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DOWN FOR A BREAKDOWN OF HURRICANE MECHANICS
Please send any questions or comments to cat5hurricane@hotmail.com
Tropical thunderstorms that organize around the center of a
hurricane act like carburetors, injecting fuel into the hurricane's eye, the
nearly circular cylinder of relative calm at the center of the storm. The fuel
injected into the eye is latent heat, a high-premium grade released when
rising, invisible water vapor condenses into cloud droplets within
thunderstorms. Interrupt or lessen the flow of latent heat into the eye and the
engine sputters or stalls. Before we tinker with the parts of this engine, we
need to read its operating manual.
Just as the operating manual for your car's engine offers
tips for peak performance under various weather conditions, there are optimum
conditions that promote and support the ignition (and subsequent smooth
running) of the heat engine of a hurricane. Here is the tropical manual for
successfully starting up a hurricane's heat engine:
Of the three guidelines, warm ocean water is the most
important for ignition. Water evaporating from warm tropical oceans tops off
the lower and middle troposphere's gas tank of water vapor. A full tank ensures
a great release of latent heat once thunderstorms organize around the center of
a developing hurricane, helping to push the heat engine to full throttle. In
coming sections, we will learn how the release of latent heat drives the
development of a hurricane.
For maximum efficiency, water beneath the surface of tropical oceans needs
to be warm as well. As a hurricane develops, its revving winds stir the ocean,
mixing warm surface water with water below. If the water below the surface is
also warm, the mixing will keep surface water temperatures high, thereby
maintaining high rates of evaporation and guaranteeing a rich supply of water
vapor.
The heat engine of a hurricane has a rotary motor in which fuel-injected
thunderstorms spiral around a cylindrical-shaped eye. For engine parts to turn,
the Coriolis effect must be sufficiently strong. Climatological studies of the
tropics reveal that ignition of hurricane heat engines does not occur within 5ø
latitude of the equator, where the Coriolis effect is simply too weak to induce
a circulation (note that the basins of hurricane genesis do not extend to the
equator in Figure 11.2).
Ignition becomes increasingly more likely as latitudes increase from 5o
to 15o. Without a contribution from the Coriolis effect, winds would
blow almost directly from high to low pressure, precluding the development of
the circulation needed to organize thunderstorms around an eye.
It is also crucial to the ignition and smooth running of a hurricane for the
engine's cylinder to receive an uninterrupted supply of high-octane latent
heat. If upper-level winds are too strong, latent heat released from the tops
of thunderstorms will be blown away from the eye. Essentially, strong
upper-level winds blowing latent heat away from the center of a hurricane can
be likened to a leak in the fuel line of a hurricane.
In addition, winds blowing from a uniform direction throughout the
troposphere are important for ignition and smooth operation. Later in the
chapter, we will learn that many hurricanes are steered from east to west on
the equatorward flanks of subtropical high pressure systems. Sometimes,
however, high-level winds at or around the 250-mb level may blow against the
grain from west to east. These westerlies shear the tops off hurricanes,
decapitating thunderstorms and interrupting the flow of fuel. Warning: tropical
heat engines may sputter and stall if steered into regions with strong
high-level westerlies.
If you were to take a sharpened pencil to the task of isoplething
temperature and pressure in the tropics, the point would probably not get very
dull (assuming there aren't any hurricanes). You wouldn't draw many isotherms
or isobars because the tropics, in general, have uniform distributions of
surface temperature and pressure. Therefore, recalling earlier mid- latitude
analyses, you might conclude that the tropics should be devoid of low pressure
systems. After all, low pressure systems in the mid-latitudes derive their
strength from horizontal gradients of temperature and moisture in the presence
of strong upper-level divergence and well-defined fronts. How can a
pencil-dulling, isobar-packed low pressure system like a hurricane emerge from
such uniformity?
As discussed in Chapter 7, the Intertropical Convergence Zone (ITCZ)
meanders around the globe in tropical regions. The ITCZ is a necklace of
showers and thunderstorms that girdles equatorial regions, evidence of the
powerful convergence and lifting that occurs there. Occasionally, a cluster of
thunderstorms breaks away from the ITCZ and provides the spark to ignite the
heat engine of a hurricane.
Another spark that can ignite the hurricane heat engine is an easterly wave,
a cluster of thunderstorms that travels from east to west (hence, its name) in
the tropical troposphere. In the equable tropics, locating the pressure
signature of an easterly wave on surface weather maps can be rather difficult.
Instead, meteorologists look at maps of low-level streamlines to locate areas
of wind convergence associated with easterly waves (see Figure 11.4). These areas
of convergence promote clusters of thunderstorms that can, under the operating
conditions outlined in the heat engine manual, fuel a hurricane.
Sometimes meteorologists lump disorganized clusters of tropical
thunderstorms into the generic classification of tropical disturbances.
Assuming that the optimum conditions outlined in the operating manual apply, we
will now discuss the nuts and bolts of how tropical disturbances evolve into
the highly structured tropical system we call a hurricane.
As latent heat is released from clusters of thunderstorms
that form a tropical disturbance, ill-defined areas within the cluster begin to
warm. In response, air density lowers, prompting surface pressures to fall.
Embryonic tropical disturbances often toss the point of lowest pressure around
like a hot potato. But eventually, an area of weak low pressure will emerge if
thunderstorms within an especially intense cluster congregate and combine their
latent heat to establish a central warm core of air.
In response, tropical surface winds begin to increase in speed and converge
around the incipient low, importing richer supplies of moisture toward the
center. In turn, thunderstorms increase in intensity and begin to multiply in
number. The release of latent heat now escalates, and surface pressure,
responding to the warming, falls even more, causing converging, moist winds to accelerate
further. If the system is far enough from the equator (generally at least 8 to
9o of latitude), the Coriolis effect will induce these fledgling
winds to circulate counterclockwise inward towards the area of lowest pressure.
When sustained winds reach 37 km/hr (23 mph), the tropical disturbance
graduates into a tropical depression. The heat engine of the budding hurricane
begins to chug to life.
Meanwhile, air pressures near the tropopause, in response to the warming
from latent heat release, begin to increase (recall Chapter 4). In response to
higher pressure aloft, air begins to flow outward (that is, diverge) around the
top of the center of the tropical depression. Like a chimney, this upper-level
area of high pressure vents the tropical depression, preventing air converging
at lower levels from piling up around the center (which would raise surface air
pressures and squelch the storm). Assuming optimum conditions in the heat
engine manual still apply, this feedback process between the release of latent
heat, the subsequent drop in surface pressure, and the corresponding increase
in surface winds, will continue. When winds become sustained at 62 km/hr (39
mph) or greater, the tachometer mounted on the heat engine now reads tropical
storm. Figure 11.5 is a
visible photograph of Tropical Storm Iniki (1992), taken from the space
shuttle. The circulation around a center is clearly visible in the cloud
pattern, but there is no eye (a lack of an eye is characteristic of a tropical
storm).
Once the system reaches tropical storm status, it is given a name, a
tradition started in 1950 (for storms in the Atlantic Basin) with the use of
World War II vintage code names such as Able, Baker, Charlie, Dog, and Easy.
Female names were first used in 1953, and the alternation of male and female
names for Atlantic Basin storms began in 1979. There are separate lists of
names for storms forming in the various basins. The National Hurricane Center
(NHC) near Miami has responsibility for keeping tabs on storms in the north
Atlantic and eastern Pacific Oceans. Between longitude 140oW and the
International Date Line, the Hurricane Center in Honolulu assumes
responsibility for monitoring tropical systems. Once a storm moves west of the
Date Line, the Hurricane Center on Guam takes over. These three hurricane
centers all shared responsibility in tracking Hurricane John in August and
early September of 1994. John, whose track is shown in Figure 11.6, was the
longest-lived named storm on record and the most powerful hurricane ever
observed in the central Pacific, at one time packing winds of 276 km/hr (170
mph).
Like great baseball players, names used for hurricanes can be retired if the
storm is exceptionally noteworthy. No future Atlantic storm will ever bear the
infamous names of Camille (1969), Agnes (1972), Hugo (1989), Bob (1991), Andrew
(1992) or Opal (1995), to name a few. Names not retired are rotated into use
every six years. Table 11.2 shows the names selected for tropical storms
through the year 2000 in both the Atlantic and eastern Pacific Basins
We are now ready to bring our test engine to full throttle. Once again,
assuming optimum conditions still prevail as outlined in the hurricane
operator's manual, the feedback process between the release of latent heat,
decreasing air pressure at the surface, and increasing surface winds will
ultimately upgrade sustained wind speeds to 119 km/hr (74 mph). The heat engine
has gone from the high gear of a tropical storm to the turbo-charged overdrive
of a hurricane.
Conditions across the entire Atlantic basin were certainly near optimum in
late August 1996 when the visible satellite image of Figure 11.7 was taken. This
view of the Atlantic basin shows formidable Hurricane Edouard (packing maximum
sustained winds of 190 km/hr (120 mph)), strong Tropical Storm Fran (with winds
of nearly 110 km/hr (70 mph)), weak Tropical Storm Gustav (with winds of barely
65 km/hr (40 mph), on the verge of weakening to a tropical depression), and a
tropical disturbance off the coast of Africa (which, nearly two weeks later, would
skirt the eastern Bahamas as Hurricane Hortense with winds of 215 km/hr (135
mph)). Situations when four tropical systems are engaged in the Atlantic Basin
are rather unusual, although the record-setting hurricane season of 1995 also
produced several windows to catch four storms (or budding storms)
simultaneously.
A look under the hood at the heat engine of a full-fledged
hurricane reveals its working parts (see Figure 11.8). The eye,
undoubtedly the most distinctive feature of a hurricane, is an island of
tranquility in the midst of a sea of storminess. The eye is a nearly circular
cylinder of calm or light winds and partly cloudy skies, usually with a diameter
ranging from 8 to 80 km (5 to 50 mi). The sunny breaks that often develop in
the eye are caused by compensating subsidence from the powerful thunderstorms
that surround the eye. Sinking air warms by compressional heating. Working in
tandem with latent heat, compressional warming evaporates cloud droplets within
the eye. The notion that heating is greatest in the cylinder of the eye
(forming the warm core) is supported in Figure 11.8 by the relatively
high altitude of the 32oF (0oC) isotherm above the eye.
To explain the relative calm in the eye, consider a parcel of air spiraling
in toward the center of a hurricane. As the air parcel gets closer to the
center, its velocity increases because it will tend to conserve its angular
momentum (defined in Chapter 7). Like ice skaters whose bodies spin more
rapidly as their arms are drawn inward, parcels near the surface attempt to
speed up as they spiral in towards the center of the hurricane. Suppose, for
sake of argument, we allowed the parcels to spiral inward to the exact center
of the eye. By the law of conservation of angular momentum, the velocity of
these parcels would become infinite. But a hurricane (or any weather system)
doesn't have an infinite amount of energy to support such speeds, because the
hurricane's maximum output of energy is fixed by the temperatures of the
tropical oceans over which it moves. For water temperatures in the 80 to 90oF
(27 to 32oC) range, maximum sustained winds seldom exceed 325 km/hr
(200 mph). So in order not to violate the conservation of energy, the parcel
must stop short of reaching the center, creating a cylinder of relative calm.
Typically, in an intense hurricane that has abundant energy, parcels can
spiral closer to the center without violating the law of conservation of
energy, thereby narrowing the diameter of the eye. Usually, a small,
well-defined eye is the signature of a powerful hurricane. This general rule
certainly was true in the case of Hurricane Gilbert (1988), which struck
Jamaica, then crossed the Yucatan Peninsula and finally came ashore in northern
Mexico. The satellite image of Gilbert in Figure 11.9a shows the
hurricane at 13Z on September 12, 1988, when its central pressure was 960 mb
and its maximum sustained winds were about 200 km/hr (125 mph). The eye was
approximately 55 km (35 mi) in diameter. Figure 11.9b shows Gilbert
almost 36 hours later, at 2330Z on September 13, when its central pressure had
fallen to 888 mb, with maximum sustained winds of 296 km/hr (184 mph). This
pressure of 888 mb set the record for the lowest sea-level pressure ever
observed in the Western Hemisphere. Notice how at this time, the eye had shrunk
to only 15 km (9 mi) in diameter.
To further probe the characteristics of the eye of a hurricane, consider Figure 11.10, an aerial view
of Beaver Stadium on the University Park campus of Penn State University. No,
the authors' zeal for Nittany Lion football has not distracted us. Rather,
there is an inkling of tropical meteorology in the way successive rows of seats
slope upward and away from field level. Indeed, this inclined profile of Beaver
Stadium bears a striking resemblance to the eye structure of some hurricanes.
For proof, look at Figure
11.11, a close-up view of the eye of a Pacific typhoon in 1988. Note how
the clouds surrounding the eye slant upward and away from the storm's center as
altitude increases, forming a caricature of a stadium (please use a little
imagination here).
To explain this so-called "stadium effect," we rely on the
observation that, above approximately the 700-mb level, the strength of a
hurricane typically decreases with increasing altitude (that is, the strongest
winds surrounding the eye are always found in the lower troposphere, with wind
speeds falling off at high altitudes). As evidence, consider Color Plate 38, which shows
a series of horizontal radar slices at various altitudes through Hurricane
Erin, taken when the hurricane swirled near Florida on August 2, 1995. Note how
the circular, rain-free zones associated with the eye expand with increasing
altitude. The cheerleader for this megaphone pattern of rain-free rings is the
conservation of angular momentum. Because winds circulating around the eye
taper off above 700 mb, parcels of cloudy air, which are sworn to conserve
their angular momentum, stop their inward spirals farther and farther from the
center of the storm as altitude increases. The end result is that the cloudy
walls of the eye slope upward away from the storm's center, forming a stadium
built of clouds.
Even within the relative calm of the eye, conditions can, at times, be
turbulent. Figure 11.12
is a close-up of the eye of Hurricane Emilia, taken from the space shuttle in
July 1994. At the time, Emilia was fast becoming the strongest hurricane ever
observed in the central Pacific (Gilma and John eclipsed Emilia later in August
1994, closing the books on one of the most active periods ever recorded in the
central Pacific). The turbulent look to the clouds in Emilia's eye suggests
that low-level vortices are disrupting the relative calm of the eye. Emilia's
eye is also not clear, but there are breaks in the overcast. Sometimes, clouds
can even completely fill the eye of a hurricane (even a powerful one), as was
the case with Hurricane Opal. As Opal was an hour from landfall in Figure 11.13 at 21Z on
October 4, 1995, it packed maximum sustained winds of around 200 km/hr (125
mph) and a central pressure of 940 mb, yet there were few (if any) breaks in
the clouds in its eye.
Figure 11.14 shows
the three-dimensional air flow in a computer model of a hurricane,
demonstrating how parcels of air near the surface spiral inward toward the eye
of the storm, stop short of the center, and rise, eventually flowing outward
from the eye at the top of the hurricane (also see the cross section in Figure 11.8). These rising
parcels of air support powerful thunderstorms that surround the eye. This
fierce doughnut of thunderstorms, called the eye wall, contains the hurricane's
strongest winds and heaviest rains. Figure 11.15 is a visible
image of Hurricane Elena (1985) over the Gulf of Mexico. The eye wall appears
as the elevated area of intense convection and overshooting tops surrounding
the eye. As a hurricane develops, birds sometimes get trapped in the eye by the
towering, fierce storms in the eye wall. In effect, the eye wall becomes a
tropical bird cage until the hurricane begins to fizzle. In September 1985,
thousands of birds, presumably trapped by the eye wall, were observed in the
eye of Hurricane Gloria as the storm came ashore in southern New England.
Another distinctive feature of a hurricane is its spiral bands, tentacles of
thunderstorms that pinwheel cyclonically around and into the center of the
hurricane (these bands often bear a striking resemblance to the spiral arms of
some galaxies). Hurricane Felix, seen in Figure 11.16 spinning in
the Sargasso Sea on August 14, 1995, provides a textbook example of spiral
bands. At the time, Felix was a weak hurricane, packing winds of about 135
km/hr (85 mph). Often, waves of narrow spiral bands will precede the arrival of
a hurricane, producing fitful rains and gusty winds as the bands come and go.
Hurricanes often spawn tornadoes when they make landfall (as will be discussed
in a coming section), and about 80% of these twisters form from thunderstorms
in spiral bands.
Look once again at the storm track of Hurricane John shown
in Figure 11.6. John's
path closely follows the clockwise circulation around the robust subtropical
high pressure system that resides over the North Pacific Ocean during the
summer. Among their many atmospheric duties, subtropical highs provide the
steering currents for many tropical systems. The subtropical high's clockwise
(in the Northern Hemisphere) flow of air near the 500-mb lvel is crucial to a
meteorologist's assessment of the direction of movement of a tropical storm or
hurricane. Often, hurricanes moving westward on the equatorial side of a
subtropical high curve poleward (like Hurricane John did). When and where a
hurricane will pivot poleward and recurve is often difficult to forecast,
marking a period of restlessness and uncertainty for meteorologists, especially
if the storm is nearing land.
In the North Atlantic Ocean, the Bermuda high provides the steering currents
that escort many tropical systems in a predictable, recurving path. A good
example is that of Hurricane Gilbert, whose rampage through the Caribbean and
Gulf of Mexico is shown in Figure
11.17. Other infamous storms, however, have deviated from the original
course set by the Bermuda high. Such deviations are often caused by upper-level
low pressure systems that sometimes wield more influence than the Bermuda high.
This was the situation in the case of Hurricane Hugo.
Hurricane Hugo: September 1989
Until Andrew struck southern Florida in 1992, Hugo was the costliest
hurricane in United States history, causing $7 billion in damages. Hugo was
especially devastating to areas near Charleston, SC which bore the brunt of the
powerful storm. Figure
11.18a shows Hugo's storm track, while Figure 11.18b is a visible
satellite picture of Hugo during the morning of September 21, 1989. Figure 11.19 shows the
500-mb charts during the three- day period surrounding Hugo's entry into the
United States. Arrows show the subsequent movement of the storm. The upper-
level steering currents for Hugo were constant for the duration of the storm's
approach to land: the combination of the Bermuda high and an upper-level low
pressure area in the north-central Gulf of Mexico coaxed Hugo into the South
Carolina coast. Hugo was then absorbed by an approaching trough of low pressure
after coming ashore, helping to zip the remnants of the storm quickly to the
northeast.
Hurricane Elena: Late August and early September 1985
The strength and reach of subtropical highs varies over time, creating
situations where hurricanes are left to wander somewhat aimlessly. The result
can be a hurricane path that resembles the crayon scribble of a four-year old.
In a coloring book, that's not a problem. But with a major hurricane, as was the
case with Hurricane Elena, the result is unsettling - residents from Tampa to
New Orleans fled the coast in the largest peacetime evacuation in United States
history. Elena's track is shown in Figure 11.20 (and recall
the satellite image of Elena in Figure 11.15).
Hurricane Elena carved out an erratic path over the Gulf of Mexico during
Labor Day weekend, 1985. Figure
11.21 shows the 500-mb chart over four of the five days that Elena
threatened the Gulf Coast. The arrows indicate the subsequent movement of the
storm. Elena developed near central Cuba and moved northwestward through the
Gulf of Mexico, steered by the westward-shifting Bermuda high near the Georgia
coast. On August 30, 1985, however, Elena was drawn toward a trough of low
pressure that had quickly dipped into the eastern United States. As this trough
moved eastward, Elena made a 90o turn and headed east, threatening
the west coast of Florida. But the course change was temporary, given that the
upper-level trough zipped to the northeast. Elena could not keep up. Abandoned
by steering winds, Elena remained stationary for more than 24 hours. In
response, the storm's battalion of hurricane-force winds were camped out just
off the coast of the Tampa Bay area. Finally, late on September 1, Elena began
to be influenced by a 500-mb high pressure system building to its north, which
nudged the storm into Mississippi and Louisiana. This last maneuver forced many
residents in those states to evacuate a second time.
Subtropical highs lead many hurricanes to their demise, eventually luring
them poleward toward land, colder water and strong westerlies. All three are
lethal. As a hurricane succumbs to the alien environment over middle latitudes,
it releases its energy and moisture to the surroundings. In this way, a
hurricane transfers energy and moisture out of the tropics and into the middle
latitudes. In effect, hurricanes aid the earth's general circulation in
mitigating the large temperature contrasts between the poles and equatorial
regions. Although hurricanes and tropical storms, taken as a whole, are
responsible for only about 2% of energy transport out of the tropics, they
account for as much as 30% of the energy transport during the peak of hurricane
season.
Land is where hurricanes sometimes go to die. But as the eye
of a hurricane comes onshore (that is, as the hurricane makes landfall), it
does not give up without a ferocious fight. Heavy rain, powerful winds, and
tornadoes are weapons that a hurricane brings to bear on coastal communities.
But its most destructive weapon is the storm surge, a rise in ocean levels of
up to 9 meters (about 30 ft) that accompanies the landfall of a hurricane.
Hurricane Andrew's ferocious winds caused considerable damage in southern
Florida, but the brute force of the storm surge, as evidenced in Color Plate 39, was truly
remarkable. When people think of a storm surge, they often envision dramatic
"Hawaii Five-O" style tidal waves ripping into the coastline. Some
Hollywood movie producers perpetuate a similar notion that is riddled with
misconceptions about storm surges. The 1979 film Hurricane featured defiant
beach condominium owners ignoring orders to evacuate as they partied against
the wind. Then, in dramatic fashion, a distant rumble signalled the approach of
a mountain of water that slammed into the condo, instantaneously wiping out the
rebellious revelers.
The storm surge does indeed have destructive impact upon the coast, but it
makes its presence felt in a more gradual manner. Over the open ocean, a
hurricane's violent winds push and churn up surface waters, creating waves of
many sizes. These waves propagate away from the hurricane, eventually
organizing into swells that break on distant shores, foretelling the approach
of the storm (for example, during hurricane season, surfers flock to the
southern shores of Hawaii to ride the big waves generated by storms passing
south of the Islands). Likewise, in the vicinity of a seafaring hurricane,
there is little rise in ocean levels. There is virtually no storm surge.
It's a different matter once the storm approaches shore. Building onshore
winds start to push water toward land. As water approaches the coast, it
"feels" the bottom and starts to slow down and pile up near shore.
Slowly but surely, ocean waters rise and, like a swelling monstrous tide, swamp
everything in their path. This "surge" of water, with battering waves
rolling on top of it, is very powerful, capable of leveling houses and
small-story buildings. When Hurricane Camille struck the central Gulf Coast in
1969, large ships were carried as much as 1.6 km (1 mi) inland by a storm surge
that reached 7 meters (23 ft) near Biloxi, MS.
Destruction is greatest when the storm surge arrives around the time of high
tide. As Hurricane Gloria bore down on southern New England in 1985,
destruction was expected to be catastrophic. Though damage was extensive,
Gloria's landfall did not coincide with high tide, lessening the impact of the
storm surge. It could have been a lot worse.
The storm surge is always highest on the side of the eye corresponding to
onshore winds, which is on the right side of the point of landfall in the
Northern Hemisphere (point R is in this region in Figure 11.22a). On this
side of the hurricane, the forward motion of the storm also contributes to a
larger storm surge. To understand why the storm's forward motion makes a
difference, consider a train robber running at 5 km/hr on top of a train moving
at 80 km/hr. If the robber is running in the same direction as the train is
moving, his speed relative to the ground is actually 85 km/hr. Similarly, a
hurricane moving at 40 km/hr with peak winds measured at 160 km/hr in the right
front quadrant of the storm (Figure
11.22a) will effectively have a maximum wind speed of 200 km/hr. The
combined rise in ocean waters inundates coastal locations as shown in Figure 11.22(b-c).
On the opposite side of the point of landfall of the storm (point L is in
this region in Figure
11.22a), the water level sometimes actually decreases as the storm makes
landfall. Here, winds are offshore, opposing the direction of the storm's
movement (if the train robber had been running opposite the direction of the
train's movement, his speed relative to the ground would have been only 75
km/hr). The shallow sounds of eastern North Carolina (with average depths
around 2 meters (6 ft)) have actually been observed to nearly empty as a
hurricane makes landfall north of North Carolina. When Hurricane Hugo slammed
into South Carolina in September 1989, a low water warning was issued for the
coastal waters off Jacksonville, FL, about 300 km (186 mi) south of the point
of landfall.
In the early 1970s, a system was designed by Herbert Saffir, a consulting
engineer, and Robert Simpson, then the director of the National Hurricane Center,
to quantify (for disaster agencies) what level of damage to expect from a
hurricane. Using a mix of structural engineering and meteorology, they
constructed the Saffir-Simpson Scale, which consists of five categories which
correspond to a hurricane's central pressure, maximum sustained winds, and
storm surge, as given in Table 11.3. Categories 3, 4 and 5 are intense
hurricanes, with the potential to inflict great damage and loss of life. Since
1988, several intense hurricanes have made landfall into the United States,
including Hugo (1989), Andrew (1992), and Iniki (1992), all category 4 storms.
Camille (1969) and the Florida Keys' Labor Day Hurricane of 1935 are the only
two category 5 hurricanes to strike the United States in this century.
Table 11.3: Saffir-Simpson Hurricane Damage Potential Scale
Category Pressure (mb; inches mercury) Wind (knots; mph) Storm Surge (m; ft)
1: Minimal 980 28.94 64-82 74-95 1.0-1.7 4-5
2: Moderate 965-979 28.50 - 28.91 83-95 96-110 1.8-2.6 6-8
3: Extensive 945-964 27.91 - 28.47 96-113 111-130 2.7-3.8 9-12
4: Extreme 920-944 27.17 - 27.88 114-135 131-155 3.9-5.6 13-18
5: Catastrophic < 920 < 27.17 >135 >155 >5.6 >18
Another danger that a hurricane poses when it reaches land
is the potential for tornadoes. A tornado watch is usually issued when a
hurricane comes ashore, especially for areas in the right front quadrant of the
storm (see Figure 11.22a).
Here, air moving from water to land experiences an increase in friction
(because the land is rougher), causing surface winds to slow and thus cross the
isobars at a larger angle (recall that as the wind slows, the magnitudes of
both friction and the Coriolis effect are reduced, so the pressure gradient
force has more of an upper hand, pushing parcels more directly towards lower
pressure). Wind speeds and directions above the surface are less affected by
friction, creating a zone of vertical wind shear that can spin up some F0 or F1
tornadoes.
About 20% of tornadoes spawned by a landfalling hurricane occur near the
outer edge of the eye wall. The rest form in bands of thunderstorms that lie
farther from the eye. Some of the spiral bands of Hurricane Allison, which made
landfall near Apalachee Bay, FL in June 1995, spun up tornadoes across northern
Florida and southern Georgia. Allison's penchant for spin is clearly evident in
Figure 11.23. One of
Allison's twisters seriously damaged an elementary school in Jacksonville, FL.
Unlike "traditional" twisters that develop primarily in the heat of
the afternoon and evening when severe thunderstorms are most common, there is
no preferred time for the formation of tornadoes associated with a tropical
storm or hurricane. These tornadoes typically develop when the storm makes
landfall, whether it's day or night.
The guidelines in the manual for the smooth ignition and
operation of the heat engine of a hurricane are akin to the legs that support a
table - if one is kicked out, the table almost always topples. When hurricanes
die over land, their demise is often attributed to the greater roughness of the
land which causes the winds to slow. Indeed, friction eventually helps to sap
hurricane winds. But the primary reason that hurricanes dissipate over land or
over higher latitude waters is that they are removed from their one true source
of energy þ warm, tropical waters. The weakening effect that landfall had on
Hurricane Bertha in July 1996 can be seen by comparing the two Doppler radar
reflectivity images in Color
Plate 40 and Color Plate
41, taken seven hours apart surrounding Bertha's landfall just east of
Wilmington, NC. Although heavy rain persists in the later image, primarily
north of the storm center, Bertha's circulation is clearly more ragged just a
few hours after coming ashore.
Colder water takes an obvious toll by lowering evaporation rates and thus
reducing the amount of available water vapor. A hurricane moving north up the
East Coast of the United States over progressively cooler coastal waters must
move quickly if it is to hit New England without being sapped of its full fury.
A slow-moving storm inevitably weakens following such a path (provided it
doesn't move over the relatively warm waters of the nearby Gulf Stream - then
all bets are off!).
In the Pacific, relatively cool water south and east of Hawaii usually
protects the Islands from the full fury of hurricanes. This cool pool of water
owes its existence to the high rates of evaporation and low annual rainfalls in
the latitudes of the subtropical highs. This combination makes surface water
rather salty and relatively dense. Owing to its density, the saline surface
water periodically sinks and cooler water from below rises to take its place.
This cooler water then acts as an outer defense for the Islands. When storms
approach the Islands from the east (guided by the Pacific subtropical high),
they cannot avoid these cool seas and are gradually sapped of strength.
For a hurricane to deliver a devastating blow to Hawaii, it must be sneaky
and fast. On occasion, while a hurricane is seemingly passing safely south of
the Islands over warmer water, a trough of low pressure from higher latitudes
can dip southward and latch onto the storm. In response to the relatively
strong southerly and southwesterly steering winds ahead of the trough, the
hurricane can turn and accelerate northward, its faster pace limiting the time
spent over cool water. With most of its power intact, the storm can then deal
Hawaii a big blow. Hurricane Iniki (1992) was such a storm, breaking through
these outer defenses and striking the western part of the island of Kauai (see Color Plate 42 taken during
landfall) with sustained winds of 210 km/hr (130 mph). Iniki was the worst
hurricane to strike Hawaii this century. At the time, Steven Spielberg's
Jurassic Park was being filmed on Kauai, and the production was halted by the
dramatic actions of the hurricane.
Finally, wind shear can cause a hurricane's heat engine to sputter and
stall, given that latent heat released in the upper part of the storm can
become separated from the low-level circulation, disassembling the hurricane's
engine. In late autumn, strengthening westerlies, in combination with cooling
tropical seas, help to close the door on the Atlantic hurricane season. In
November 1994, however, Hurricane Gordon challenged late-autumn odds and formed
off the coast of Nicaragua (see Figure 11.24). Over the
Caribbean, searching westerlies eventually found the storm and sheared off its
top. Defiantly, the tropical storm regrouped near Key West, FL, cut across the
Sunshine State, and then headed north off the Atlantic Coast. Gordon
intensified into a hurricane as it passed over the warm waters of the Gulf
Stream and approached Cape Hatteras on November 18 (see Figure 11.25), marking the
closest approach by a late-season hurricane to the Outer Banks since December
2, 1925. Inevitably, the bullying winds of the mid- latitude jet stream forced
Gordon to retreat and die, shearing off the tops of its thunderstorms. November
is a hostile month for tropical storms and hurricanes.
Hurricanes certainly do not die gracefully. Though eventually downgraded to
tropical depressions after striking land, hurricanes and tropical storms often
remain dangerous. As tropical systems enter the alien world of the
mid-latitudes, they are like cornered bulls, tormented by their extratropical
surroundings. Depending on the nature of these surroundings, a hurricane
sometimes charges, accelerating through the mid- latitudes and spreading strong
winds forward. For example, Hurricane Hazel (1954) stampeded up the East Coast,
with its horns lowered in a forward charge until mid-latitude conditions
corralled its center of circulation near Toronto, Ontario. Other times, a
hurricane can stop dead in its tracks, menacingly pawing its tropical rain over
the same ground and goring a region with massive flooding. The moisture-laden
remnants of Hurricane Camille (1969), which stalled over the central
Appalachians, dumped as much as 63.5 cm (25 in) of rain, causing devastating
floods that killed 150 people. Sometimes a tropical storm will interact with
fronts in the mid-latitudes, as the remnants of Hurricane Agnes did in June
1972. The joining was catastrophic, leading to torrential rains that produced
all-time record flooding in eastern Pennsylvania, Maryland and northeastern
West Virginia (see Chapter 15 for more on Agnes).
THE END