LECTURE 5
HURRICANES
Hurricanes are known as the "mighty middle
sized storm" in that they are not as large as the mid-latitude
wave cyclones which can be over a thousand miles in diameter and
they are larger than the plains tornado which may be up to a mile
in width. They are the "middle sized storm" in that
while their winds are not as intense as a mature tornado, they
are on average faster than those observed inthe larger wave cyclone.
The word hurricane is derived from the term
urican or unrican used by the ancient Carib Indians to describe
the big autumn storms that plauged the Caribbean Sea. Hurricanes
are called typhoons in the western North Pacific Ocean, cyclones
in the Indian Ocean and off the coast of Australia. They have
been observed in every tropical ocean except the South Atlantic
(one of meterology's mysteries-it could be because the water temperature
is too cold or that the Intertropical Convergence Zone (ITCZ)
does not penetrate too far to the south of the Equator in order
to take advantage of Coriolus Force necessary to initiate ratation).
The Atlantic tropical cyclone season (including
the Gulf of Mexico and the Caribbean Sea) typically peaks during
late summer and early fall. This is generally the time that the
ocean's water temperature is the warmest. (The same is true for
the southern Hemisphere-the seasons are reversed from those observed
in the northern Hemisphere) The critical temperature for tropical
cyclone formation is 80 degrees, while 85 degrees and higher is
necessary for tropical cyclones to intensify into severe hurricanes.
There are several geographic regions in the tropics
that deviate from the seasonal pattern exhibited by the tropical
Atlantic and other tropical oceans in the world. The western North
Pacific is home to the warmest water in the world (sometimes known
as the West Pacific Warm Pool) and therefore the typhoon season
here lasts year-round although peak typhoon activity is observed
from May to December. In the northern Indian Ocean, tropical cyclone
activity is observed during the months of April and May and again
from September through November. This is due to the fact that
the ITCZ (responsible for enhancing monsoon-induced precipitation
and hatching disturbances that develop into tropical cyclones)
is over India and Bangladesh during June through August. Hurricanes
don't develop over land although a disturbance which eventually
develops into a hurricane can originate over land.
There are a variety of disturbances that can develop
into a hurricane and during a typical hurricane season there may
be 100 or more of these different types of distubances which may
develop into a tropical cyclone. It is critical that the developing
storm be over warm water (at least 80 dgrees as discussed earlier)
and at least 10 degrees either side of the Equator in order for
the Coriolus Force to initiate enough spin for the developing
storm to rotate. If the above criteria have been met, hurricanes
can develop from a number of pre-existing disturbances-hundreds
of these disturbances are present during the hurricane season,
but on average only 10 of these will develop into tropical storms
or hurricanes.
Easterly Waves
Easterly waves are small
westward moving disturbances in the tropics that experience fair
weather and northeast winds in advance of the trough and southeast
winds and rain squalls behind the trough. If enough rotation is
available, an easterly wave may develop a closed circulation (which
is why the report of westerly winds at the surface in the tropics
is a sign of trouble) and eventually develop into a tropical storm.
Hurricane Iris (Oct 4-12, 2001) which violently struck the coast
of Belize developed from an easterly wave.
Mesoscale Convective Complexes
(MCC's).
The tropics also experience
MCC's similar to those described in our lecture re thunderstorms.
These typically develop in the monsoon trough (aka the ITCZ) and
will migrate westward and can last up to a week. MCC's are numerous
in the African savannas during their wet season and some may move
into the African Sahel a tropical steppe region that is subject
to drought and flood. Eventually these disturbances will move
off of the western coast of Africa near the Cape Verde Islands
(especially during the months of August and September) and intensify
into a tropical cyclone. The most intense and long-lived hurricanes
observed in the Atlantic Basin form in this region and are known
as Cape Verde Hurricanes. Hurricanes Hugo (Sept 8-22, 1989), Andrew
(Aug 16-28, 1992), Luis (Aug 26-Sept 10, 1995) and Floyd (Sept
7-17, 1999) are examples of Cape Verde Hurricanes. All of these
storms had winds of at least 140 mph at their peak intensity.
MCC's can also originate in the Amazon basin during the latter
portion of the tropical cyclone season and migrate into the Caribbean.
Hurricane Mitch (Oct-21-Nov 3, 1998) a 180mph monster and the
deadliest (over 11,000 killed) hurricane to strike North America
in the 20th Century, developed from a disturbance that can be
traced back to the Amazon basin.
The base of an old frontal
trough extending into the tropics.
Tropical cyclones that develop
during June and then again during the months of October and November
can often be traced to the base of an old dissipating cold front
that washes out in the tropics usually in the Gulf of Mexico and
the western Caribbean Sea. From July through September little
if any frontal activity penetrates the tropics as the Azore-Bermuda
High reaches its greatest strength and frontal activity from Canada
is not strong enough to reach the Gulf and the eastern seaboard
from South Carolina to Florida. It takes a while, but usually
within three days if environmental conditions warrant a tropical
cyclone will develop although these on average are not as strong
as the MCC-induced tropical cyclones. There are some exceptions,
Hurricane Audrey (June 24-28, 1957) which formed at the base of
an old frontal trough in the Gulf of Mexico, was the strongest
hurricane ever observed in June.
Non-tropical upper-level
lows that move into the tropics and become cut off.
Non tropical upper-level
lows are the remnants of old cold fronts which move into the tropics.
These are not tropical as the center of the low is cold (all tropical
cyclones have a warm core) and while they may cause significant
precipitation in the tropics they normally do not develop into
a tropical cyclone. When it does occur, the process is long and
the storm that develops usually is not as strong as others previously
discussed. Hurricane Karen developed in this manner near Bermuda
in October of 2001.
Strong midlatitude cyclone
which developes a warm core.
This is a relatively rare
occurence and usually occurs near the end of the tropical season
in the North Atlantic. In October 1991, a strong midlatitude cyclone
moved from the Great Lakes into the North Atantic essentially
"eating" Hurricane Gladys and then wandering back towards
Cape Hatteras. As the storm eventually began to move back to the
northeast, its passage over the warm waters of the Gulf Stream
transformed the midlatitude cyclone into an unnamed hurricane
with winds of over a 100mph. This was the scenario Sebastian Junger
used to write The Perfect Storm. This was one of the worst Nor'easters
ever to strike the east coast which did a tremendous amount of
beach erosion due to its slow movement. Some meteorologists believe
that there should be a distinction between these types of "hypercanes"
or "neutercanes" and other storms which have tropical
and non-tropical characteristics.
Destructive Forces
Hurricanes have a number of features that contribute
to its destructive capabilities. These include storm surge, winds,
rain/flooding and tornadoes.
Storm Surge
In a mature hurricane (usually
120 mph or more) this is the deadliest and the most destructive
feature. Storm surge is the rise in the water level above the
mean tidal level and sometimes erroneously called a tidal wave.
The low barometric pressure causes a rise of the sea level (the
French call it a raise de maire). As the hurricane approaches
the continental shelf, the water encounters the bottom and begins
to lift up further. If a hurricane approaches the coastline at
a 45 degree angle or less, the water will pile up on the right
side of the hurricane and storm surges of 15-20 feet above normal
and even 30 feet above normal can be expected depending on the
intensity of the storm. In addition, the fetch of the winds blowing
across the water and its counterclockwise movement around the
center result in the water piling up on the right side of the
storm. A cubic yard of water weighs three-quarters of a ton and
with breaking waves superimposed on the top of the water level,
even the most well-built structure will not survive a storm surge.
90% of the deaths in most hurricanes can be attributed to the
storm surge. After Hurricane Hazel struck the North and South
Carolina coast in October of 1954, every single beach community
was destroyed from Myrtle Beach South Carolina to Cape Hatteras
North Carolina. In many cases all traces of homes were swept away
foundation included-destruction far worse than that of an F5 tornado.
Similar destruction was noted during Hurricanes Carla (1961),
Camille (1969) and Hugo (1989). Some of the worse natural disasters
humankind has experienced featured tropical cyclones with massive
storm surges. The worst natural disaster in U.S. history was The
Galveston Hurricane of 1900 which killed between 6,000 and 8,000
people. One of the worst natural disasters in the world occurred
in Bangladesh in November of 1970 when 500,000 people were killed
in a cyclone which struck at the head of the Bay of Bengal. The
geographic nature of the coast which allows water to be funneled
into a narrow point and the vulnerable low-lying Ganges and Brahmaputra
deltas coupled with the region's endemic poverty, makes this region
the most deadly area of the world to experience tropical cyclones.
Winds
This is not the most destructive
aspect of a hurricane, but significant damage can occur from winds
especially once they exceed 100mph. Some of the highest winds
ever recorded on Earth have occurred in a hurricane as the winds
of a tornado are too strong and only a few mountain stations are
in place to observe winds near jet stream level such as Mt. Washington
New Hampshire which recorded winds of 234mph in a snowstorm in
April of 1934. In Hurricane Andrew of 1992, the wind equipment
at the National Hurricane Center failed at 168mph. Peak wind gusts
of 175mph were observed at Port O'Connor on the middle Texas Coast
on Sept 11, 1961 (another Sept 11 when bad things happened). During
Hurricane Celia in August 1970 at Corpus Christi, peak wind gusts
of 180mph were estimated after the wind equipment failed at 161mph.
Hurricane Camille is estimated to have had winds in excess of
200mph when it struck Pass Christian Mississippi in August of
1969. A typhoon in the western North Pacific near Wake Island
in 1979 had sustained winds (sustained winds are averaged over
5 minutes, gusts are instantaneous bursts of wind) in excess of
215mph and gusts in excess of 250mph -the most intense tropical
cyclone (along with the lowest pressure observed in a tropical
cyclone) observed since people have been flying into these storms
since 1943. While winds are usually decreasing in intensity at
the time of landfall, (due to friction with the land surface and
removal from its ocean source of energy) a storm with sustained
winds of 125mph or more can do a great deal of destruction. The
winds will get under any extension such as the eaves of homes
and cause them to become airborne in the same manner that an airplane
will lift up when its airspeed exceeds a critical value. This
and other airborne debris, act as missles and cause additional
damage when impacting other structures. This is one reason why
flat-roofed homes (if the community enforces the hurricane code-along
the Gulf coast it is known as the Southern Building Code) should
be avoided in hurricane-prone regions. A well-constructed home
should have a 45 degree pitch and boxed eaves so the wind can't
get under them and storm or hurricane shutters is ideal enabling
it to survive all but the most intense hurricanes.
The relationship between the barometric
pressure and the winds is almost a one-to-one relationship. The
lower the barometric pressure, the stronger the winds or "the
lower it goes, the harder she blows" as mariners say. There
have been several formulas used by meteorologists in order to
predict the maximum winds based on the barometric pressure. One
formula used by Miller and Dunn (1964), takes the square root
of the sum of the highest closed isobar at the edge of the hurricane
(usually 1010 millibars-you must use millibars in order for the
formula to work) minus the observed barometric pressure (we'll
say 930 mb (27.46 inches)- a fairly severe hurricane) at the center
of the storm times a constant (16). The result is the sustained
wind speed in knots (143kts) -I think this is a bit too high (in
miles per hour it would be 169), but if you use miles per hour
instead of knots (nautical miles), it is a pretty good model to
use in order to correlate pressure with wind speed. A hurricane
with a barometric pressure of 930 mb usually has sustained winds
of 145mph -close to the value given by the above formula.
Floods/Rain
This aspect of a hurricane is the next most destructive
feature of a hurricane and many times the most destructive aspect
of a hurricane. Some of the worst natural catastrophes have been
hurricanes in which flooding was the dominant destructive force.
Flora (Sept-Oct 1963), Fifi (Sept 1974) and Mitch (Oct-Nov 1998)
were major hurricanes of at least Category 3 magnitude or greater,
but the floods produced by each of these storms killed at least
30,000 or more. Hurricanes supply a tremendous amount of unstable
warm tropical air (it is so unstable that some meteorologists
have proposed labeling tropical air associated with a hurricane
a distinctive air mass, maritime Equatorial (mE). When this tropical
air mass encounters an old frontal boundary or the rising topography
over land, the increased lift wrings out even more precipitation
than would ordinary fall. A mature hurricane is not necessary
for a major flood disaster such as Tropical Storm Allison of June
2001 along the upper Texas coast and Tropical Storm Amelia in
August of 1978 in the Texas Hill country so aptly demonstrated.
Heavy rains from a dying tropical storm is not always bad news.
Many times rainfall from a dissipating hurricane have ended or
alleviated drought conditions in the tropics and the midlatitudes.
The drought and heat wave of 1998 ended in August when Tropical
Storm Charlie brought beneficial rains of 5-6 inches to South
Central Texas (although in Del Rio 17 inches in 24 hours was too
much and devastating floods occurred). In some regions, tropical
activity is necessary to fill up reservoirs and aquifers, and
if the season is weaker than normal, severe drought conditions
can develop as they do in the Caribbean when the tropical cyclone
season is slower than usual.
Tornadoes
Tornadoes are not just a midlatitude phenomenon,
they also occur in association with landfalling hurricanes. They
are typically weaker than the kind found on the Great Plains and
do not last as long. Tornadoes form in response to the wind shear
present in the heavier thunderstorms near the eyewall as the airflow
and convergence is greatest here. As these thunderstorm towers
crash into each other, the horizontal vorticity (the tendency
of an air parcel to rotate) is tilted into the vertical, and small
short-lived tornadoes can quickly develop with winds up to 150mph.
Hurricane Allen in August of 1980 produced 29 tornadoes as it
moved onshore in deep South Texas. Some of the tornadoes associated
with this hurricane were amoung the strongest hurricane-produced
tornadoes observed along with damage paths in excess of 50 miles.
Tornadoes did over 150 million dollars in damage at Robert Muller
Municipal Airport (now closed) in Austin Texas and 50 million
dollars in damage in San Marcos Texas (I witnessed the latter)
on August 10, 1980. Hurricane Beulah in September 1967 produced
the most tornadoes by any hurricane -a total of 115 on September
21, 1967. This is also the record for the most tornadoes observed
in a single day.
Hurricane formation, maintenance
and dissipation
As previously discussed, hurricanes are the product
of a pre-existing disturbance. If the storm is far enough away
from the Equator in order to take adavantage of the presence of
Coriolis Force, there will be enough rotation and a closed low
(at least one circular isobar) will develop. If the winds reach
35mph, the storm is classified as a tropical depression. When
the winds reach 40mph, the storm is given a name (more on that
later). If the winds speed reaches 74mph or greater, the storm
becomes a hurricane. There are two schools of thought on how a
hurricane utilizes the available potential energy of the oceans
in order to maintain itself and intensify. These are known as
the Organized Convection Theory and the Hurricane as Heat Engine
which are described in detail by your text and you should read
them now. It is difficult for a storm to develop into a mature
hurricane even in a favored tropical oceans because of the presence
of a number of hostile features which can inhibit tropical cyclone
development and can weaken even the strongest of hurricanes.
Wind Shear
This is the most hostile feature that can inhibit
tropical cyclone development and even in the tropical oceans it
is quite common. A tropical cyclone needs to have an environment
of fairly weak winds at all levels of the atmosphere in order
for to the storm to able to properly "vent" itself in
order to intenify. Winds at 10,000 to 20,000 feet will shear apart
a developing storm by removing and tearing up the developing cumulonimbus
towers from the center literally toppling or knocking over the
storm in the process. Even a mature hurricane can be weakened
in short order by shearing winds in the midlatitudes. Any meterological
feature present that results in wind shear over a tropical cyclone
is detrimental to its development. An example of this is an upper-level-low
located west of a westward moving tropical cyclone which will
bring upper-level southwesterly winds over the storm inhibiting
its development. Others include southerly winds in advance of
a strong mid-latitude trough which can tear apart a cyclone (often
serves to recurve the storm away from the U.S. mainland) and the
mid level winds that are produced from the thunderstorm complexes
that develop as a result of El Nino conditions in the central
and eastern Pacific Ocean.
Cold Water
As previously mentioned, tropical cyclones need
water temperatures of at least 80 degrees farenheight in order
to maintain and intensify themselves. During the tropical season,
a mature hurricane may pass over an area of the tropical ocean
and the strong winds blowing across the ocean's surface allows
colder water from the subsurface of the ocean to migrate to the
ocean's surface. This is called upwelling and usually for up to
two weeks after the passage of a major hurricane, the water temperature
may be as much as 5 to 8 degrees below what they were prior to
the passage of the hurricane. This colder water will weaken a
topical storm or hurricane that follows the path of the hurricane
that leaves this cold water in its wake. In 1989, Hurricane Iris
followed the path of Hurricane Hugo north of Puerto Rico and crossed
over the cold water left behind by Hugo. Along with shearing winds
at the upper level from Hugo (another reason why strong hurricanes
don't strike to close to each other) the hurricane ultimately
dissipated while Hugo went on to ravage South Carolina. When Tropical
Storm Jerry followed Hurricane Iris in October of 2001, the same
thing occurred; cold water upwelling and shearing winds tore up
the developing cyclone. A hurricane moving over the cold waters
of the North Atlantic Ocean will also weaken rapidly.
Westerlies
When a hurricane approaches the midlatitudes, it
comes under the influences of the upper level westerly flow that
we learned about in an earlier lecture. Like the mid level winds
encountered in the tropics, these winds will tear apart a mature
hurricane in short order leading to its dissipation or at least
weakening it somewhat. There is an exception to this in that a
hurricane approaching the midlatitudes will make contact with
a deep long-wave trough with a pool of extremely cold air aloft
which will serve to initiate a wave cyclone. The hurricane "becomes"
the cyclone and takes the place of a cyclone that was likely going
to develop anyway. In this transformation, the hurricane becomes
an extratropical cyclone and the cold pool of air aloft serves
as a new source of energy with the storm maintaining hurricane
force winds well into the midlatitudes. These conditions often
occur along the U.S east coast (and the Japanese coast in the
Pacific). This is a difficult process to forecast which is made
more difficult in that the hurricane is often accelerating up
the coast at better than 50mph. Hurricane Hazel in October of
1954 was an example of a hurricane which was transformed into
a severe extratropical storm with hurricane force winds from South
Carolina to Toronto Canada.
Hurricane Intensification
and dissipation
This is one of the most critical areas of hurricane
forecasting particularly if the intensification occurs just prior
to landfall along with forecasting the location where the eye
will cross the coastline. Dramatic hurricane intensification prior
to landfall occurred with Hurricane Andrew (1992), Celia (1970)
and Iris (2001) just to name a few. Conditions for a hurricane
to reach its greatest intensity usually involve moving over very
warm water, a very large anticyclone in the tropics positioned
such that it removes a significant amount of air out of the center
of the storm (usually to the northeast if the storm is moving
to the west). An intensifying tropical cyclone needs an upper-level
anticyclone to develop over the top of the incipient storm in
order to "ventilate" it. And no wind shear intrusion
from upper-level lows or some other type of system should be present.
Once a hurricane reaches its greatest intensity it can hold it
for several days providing no other weather system interfers.
The storm is usually quite small in diameter (100 miles), but
after a few days and particularly if the storm moves into the
midlatitudes, the hurricane will expand in diameter while not
necessarily intensifying anymore sometimes up to 1000 miles. When
a hurricane enters the dissipation stage, it is either over cold
waters of the midlatide oceans or has passed over land. The rapid
weakening of the winds is due to the fact that the storm has been
removed from its source of energy, the tropical oceans and to
a lesser effect, the friction of the land surface. Mature hurricanes
passing over the high mountains of eastern Cuba or Hispanola (up
to 10,000 feet in altitude) can be torn up in short order as the
high mountains tear up the storm similar to that of wind shear.
Hurricane Names
As previously discussed, when a tropical cyclone
reaches 40mph, it is given a name by the Tropical Prediction Center
in Miami. Naming tropical cyclones is necessary, because significant
confusion can exist if there is more than one storm (as was the
case prior to 1950). If the storm becomes a significant hurricane
producing significant damage and destruction the country that
experienced the storm, will petition to have the storm "retired"to
a rouge's hall of fame never to be used again. For example in
1992, Hurricane Andrew (the most destructive hurricane to hit
the U.S.-over 30 billion in damage) was retired and replaced in
1998 with the name Alex. The national Hurricane Center alternates
Men and Women's names and has a six sets of names. For example,
the names that were used this year were used in 1995 (retired
storms Luis, Marilyn, Opal and Roxanne), 1989 (Hugo) and 1983
(Alicia). Those that were not retired will be used again in 2007
(see the NHC website for previous and upcoming named storm lists).
Any storm deemed severe enough (probaby Allison,Iris) will be
retired but the other names will be used again if they were weak
or stayed out to sea not harming anyone. This system has been
in use since 1979. Prior to then, only women's names were used
which began in 1953 (Here again hurricane names were retired if
they were deemed severe enough) . Between 1950 and 1952 the phonetic
alphabet (Able, Baker, Charlie, etc.) was used and was abandoned
because of the confusion in using the same names every year.
RETIRED HURRICANES SINCE 1953
1954 Carol, Edna, Hazel
1955 Connie, Dianne, Ione, Janet
1957 Audrey
1959 Gracie
1960 Donna
1961 Carla, Esther, Hattie
1963 Flora
1964 Cleo, Dora, Hilda
1965 Betsy
1966 Inez
1967 Beulah, Doria
1969 Camille
1970 Celia
1972 Agnes
1974 Carmen, Fifi
1975 Eloise
1977 Anita
1979 David, Fredrick
1980 Allen
1983 Alicia
1984 Diana
1985 Elena, Gloria, Jaun
1988 Gilbert, Joan
1989 Hugo
1991 Bob
1992 Andrew
1995 Luis, Marilyn, Opal, Roxanne
1996 Fran
1998 Bonnie, George, Mitch
1999 Floyd
2000 Keith
For more information:
Tropical
Meteorology Project- website headed by Dr. William Gray a
pioneer in seasonal hurricane forecasting
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