C) Tropical cyclones myths and transformation

C1) Doesn't the low pressure in the tropical cyclone center cause the storm surge?
C2) Doesn't the friction over land kill tropical cyclones?
C3) Aren't big tropical cyclones also intense tropical cyclones?
C4) Has there ever been an attempt or experiment to reduce the strength of a hurricane ?
C5) Why don't we try to destroy tropical cyclones by:

seeding them with silver iodide ?
placing a substance on the ocean surface ?
nuking them ?
adding water absorbing substances ?
cooling the surface waters with icebergs or deep ocean water?
harnessing their energy ?
other means ?

C6) 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 ?
C7) Should I tape my windows when a hurricane threatens ?

Subject C1) Doesn't the low pressure in the tropical cyclone center cause the storm surge ?

Contributed by Chris Landsea.

NB: storm tide is storm surge which is added to the astronomical tide (subject A6).

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.

Subject C2) 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).

Subject C3) Aren't big tropical cyclones also intense tropical cyclones?

Contributed by Chris Landsea.

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.

Subject C4) 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).

Subject C5) 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 Project STORMFURY, 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 a water absorbing substance ?

Contributed by Hugh Willoughby (FIU)

"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.

6. ... harnessing the energy of tropical cyclones?

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. ... etc ... ?

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.

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 need 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.

Subject C6) 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.

Subject C7) Should I tape my windows when a hurricane threatens?

Contributed by Chris Landsea

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.