What
is a hurricane, typhoon, or tropical cyclone?
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 subtropical waters with organized
convection (i.e. thunderstorm activity) and definite cyclonic surface
wind circulation.
Tropical
cyclones with maximum sustained surface winds 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 they are assigned
a name. If the cyclone’s winds reach 33 m/s (64 kt), it will
be classified as a "hurricane" (in the North Atlantic Ocean,
in the Northeast Pacific Ocean east of the dateline,or in the South
Pacific Ocean east of 160E); a "typhoon" (in the Northwest
Pacific Ocean west of the dateline); a "severe tropical cyclone" (in
the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean
east of 90E); a "severe cyclonic storm" (in the North Indian
Ocean); and a "tropical cyclone" (in the Southwest Indian
Ocean).
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, since using a smaller averaging period may slightly raise
the number of occurrences.
What
are "Cape Verde" type hurricanes? Cape Verde-type
hurricanes are those Atlantic basin tropical cyclones that develop
into tropical storms fairly close (<1000km or so) to the Cape Verde
Islands and then become hurricanes before reaching the Caribbean. (there
may be other definitions). Typically, this occurs 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.
What is a
super-typhoon?
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.
Where do these easterly waves come from and what causes them?
It has been
recognized since at least the 1930s 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. In 1945, Riehl 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 in 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. These "waves" are
actually 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 are 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 intense (or major) hurricanes have their origins
as easterly waves. It is suggested that nearly all of the tropical cyclones
that occur
in the Eastern Pacific Ocean can also be traced back to Africa.
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).
What is a
sub-tropical cyclone?
A subtropical
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 they 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 subtropical cyclones
have not been observed to be stronger than about 64 kt (33 m/s).
Often
these subtropical storms transform into true tropical cyclones.
A recent example from November, 1994 is the Atlantic basin's Hurricane
Florence,
which began as a subtropical cyclone before becoming fully tropical.
Note that there has been at least one occurrence of a tropical cyclone
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) is classified as
a "subtropical
depression", greater than or equal to 34 kt (18 m/s) is classified
as a "subtropical storm". While these are not given names,
the National Hurricane Center does issue warnings and forecasts as
they would
for tropical cyclones in the region.
How are tropical cyclones different from mid-latitude storms?
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. It has associated condensation in convective
clouds concentrated near its center. 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.
How are
tropical cyclones different from tornadoes?
Tropical cyclones and
tornadoes are both atmospheric vortices, but 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 100’s 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—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 when the pressure was 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.
What is
the "eye"?
How is it formed and maintained?
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 in the eye, 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 more
than 10°C (18°F) warmer at an altitude of 12 km (8 mi) than the
surrounding environment, but only 0-2°C (0-3 F) warmer at the surface
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.
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 and occasionally strong
updrafts and downdrafts. The eye's warm temperatures are due to compressional
warming of the subsiding air. Most soundings taken within the eye show
a low-level layer which is relatively moist, with an inversion above.
This suggests 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 subject.
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 that supergradient wind flow (i.e. swirling winds that are
stronger than what the local pressure gradient can typically support)
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 found that the swirling winds within several tropical
storms
and hurricanes were within 1-4% of gradient balance. It may be that the
amount of supergradient 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 at 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 fall in pressure across the
band
since warm air is lighter than cold air. Because of the pressure drops
on the
inside, the tangential winds around the tropical cyclone increase due
to the increases in the pressure gradient. Eventually, the band moves
toward
the center and encircles it and the eye and eyewall form.
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. 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 increasing pressure due to the destruction of the inner eyewall is
usually
more rapid than the decreasing pressure caused by the intensification
of the outer eyewall, causing the cyclone to weakens for a short period
of
time.
Doesn't the
low pressure in the tropical cyclone center cause the storm surge?
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, the effect caused
by a tropical cyclone with a 900 mb central pressure would be only 1
m (3 ft). The total storm surge for a tropical cyclone of this intensity
can be 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 ambient pressure outside the storm) determines the wind strength,
the
central pressure indirectly indicates the height of the storm surge.
Note also that individual storm surges are dependent upon the coastal
topography,
angle of incidence of landfall, speed of tropical cyclone motion, and
wind strength. |
