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| "I myself have experienced only one real disaster, Hurricane Andrew, and it was considerably different from the disaster movies that I've seen. For one thing, in the movies, there's always some kind of romance interest; whereas after Hurricane Andrew, nobody in the affected area was able to take a shower for approximately two months. Everybody smelled like a cologne named Eau de Dead Goat. The most romantic thing people did during that time was refuel each other's generators." --- Dave Barry, 1997 |
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FAQ: HURRICANES, TYPHOONS, AND TROPICAL CYCLONES
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OUTLINE
Part I:
Section A : BASIC DEFINITIONS
Section B : TROPICAL CYCLONE NAMES
Section C : TROPICAL CYCLONE MYTHS
Part II
Section D : TROPICAL CYCLONE WINDS
Section E : TROPICAL CYCLONE RECORDS
Part III
Section F : TROPICAL CYCLONE FORECASTING
Section G : TROPICAL CYCLONE CLIMATOLOGY
Section H : TROPICAL CYCLONE OBSERVATION
Part IV
Section I : Real Time Information
Section J : Historical Information
Section K : Preparedness Information
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The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).
Tropical cyclones with maximum sustained surface winds (see note below) of less than 17 m/s (34 kt) are called "tropical depressions". (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-) Once the tropical cyclone reaches winds of at least 17 m/s they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt), then they are called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993).
Note that just the definition of "maximum sustained surface winds" depends upon who is taking the measurements. The World Meteorology Organization guidelines suggest utilizing a 10 min average to get a sustained measurement. Most countries utilize this as the standard. However the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) of the USA use a 1 min averaging period to get sustained winds. This difference may provide complications in comparing the statistics from one basin to another as using a smaller averaging period may slightly raise the number of occurrences (Neumann 1993).
Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000km or so) of the Cape Verde Islands and then become hurricanes before reaching the Caribbean. (That would be my definition, there may be others.) Typically, this may occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2.
A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center in Guam for typhoons that reach maximum sustained 1-minute surface winds of at least 130 kt (240 km/h). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.
It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km with a maximum at 3 km) westward traveling disturbances often serve as the "seedling" circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability - known as baroclinic-barotropic instability - is where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast.
The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km, typically (Burpee 1974). One should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year.
While only about 60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the major hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995).
It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic (and East Pacific).
A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 60-125 miles [100-200 km] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 64 kt (33 m/s).
Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).
Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds: less than 34 kt (18 m/s) - "subtropical depression", greater than or equal to 34 kt (18 m/s) - "subtropical storm". Note that while these are not given names, they are warned on and forecasted for by the National Hurricane Center similar to the treatment received by tropical cyclones in the region.
The tropical cyclone is a low-pressure system which derives its energy primarily from evaporation from the sea in the presence of high winds and lowered surface pressure and the associated condensation in convective clouds concentrated near its center (Holland 1993). Mid-latitude storms (low pressure systems with associated cold fronts, warm fronts, and occluded fronts) primarily get their energy from the horizontal temperature gradients that exist in the atmosphere.
Structurally, tropical cyclones have their strongest winds near the earth's surface (a consequence of being "warm-core" in the troposphere), while mid-latitude storms have their strongest winds near the tropopause (a consequence of being "warm-core" in the stratosphere and "cold-core" in the troposphere). "Warm-core" refers to being relatively warmer than the environment at the same pressure surface ("pressure surfaces" are simply another way to measure height or altitude).
While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of *kilometers* and is comprised of several to dozens of convective storms. Additionally, while tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis, tropical cyclones require very low values (less than 10 m/s or 20 kt) of tropospheric vertical shear in order to form and grow. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). In contrast, tropical cyclones are purely an oceanic phenomena - they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes.
An interesting side note is that tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker than the winds at, say, 850 mb. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone's right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone's left side - due to the reverse spin of southern hemisphere storms. (Novlan and Gray 1974)
Tropical cyclones spawn tornadoes when certain instability and vertical shear criteria are met, in a manner similar to other tornado producing systems. However, in tropical cyclones, the vertical structure of the atmosphere differs somewhat from that most often seen in mid-latitude systems. In particular, most of the thermal instability is found near or below 10,000 feet altitude, in contrast to mid-latitude systems, where the instability maximizes typically above 20,000 feet. Because the instability in TC's is focused at low altitudes, the storm cells tend to be smaller and shallower than those usually found in most severe mid-latitude systems. But because the vertical shear in TC's is also very strong at low altitudes, the combination of instability and shear can become favorable for the production of small supercell storms, which have an enhanced likelihood of spawning tornadoes compared to ordinary thunderstorm cells (Novlan and Gray 1974, Gentry 1983, McCaul 1991).
Almost all tropical cyclones making landfall in the United States spawn at least one tornado, provided enough of the TC's circulation moves over land. This implies that Gulf coast landfalling TC's are more likely to produce tornadoes than Atlantic coast TC's that "sideswipe" the coastline. The rate at which TC's produce tornadoes (waterspouts) over the ocean is unknown, although Doppler radars have identified many cases where storm cell rotation suggestive of the presence of tornadoes was observed over water (Novlan and Gray 1974, Spratt et al. 1997).
In the northern hemisphere, the right-front quadrant (relative to TC motion) is strongly favored. In the southern hemisphere, the left-front quadrant presumably is favored, although there is little research on this point. Most of the tornadoes form in outer rainbands some 50-200 miles from the TC center, but some have been documented to occur in the inner core, or even in the TC eyewall (Novlan and Gray 1974, McCaul and Weisman 1996, Spratt et al. 1997).
TC's may spawn tornadoes up to three days after landfall. Statistics show that most of the tornadoes occur on the day of landfall, or the next day. The most likely time for tornadoes is during daylight hours, although they can occur during the night too (Novlan and Gray 1974, Gentry 1983).
In general, it appears that TC tornadoes are somewhat weaker and briefer than mid-latitude tornadoes. There have been no F5-rated TC tornadoes in the past 50 years of reliable data, and only two F4's. There have, however, been numerous F3's, and some of these have caused many casualties and much damage. Of course, we cannot rule out the possibility that a future TC might spawn an F5 tornado (Gentry 1983, McCaul 1991).
Hurricane Beulah spawned a reported 141 tornadoes in southeast Texas during the first several days after its landfall in September 1967 (Orton 1970). This number of tornadoes represents one of the largest tornado outbreaks of any kind in the U. S. tornado climatology. In 1992, Hurricane Andrew spawned 62 tornadoes. It is difficult to predict which TCs will produce large tornado outbreaks, although there is some indication that the likelihood of a major outbreak increases as TC size and intensity increase.
What is the deadliest single TC-spawned tornado?One of the tornadoes spawned in October 1964 by Hurricane Hilda killed 22 people in Larose, LA (Novlan and Gray 1974).
What is the most damaging single TC-spawned tornado?One of the tornadoes produced by Hurricane Allen in 1980 did about $100 million damage, in recent dollars, in the Austin, TX, area (Gentry 1983).
TC tornadoes are often spawned by unusually small storm cells that may not appear particularly dangerous on weather radars, especially if the cells are located more than about 60 miles from the radar. In addition, these small storms often tend to produce little or no lightning or thunder, and may not look very threatening visually to the average person. Furthermore, the tornadoes are often obscured by rain, and the storm cells spawning them may move rapidly, leaving little time to take evasive action once the threat has been perceived. (McCaul et al. 1996, Spratt et al. 1997).
"CDO" is an acronym that stands for "central dense overcast". This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches hurricane strength (64 kt or 33 m/s), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO's are indicative of favorable, low vertical shear environments.
A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an "upper cold low". TUTTs are different than mid- latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of harmful vertical wind shear over tropical disturbances and tropical cyclones. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995).
To undergo tropical cyclogenesis, there are several favorable precursor environmental conditions that must be in place (Gray 1968, 1979):
Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages: stage 1 occurs when the MCC produces a mesoscale vortex and stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.
(A good portion of this section was written by Sim Aberson.)
The "eye" is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10 C [18 F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2 C [0-3 F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988). The eye is surrounded by the eyewall, the roughly circular area of deep convection which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of of the subsiding air. Most soundings taken within the eye show a low-level layer which is relatively moist, with an inversion above - suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km of the surface.
The general mechanisms by which the eye and eyewall are formed are not fully understood, although observations have shed some light on the problem. The calm eye of the tropical cyclone shares many qualitative characteristics with other vortical systems such as tornadoes, waterspouts, dust devils and whirlpools. Given that many of these lack a change of phase of water (i.e. no clouds and diabatic heating involved), it may be that the eye feature is a fundamental component to all rotating fluids. It has been hypothesized (e.g. Gray and Shea 1973, Gray 1991) that super-gradient wind flow (i.e. swirling winds that are stronger than what the local pressure gradient can typically support) present near the radius of maximum winds (RMW) causes air to be centrifuged out of the eye into the eyewall, thus accounting for the subsidence in the eye. However, Willoughby (1990b, 1991) found that the swirling winds within several tropical storms and hurricanes were within 1-4% of gradient balance. It may be though that the amount of super-gradient flow needed to cause such centrifuging of air is only on the order of a couple percent and thus difficult to measure.
Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are sometimes called spiral bands. Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995).
Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary.
Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation center of the storm (Willoughby et al. 1982, Willoughby 1990a). Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.
Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Since the storms can often last a week or longer and that more than one can be occurring in the same basin at the same time, names can reduce the confusion about what storm is being described. According to Dunn and Miller (1960), the first use of a proper name for a tropical cyclone was by an Australian forecaster early in this century. He gave tropical cyclone names "after political figures whom he disliked. By properly naming a hurricane, the weatherman could publicly describe a politician (who perhaps was not too generous with weather-bureau appropriations) as 'causing great distress' or 'wandering aimlessly about the Pacific.'" (Perhaps this should be brought back into use ;-)
During World War II, tropical cyclones were informally given women's names by USA Air Force and Navy meteorologists (after their girlfriends or wives) who were monitoring and forecasting tropical cyclones over the Pacific. From 1950 to 1952, tropical cyclones of the North Atlantic Ocean were identified by the phonetic alphabet (Able-Baker-Charlie-etc.), but in 1953 the USA Weather Bureau switched to women's names. In 1979, the World Meteorological Organization (WMO) and the USA National Weather Service (NWS) switched to a list of names that also included men's names.
The Northeast Pacific basin tropical cyclones were named using women's names starting in 1959 for storms near Hawaii and in 1960 for the remainder of the Northeast Pacific basin. In 1978, both men's and women's names were utilized.
The Northwest Pacific basin tropical cyclones were given women's names officially starting in 1945 and men's names were also included beginning in 1979.
The North Indian Ocean region tropical cyclones are not named.
The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season.
The Australian and South Pacific region (east of 90E, south of the equator) started giving women's names to the storms in 1964 and both men's and women's names in 1974/1975.
Each of the links below to the actual Storm Names will open to a new browser window. This is to save the time of having to reload this page as it is rather large. When done looking at the storm names, simply close that new browser to return here.
Northern Hemisphere Tropical Cyclone Names
Atlantic, Gulf of
Mexico, Caribbean Sea
Eastern North Pacific (east of 140W)
Central North Pacific (from the dateline to 140W)
Western North Pacific
(west of the dateline)
North Indian Ocean -
Tropical cyclones in this region are not named.
Southern Hemisphere Tropical Cyclone Names
Southwest Indian
(west of 90E)
Western Australian region (90E to 125E)
Northern Australian region (125E to 137E)
Eastern Australian region (137E to 160E, south of ~10S)
Fiji Area (160E to 120W)
Papua New Guinea
(140E to 160E, north of ~10S)
In the Atlantic basin, tropical cyclone names are "retired" (that is, not to be used again for a new storm) if it is deemed to be quite noteworthy because of the damage and/or deaths it caused. This is to prevent confusion with a historically well-known cyclone with a current one in the Atlantic basin. The following list gives the names that have been retired through the year 1998 and the year of the storm in question. (Kindly provided by Gary Padgett, Jack Beven and James Lewis Free).
"HURRICANE...derived from 'hurican', the Carib god of evil...
alternative
spellings: foracan,
foracane, furacana, furacane, furicane,
furicano, haracana, harauncana, haraucane,
haroucana, harrycain, hauracane, haurachana,
herican, hericane, hericano, herocane, herricao,
herycano, heuricane, hiracano, hirecano, hurac[s]n,
huracano, hurican, hurleblast, hurlecan, hurlecano,
hurlicano, hurrican, hurricano, hyrracano, urycan,
hyrricano, jimmycane, oraucan, uracan, uracano"
From the _Glossary of Meteorology_
It should be noted that 'hurican' was derived from the Mayan 'Hurakan', one of their creator gods, who blew his breath across the Chaotic water and brought forth dry land.
No. Many people assume that the partial vacuum at the center of a tropical cyclone allows the ocean so 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 1.0 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.
(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).
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] 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) sustained winds at landfall in Florida) that was also relatively small (15 m/s winds extended out only about 150 km from the center). Weatherford and Gray (1988) also showed that changes of both intensity and size are essentially independent of one another.
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 idea was 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 or 100 kt). 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. 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 re-intensified.
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 as it was called, read Willoughby et al. (1985). To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990a).
As for the other ideas, 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 detrimental 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.
Lastly, there always appears ideas during the hurricane season that one should simply use nuclear weapons to try and destroy the storms. Apart from the concern that this might not even alter the storm, this approach neglects the problem that the released radiation would fairly quickly move with the trade winds to over land. Needless to say, this is not a good idea.
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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, enforce building codes that can have houses stand up to the force of the tropical cyclones. Also the people that choose to live in these locations should 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.
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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.
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.
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Hurricane FAQ is Provided Courtesy of:
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Chris Landsea .
NOAA AOML/Hurricane Research Division Voice: (305) 361-4357
4301 Rickenbacker Causeway Fax: (305) 361-4402
Miami, Florida 33149 Internet: landsea@aoml.noaa.gov
http://www.aoml.noaa.gov/hrd/Landsea/landsea_bio.html
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Last Revised: November 01, 2006 04:59 PM.
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