was hoped, would compete with the natural circulation of the storm and
weaken it. The idea was that the silver iodide would enhance the thunderstorms
of a rainband by causing the supercooled water to freeze, thus liberating the
latent heat of fusion and helping a 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. For cloud seeding to be successful, the
clouds must contain sufficient supercooled water (water that has remained
liquid at temperatures below the freezing point, 0°C/32°F). Neat idea, but it,
in the end, had a fatal flaw. Observations made in the 1980s showed that most
hurricanes don't have enough supercooled water for STORMFURY seeding to work -
the buoyancy in hurricane convection is fairly small and the updrafts
correspondingly small compared to the type one would observe in mid-latitude
continental super or multicells.
In addition, it was found that unseeded hurricanes form natural outer eyewalls
just as the STORMFURY scientists expected seeded ones to do. This phenomenon
makes it almost impossible to separate the effect (if any) of seeding from
natural changes. The few times that they did seed and saw a reduction in
intensity was undoubtedly due to what is now called "concentric eyewall cycles." Thus nature accomplishes what NOAA had
hoped to do artificially. No wonder that the first few experiments were thought
to be successes. Because the results of seeding experiments were so
inconclusive, STORMFURY was discontinued. A special committee of the National
Academy of Sciences concluded that a more complete understanding of the
physical processes taking place in hurricanes was needed before any additional
modification experiments. The primary focus of NOAA's Hurricane Research
Division today is better physical understanding of hurricanes and improvement
of forecasts. To learn about the STORMFURY project as it was called, read
Willoughby et al. (1985).
Why don't we try to destroy tropical cyclones by:
1. 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
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).
2. placing a substance on the ocean surface?
Contributed by Chris Landsea
There has been some experimental work in trying to develop a liquid that when
placed over the ocean surface would prevent evaporation from occurring. If this
worked in the tropical cyclone environment, it would probably have a limiting
effect on the intensity of the storm as it needs huge amounts of oceanic
evaporation to continue to maintain its intensity
(Simpson and Simpson 1966). However, finding a substance that
would be able to stay together in the rough seas of a tropical cyclone proved
to be the downfall of this idea.
There was also suggested about 20 years ago
(Gray et al. 1976) that the use of carbon black (or soot) might
be a good way to modify tropical cyclones. The idea was that one could burn a
large quantity of a heavy petroleum to produce vast numbers of carbon black
particles that would be released on the edges of the tropical cyclone in the
boundary layer. These carbon black aerosols would produce a tremendous heat
source simply by absorbing the solar radiation and transferring the heat
directly to the atmosphere. This would provide for the initiation of
thunderstorm activity outside of the tropical cyclone core and, similarly to
STORMFURY, weaken the eyewall convection. This suggestion has never been
carried out in real-life.
3. nuking them?
Contributed by Chris Landsea
During each hurricane season, there always appear suggestions that one should
simply use nuclear weapons to try and destroy the storms. Apart from the fact
that this might not even alter the storm, this approach neglects the problem
that the released radioactive fallout would fairly quickly move with the
tradewinds to affect land areas and cause devastating environmental problems.
Needless to say, this is not a good idea.
Now for a more rigorous scientific explanation of why this would not be an
effective hurricane modification technique. The main difficulty with using
explosives to modify hurricanes is the
amount of energy required. A fully developed hurricane can release heat
energy at a rate of 5 to 20x1013 watts and converts less than 10% of
the heat into the mechanical energy of the wind. The heat release is equivalent
to a 10-megaton nuclear bomb exploding every 20 minutes. According to the 1993
World Almanac, the entire human race used energy at a rate of 1013 watts
in 1990, a rate less than 20% of the power of a hurricane.
If we think about mechanical energy, the energy at humanity's disposal is
closer to the storm's, but the task of focusing even half of the energy on a
spot in the middle of a remote ocean would still be formidable. Brute force
interference with hurricanes doesn't seem promising.
In addition, an explosive, even a nuclear explosive, produces a shock wave, or
pulse of high pressure, that propagates away from the site of the explosion
somewhat faster than the speed of sound. Such an event doesn't raise the
barometric pressure after the shock has passed because barometric pressure in
the atmosphere reflects the weight of the air above the ground. For normal
atmospheric pressure, there are about ten metric tons (1000 kilograms per ton)
of air bearing down on each square meter of surface. In the strongest
hurricanes there are nine. To change a Category 5 hurricane into a Category 2
hurricane you would have to add about a half ton of air for each square meter
inside the eye, or a total of a bit more than half a billion (500,000,000) tons
for a 20 km radius eye. It's difficult to envision a practical way of moving
that much air around.
Attacking weak tropical waves or depressions before they have a chance to grow
into hurricanes isn't promising either. About 80 of these disturbances form
every year in the Atlantic basin, but only about 5 become hurricanes in a
typical year. There is no way to tell in advance which ones will develop. If
the energy released in a tropical disturbance were only 10% of that released in
a hurricane, it's still a lot of power, so that the hurricane police would need
to dim the whole world's lights many times a year.
4. adding water absorbing substances?
Contributed by Chris Landsea
"Dyn-O-Gel" is a special powder (produced by Dyn-O-Mat) that absorbs large
amounts of moisture and then becomes a gooey gel. It has been proposed to drop
large amounts of the substance into the clouds of a hurricane to dissipate some
of the clouds thus helping to weaken or destroy the hurricane.
At HRD we tried the one possible way that "Dyn-O-Gel" could weaken a hurricane
in the MM5 numerical model. We saw an effect but it was small (~1 m/s). The
argument was that the glop would make raindrops lumpy (i. e., non-aerodynamic)
they would fall slower and increase condensate loading, thus weakening the
eyewall updraft. If, by contrast, one increases the fall speed of the
hydrometeors, the storm strengthens (again by only ~1 m/s). In the numerical
experiments "decrease" meant reduce the fall velocity to half the real value,
and "increase" meant double the real value. The foregoing effect is larger than
anything one could hope to produce in the real atmosphere.
The observation that the experiment that "Dyn-O-Gel" conducted actually
"dissipated" clouds is problematic. Did they watch any unmodified clouds ?
Isolated Florida cumuli have short lifetimes, and these are just the ones an
experimenter would logically pick.
Accepting for the sake of argument that they actually did have an effect, the
descriptions seem more consistent with an increase in hydrometeor fall speed
and accelerated collision coalescence, which the numerical model results argue
would strengthen the hurricane, but not much. If this speculation proves to be
correct, "Dyn-O-Gel" might be useful for rainmaking during a dry spell, unlike
glaciogenic seeding which (in the tropics at least) tends to make rainy days
even more rainy--if it does anything at all.
One of the biggest problems is, however, that it would take a
LOT of the stuff to even hope to have an impact. 2 cm of rain falling
over 1 square kilometer of surface deposits 20,000 metric tons of water. At the
2000-to-one ratio that the "Dyn-O-Gel" folks advertise, each square km would
require 10 tons of goop. If we take the eye to be 20 km in diameter surrounded
by a 20km thick eyewall, that's 3,769.91 square kilometers, requiring 37,699.1
tons of "Dyn-O-Gel". A C-5A heavy-lift transport airplane can carry a 100 ton
payload. So that treating the eyewall would require 377 sorties. A typical
average reflectivity in the eyewall is about 40 dB(Z), which works out to 1.3
cm/hr rain rate. Thus to keep the eyewall doped up, you'd need to deliver this
much "Dyn-O-Gel" every hour-and-a-half or so. If you crank the reflectivity up
to 43 dB(Z) you need to do it every hour. (If the eyewall is only 10 km thick,
you can get by with 157 sorties every hour-and-a-half at the lower
reflectivity.)
5. cooling the surface waters with icebergs or deep
ocean water?
Contributed by Neal Dorst
Since hurricanes draw their energy from warm ocean water, some proposals have
been put forward to tow icebergs from the arctic zones to the tropics to cool
the sea surface temperatures. Others have suggested pumping cold bottom water
in pipes to the surface, or releasing bags of cold freshwater from near the
bottom to do this.
Consider the scale of what we are talking about. The critical region in the
hurricane for energy transfer would be under or near the eyewall region. If the
eyewall was thirty miles (48 kilometer) in diameter, that means an area of
nearly 2000 square miles (4550 square kilometers). Now if the hurricane is
moving at 10 miles an hour (16 km/hr) it will sweep over 7200 square miles
(18,650 square kilometers) of ocean. That's a lot of icebergs for just 24 hours
of the cyclone's life.
Now add in the uncertainty in the track, which is currently 100 miles (160 km)
at 24 hours and you have to increase your cool patch by 24,000 sq mi (38,000 sq
km). For the iceberg towing method you would have to increase your lead time
even more (and hence the uncertainty and area cooled) or risk your fleet of
tugboats getting caught by the storm.
For the bag/pipe method you would have to preposition your system across all
possible approaches for hurricanes. Just for the US mainland from Cape Hatteras
to Brownsville would mean covering 528,000 sq mi (850,000 sq km) of ocean floor
with devices.
Lastly, consider the creatures of the sea. If you suddenly cool the surface
layer of the ocean (and even turn it temporarily fresh), you would alter the
ecology of that area and probably kill most of the sea life contained therein.
A hurricane would be devastating enough on them without our adding to the
mayhem.
Last updated August 13, 2004
6. harnessing their energy?
Contributed by Neal Dorst
If someone can figure out a way to harness that energy, the more power to them. They could earn millions of dollars and the gratitude of everyone on the shore. Every dyne of energy harvested would be one less dyne blowing over trees.
The biggest technical impediment is that a hurricane's energy is low grade. It's abundant, but it's spread over a tremendous area. For energy to be high grade it should be concentrated, making it easy to gather and use. You would need a field of wind turbines covering dozens of square miles in order for it to be profitable. And it would have to be mobile, so you could intercept landfalling storms, or chase those that change direction. Of course, you have to expend energy to move them around, so you run the risk of losing money on the operation. The same is true of wave turbines plus you would need to find a way of anchoring them securely without compromising mobility.
It would be a daunting technical task, plus you have to worry about your turbines being robust enough to sustain damage from windblown debris and be able to transmit the energy gathered quickly. So after you draw up your engineering specs, you'd better have an investor or two, because it will cost you a great deal of money to build so many of these reinforced, mobile turbine units even before you collect you first erg.
7. altering their heat balance using high altitude particles?
Contributed by Neal Dorst
The idea here is to spread a layer of sunlight absorbing or reflecting particles (such as micro-encapsulated soot, carbon black, or tiny reflectors) at high altitude around a hurricane. This would prevent solar radiation from reaching the surface and cooling it, while at the same time increase the temperature of the upper atmosphere. Being vertically oriented, tropical cyclones are driven by energy differences between the lower and upper layer of the troposphere. Reducing this difference should reduce the forces behind hurricane winds.
It would take a tremendous amount of whichever substance you choose to alter the energy balance over a wide swath of the ocean in order to have an impact on a hurricane. One would hope that this substance would eventually disperse or disintegrate and not have a terrible impact on the earth's ecology. Knowing where to place it would also be tricky. You don't want to heat up the wrong area of the atmosphere or you could put more energy into the cyclone. These proposals would require a great deal of precisely-timed, coordinated activity to spread the layer, while running the risk of doing more harm than good. Many computer simulations should be run before any field test were tried.
References:
Gray, W.M., W.M. Frank, M.L. Corrin, C.A. Stokes, 1976: Weather Modification by Carbon Dust Absorption of Solar Energy, J. of Appl. Meteor., 15 4, pp. 355-386.
Last modified 11/6/2007
8. 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
9. other means?
Contributed by Chris Landsea
There have been numerous techniques that we have considered over the years to
modify hurricanes: seeding clouds with dry ice or Silver Iodide, cooling the
ocean with cryogenic material or icebergs, changing the radiational balance in
the hurricane environment by absorption of sunlight with carbon black,
exploding the hurricane apart with hydrogen bombs, and blowing the storm away
from land with giant fans, etc. (Some of these have been addressed in detail in
this section of FAQ's.) As carefully reasoned as some of these suggestions are,
they all share the same shortcoming: They fail to appreciate the size and power
of tropical cyclones. For example, when Hurricane Andrew struck South Florida
in 1992, the eye and eyewall devastated a swath 20 miles wide. The heat energy
released around the eye was 5,000 times the combined heat and electrical power
generation of the Turkey Point nuclear power plant over which the eye passed.
The kinetic energy of the wind at any instant was equivalent to that released
by a nuclear warhead. Perhaps if the time comes when men and women can travel
at nearly the speed of light to the stars, we will then have enough energy for
brute-force intervention in hurricane dynamics.
Human beings are used to dealing with chemically complex biological systems or
artificial mechanical systems that embody a small amount (by geophysical
standards) of high-grade energy. Because hurricanes are chemically simple --air
and water vapor -- introduction of catalysts is unpromising. The energy
involved in atmospheric dynamics is primarily low-grade heat energy, but the
amount of it is immense in terms of human experience.
Attacking weak tropical waves or depressions before they have a chance to grow
into hurricanes isn't promising either. About 80 of these disturbances form
every year in the Atlantic basin, but only about 5 become hurricanes in a
typical year. There is no way to tell in advance which ones will develop. If
the energy released in a tropical disturbance were only 10% of that released in
a hurricane, it's still a lot of power, so that the hurricane police would need
to dim the whole world's lights many times a year.
Perhaps some day, somebody will come up with a way to weaken hurricanes
artificially. It is a beguiling notion. Wouldn't it be wonderful if we could do
it?
Perhaps the best solution is not to try to alter or destroy the tropical
cyclones, but just learn to co-exist better with them. Since we know that
coastal regions are vulnerable to the storms, building codes that can have
houses stand up to the force of the tropical cyclones need to be
enforced. The people that choose to live in these locations should be willing
to shoulder a fair portion of the costs in terms of property insurance - not
exorbitant rates, but ones which truly reflect the risk of living in a
vulnerable region. In addition,
efforts to educate the public on effective preparedness needs to
continue. Helping poorer nations in their mitigation efforts can also result in
saving countless lives. Finally, we need to continue in our efforts to better
understand and observe hurricanes in order to more accurately predict their
development, intensification and track.
During a hurricane are you supposed to have the windows
and doors on the storm side closed and the windows and doors on the lee side
open ?
Contributed by Chris Landsea
No! All of the doors and windows should be closed (and shuttered) throughout
the duration of the hurricane. The pressure differences between inside your
house and outside in the storm do not build up enough to cause any damaging
explosions. (No house is built airtight.)
The winds in a hurricane are highly turbulent and an open window or door - even
if in the lee side of the house - can be an open target to flying debris. All
exterior windows should be boarded up with either
wooden or metal shutters.
Should I tape my windows when a hurricane threatens?
No, it is a waste of effort, time, and tape. It offers little strength to the
glass and NO protection against flying debris. After the storm passes you will
spend many a hot summer afternoon trying to scrape the old, baked-on tape off
your windows (assuming they weren't shattered). Once a Hurricane Warning has
been issued you would be better off spending your time putting up
shutters over doors and windows.