Download Tropical Cyclones, Hurricanes and Typhoons

Tropical Cyclones, Hurricanes and
Typhoons
 Motivation (Why do we care)
 Definition
 Where and when do they occur
 Formation and intensity
 Structure
 Hazards
World’s Deadliest Tropical Cyclones
 Of the 20 deadliest
tropical cyclones, 14 have
occurred in South Asia
(India, Bangladesh).
 The deadliest was the
great Bhola Cyclone
which hit Bangladesh in
1970 resulting in app.
500,000 deaths.
World’s Deadliest Tropical Cyclones
 The deadliest storm in the
Atlantic Basin occurred in
1780. (22,000 deaths)
 Deadliest US storm was
the Galveston Hurricane in
1900 which killed app.
8,000 people.
 Of the 10 deadliest storms
in the US, only 1 has
occurred since 1957
(Katrina - 1900 dead)
What About Canada?
 On average, about 4
hurricanes impact
Atlantic Canada in any
given year.
 Deadliest hurricane in
Canada occurred in 1775
when 4000 people died
along the Newfoundland
coast.
 What about the 2nd
deadliest?
Economic Impact
 The most obvious impact economically speaking is the
rebuilding of damaged infrastructure.
 However, other costs that are less often considered
include:
 The cost of evacuation
 The impact on energy production
 The cost of severe coastal erosion.
Costliest U.S. Hurricanes
 Hurricane Katrina (2005)
est. 81 billion dollars.
 Hurricane Andrew
(1992) est. 40 billion
dollars.
 If Andrew had made
landfall 20 miles to the
north, loss of life and
property would have
easily doubled.
Damage (wind)
 Wind damage with
Hurricane Andrew was
extreme with wind
speeds on land measured
at 270 kph.
What if Andrew hit Miami?
Damage (water)
 While Katrina was an
extremely strong
hurricane while offshore,
most of the damage was
associated with storm
surge and flooding.
Damage (water)
 New Orleans is
particularly vulnerable to
flooding because the city
itself is below sea level.
 With the storm center
passing to the east, most
of the flooding came
from Lake Pontchartrain.
Damage (water)
This is a picture
of flooding near
Venice,
Louisiana that
resulted from
levee failure.
Damage (water)
This is a picture
of damage from
near Gulfport,
Mississippi where
the damage was
from storm
surge. Note that
the damage
resembles wind
damage.
Erosion
Definition
 A tropical cyclone is a warm-core, low pressure system
without any "front" attached, that develops over the
tropical or subtropical waters, and has an organized
circulation with winds of at least 120 kph (74 mph).
Depending upon location, tropical cyclones have
different names around the world. In the:
 Atlantic/Eastern Pacific Oceans - hurricanes
 Western Pacific - typhoons
 Indian Ocean - cyclones
Definition
 What do we mean by
warm core?
 Literally that the
warmest air is located at
the center of the storm.
 This warm air is
generated by a couple of
different mechanisms.
 Latent Heat Release
Definition
 What do we mean by
warm core?
 Another mechanism is
subsidence. As the air in
the eye of the cyclone
sinks, it warms due to
compression.
Where and When
Note that tropical cyclones do not form near the equator due
to the lack of the coriolis effect. Also, storms tend to curve to
the north and east as they interact with the westerlies.
Where and When
 The Coriolis effect is the
apparent deflection of
air due to the rotation of
the earth.
 Air, rather than flowing
directly from areas of
high pressure to low
pressure, rotate to the
right of this direction in
the Northern
Hemisphere.
At least 4 degrees from the equator:
Coriolis force can be large enough to produce rotation
(deflecting to right in the Northern Hemisphere)
Before:
After:
Where and When
Worldwide, tropical cyclone activity peaks in late summer
when water temperatures are warmest.
Where and When
In general, sea surface temperatures are warmer along eastern coasts
than western coasts and are warmest near Indonesia accounting for the
strongest and most frequent activity.
Where and When
In general, sea surface temperatures are warmer along eastern coasts
than western coasts and are warmest near Indonesia accounting for the
strongest and most frequent activity.
Formation and intensity
 There are at least four main requirements for tropical
cyclogenesis :
 enough Coriolis force to develop a low pressure center,
 a preexisting low-level focus or disturbance
 sufficiently warm sea surface temperatures
approximately 27° C at least 60 m deep.
 low vertical wind shear
 These conditions are necessary but NOT sufficient
conditions for the formation of tropical cyclones.
Formation
A low-level disturbance is necessary to start and concentrate convection

locally.
 Most commonly in the Atlantic basin, the disturbances are either fronts,
easterly waves or the intertropical convergence zone. Storms that
develop near the coast of Africa from easterly waves are referred to as
Cape Verde storms.
Formation
 Since easterly waves account for about 60% of all
Atlantic basin tropical cyclones, there is a relationship
between West African rain and cyclone frequency.
Formation
 Why is it that most easterly waves do not develop
into hurricanes?
 Strong descending air associated with Azores high
produces an inversion inhibiting convection.
 Vertical wind shear is usually too strong over the
tropical Atlantic for the cloud systems to stay together.
 The middle layers of the atmosphere are usually too dry.
Formation
The Intertropical Convergence Zone (ITCZ), also known as the
Intertropical Front, Monsoon trough, Doldrums or the Equatorial
Convergence Zone, is a belt of low pressure girdling Earth at the equator. Air,
convergences at the surface towards this low pressure and then rises forming a
band of clouds and convection which can sometimes form tropical cyclones.
Formation
The Intertropical Convergence Zone (ITCZ), migrates with the
Sun towards the summer hemisphere. Note that the ITCZ is
slightly displaced towards the Northern Hemisphere since there
is more land mass.
Intensity
 So why do we need warm
sea surface
temperatures?
 Consider the same
schematic that we saw
earlier:
Intensity
 There is a direct relationship between the
intensity (central pressure) of the most
intense hurricanes and the temperature of
the sea-surface over which the storms are
moving.
 Note from the attached chart that the six
strongest hurricanes all occurred in the
western Pacific where the sea-surface
temperatures are warmest.
Intensity
Note that the 5
lowest
pressures ever
recorded in
tropical
cyclones all
have occurred
in the Pacific
basin.
Intensity
 Predicted maximum intensity of a hurricane based upon average ocean surface
temperatures. The agreement between theory and observations is excellent,
showing the importance of a warm ocean for hurricane formation.
Intensity
 However, we not only
need to know how warm
the ocean surface is, but
how deep is the warm
water. That is because
the storm itself can cool
the sea surface by
bringing up cooler water
through upwelling.
Intensity
 Weak wind shear:
 Energy is derived from release of latent
heat in the eyewall.
 A concentration of energy is necessary for
development
 If the clouds are carried away by the high
winds aloft, then the energy is no longer
concentrated sufficiently for the hurricane
to develop.
Intensity
 Vertical wind shear
of less than 10 m/s
(20 kts, 22 mph)
between the surface
and the tropopause
is required for
tropical cyclone
development.
Intensity
 Strong wind shear can
"blow" the tropical
cyclone apart, as it
displaces the mid-level
warm core from the
surface circulation and
dries out the mid-levels
of the troposphere,
halting development.
In what sense is the wind blowing
in the vicinity of the Azores High?
1.
2.
3.
4.
Counterclockwise
Clockwise
Directly towards the
center
Directly away from the
center
CORRECT ANSWER:
2. Clockwise
Reason: Winds blow clockwise around highs (anticyclones) and counterclockwise
around lows (cyclones and hurricanes) in the Northern Hemisphere.
What is the cause of the relative warmth in the
center (eye) of a hurricane?
1.
2.
3.
4.
Strong winds
Warm oceans
Condensation heating
Convergence
CORRECT ANSWER:
3.
Condensation heating
Reason: The phase change of condensation from water vapor to liquid water releases
heat to the surrounding air in the strong updraft (rainfall) region near the center of the
hurricane.
Where would you expect the
most significant wind damage?
1.
2.
3.
4.
5.
A
B
C
D
E
CORRECT ANSWER:
3. C
Reason: Winds blow counterclockwise
around hurricanes. Therefore, the onshore
winds would all be at points C, D, and E.
Since the strongest winds are closest to the
center, the strongest ONSHORE winds
(causing the most damage) would be at C.
A
B
C
D
E
Life Cycle
 A tropical depression is
designated when the first
appearance of a lowered
pressure and organized
circulation in the center of
the thunderstorm complex
occurs.
 Winds near the center are
constantly between 20 (37
kph) and 34 knots (23 - 39
mph).
Life Cycle
 Once a tropical depression
has intensified to the point
where its maximum
sustained winds are
between 35 (63 kph)-64
knots (39-73 mph), it
becomes a tropical storm.
It is at this time that it is
assigned a name.
 Tropical Storm Fay (2008)
 Note that you can see some
banding and symmetry
Life Cycle
 As surface pressures
continue to drop, a tropical
storm becomes a hurricane
when sustained wind
speeds reach 64 knots (74
mph or 120 kph). A
pronounced rotation
develops around the
central core.
 Large bands of clouds and
precipitation spiral from
the eye wall and are thusly
called spiral rain bands.
The Saffir-Simpson Intensity
Scale
Life Cycle (North Atlantic)
 Cape-Verde type
hurricanes usually travel
slowly westward (10-20 km
per hr) and may take a
week to cross the Atlantic.
 Once hurricanes have
reached the Caribbean or
the Gulf of Mexico, they
recurve to the north and
generally speed up.
Life Cycle
 Hurricane winds usually diminish very
quickly once landfall occurs
 The hurricane has lost its energy source (warm
water is the fuel for the latent heating).
 The increased surface roughness weakens the
system with surface pressures rising, with winds
decreasing.
 Storms rarely remain hurricanes for more than 12-24
h after landfall.
Storm Structure
 The main parts of a tropical cyclone are the rainbands, the eye,
and the eyewall. Air spirals in toward the center in a counterclockwise pattern in the norther hemisphere (clockwise in the
southern hemisphere), and out the top in the opposite direction.
In the very center of the storm, air sinks, forming an "eye" that is
mostly cloud-free.
Storm Structure
 The Eye
 The hurricane's center is a
relatively calm, generally
clear area of sinking air
and light winds that
usually doesn't exceed 15
mph (24 kph) and is
typically 20-40 miles (3264 km) across. An eye will
usually develop when the
maximum sustained wind
speeds go above 74 mph
(119 kph) and is the
calmest part of the storm.
Storm Structure
 The eyewall
 consists of a ring of tall
thunderstorms that
produce heavy rains and
usually the strongest
winds. Changes in the
structure of the eye and
eyewall can cause changes
in the wind speed, which
is an indicator of the
storm's intensity. The eye
can grow or shrink in size,
and double (concentric)
eyewalls can form.
Storm Structure
 Rainbands
 Curved bands of clouds
and thunderstorms that
trail away from the eye
wall in a spiral fashion.
These bands are capable of
producing heavy bursts of
rain and wind, as well as
tornadoes. There are
sometimes gaps in
between spiral rain bands
where no rain or wind is
found.
Storm Structure
 Tropical Cyclone Size
 Typical hurricane
strength tropical
cyclones are about 300
miles (483 km) wide
although they can vary
considerably.
 The relative sizes of the
largest and smallest
tropical cyclones on
record as compared to
the United States.
Hurricane Hazards
 A)Storm Surge
 B)Wind Damage
 C)Heavy rains (flooding)
 D)Associated tornados
 About 90% of fatalities are caused by coastal and
inland storm surge
Hurricane Hazards
 What is a storm surge?
It is an 8-160 km wide dome of water that sweeps over
the coastline during landfall.
Hurricane Hazards
 Strong onshore winds and relatively low air
pressure are responsible for a storm surge
 A sea-level rise of about .5 m for every 50 mb of
pressure loss.
 A surge is superimposed on the normal tidal
oscillation, so that the danger is greatest at high
tide
 Plus wind-driven waves up to 3 m
Hurricane Hazards
 The level of surge in a
particular area is also
determined by the slope of
the continental shelf. A
shallow slope off the coast
(right, top picture) will allow
a greater surge to inundate
coastal communities.
Communities with a steeper
continental shelf (right,
bottom picture) will not see
as much surge inundation,
although large breaking
waves can still present major
problems.
Hurricane Hazards
 SLOSH (Sea, Lake and Overland
Surges from Hurricanes) is a
computerized model run by the
National Hurricane Center
(NHC) to estimate storm surge
heights and winds resulting
from historical, hypothetical, or
predicted hurricanes by taking
into account





Pressure
Size
Forward speed
Track
Winds
Hurricane Hazards
 One of the areas most
vulnerable to storm
surge is Tampa Bay, FL.
This is of major concern,
because over 3 million
people live the region,
and it is highly
vulnerable to storm
surge-- particularly for a
storm moving northeast
or north-northeast at
landfall,
Hurricane Hazards
 Even a Category 1
hurricane can create
significant surges--up to
7' in Hillsborough
County, 6' in Manatee
County, 7' in Pinellas
County, and 9' in Pasco
County. An extreme
Category 5 hurricane can
create a storm surge of
28' in Hillsborough
County.
Hurricane Hazards
 Discussion
of
the
vulnerability of New Orleans
from
a 2004
Weather
calendar:
 The conservative estimates of
loss of life from a direct hit
from a category 5 hurricane is
25,000 deaths (recall that the
case of Katrina was a category
3 tracking to the east of
downtown New Orleans).
 Photograph (courtesy of
NASA)
Hurricane Hazards
 Examples of hurricanes with noteworthy storm surge
 Galveston, Texas (Sept. 8, 1900)
6,000 perished
 Camille (Aug. 17, 1969)
 Cat. 5 system with 186 mile per hour winds; 7.3 metre
surge
 Bangladesh (Bay of Bengal); November 13, 197o; 7
metre surge; 500,000 perished
Hurricane Hazards
Hurricane Hazards
Coastal effects of Camille (1969)
When Warnings are Ignored
The same apt. building destroyed
with 22 lives lost
36 Years Later
 Katrina’s landfall was at a
category 4, but with a
comparable storm surge to
that of Camille;
 Katrina had weakened to a
Category 4 hurricane with
maximum sustained winds
estimated at 145 mph as it
made landfall early Monday.
 However, the storm surge of
8.4 m, generally found in
category 5 storms, was
maintained during Katrina’s
weakening to a category 4.
Galveston Hurricane (1900)
Hurricane Hazards
 6,000 to 12,000 killed of
the total population of
37,000
 75% of the city was
destroyed
 Most of the fatalities were
drownings in a 7 m storm
surge
The “Wall Street” of the Southwest
 ‘High ground was only 2.5
m’
 Top winds were probably
125 mph though the
island’s only anemometer
was swept away after
recording 102 mph
Hurricane Hazards
 Warnings existed even in 1900:
 sailors arrived in port talking of stormy
seas
 Though the residents knew there was a
storm in the Gulf of Mexico, there was no
hint of where landfall would occur
Saturday, Sept 8, 1900
 Atmospheric pressures
plunged in the
morning
 Winds increased
 A steamship was torn
from its moorings and
promptly smashed
through the three
bridges to the
mainland
Saturday, Sept 8, 1900
 By evening, the winds had
shifted to easterly, bringing
in the waters of the Gulf of
Mexico
 The island was flooded,
and residents flocked to
‘higher ground’ (8 ft.) as
‘rats clinging to the sinking
mast of a ship’
Sunday Morning
 Nearly everyone in
the city lost some
family and friends
 Flood waters had
drained back to seas
exposing bodies and
ruins of the city
Isaac Cline
 Contributed to the city’s complacency by dismissing the
notion that a hurricane could destroy Galveston
 This attitude contributed to the lack of any seawall
construction
Isaac Cline
 Sent observations and warnings
throughout the hurricane’s fury
 Endured personal tragedy (wife died)
during the storm
Lessons learned
 The existence of the
new sea wall
prevented major loss
of life during a 1915
hurricane in which
eight people were
killed
Wind Damage
 The two storms causing
the most widespread
wind damage in the US
were Hurricanes Camille
(1969) and Andrew
(1992)
An entire neighborhood leveled by
Andrew (1992)
Flooding-Floyd (199)
 The hurricane produced
torrential rainfall in eastern
North Carolina, adding more
rain to an area hit by Hurricane
Dennis just weeks earlier. The
rains caused widespread
flooding over a period of several
weeks; nearly every river basin in
the eastern part of the state
exceeded 500-year flood levels.
In total, Floyd was responsible
for 57 fatalities and $4.5 billion
($5.7 billion in 2006 U.S. dollars)
in damage, mostly in North
Carolina
Flooding-Floyd (1999)
 Floyd dropped nearly 17 inches
(430 mm) of rain during the
hours of its passage and many
residents weren’t aware of the
flooding until the water came
into their homes. The National
Guard and the Coast Guard
performed nearly 1700 fresh
water rescues of people trapped
on the roofs of their homes due
to the rapid rise of the water. By
contrast, many of the worst
affected areas did not reach peak
flood levels for several weeks
after the storm, as the water
accumulated in rivers and
moved downstream.
Flooding-Floyd (1999)
 The extensive flooding
resulted in significant
crop damage.
 Around 31,000 jobs were
lost from over 60,000
businesses through the
storm, causing nearly
$4 billion (1999 USD,
$4.7 billion 2006 USD) in
lost business revenue.
Flooding-Floyd (1999)
 Runoff from the hurricane
created significant
problems for the ecology of
North Carolina's rivers and
sounds.
 Freshwater runoff,
sediment, and
decomposing organic
matter caused salinity and
oxygen levels in Pamlico
Sound and its tributary
rivers to drop to nearly
zero.
Forecasting
 There are several
elements to tropical
cyclone forecasting:
track forecasting,
intensity forecasting,
rainfall forecasting,
storm surge, and tornado
forecasting.
 The large-scale synoptic
flow determines 70 to 90
percent of a tropical
cyclone's motion. The
deep-layer mean flow is
considered to be the best
tool in determining track
direction and speed.
Forecasting
 The 1-2-3 rule (mariners'
1-2-3 rule or danger area)
is a guideline commonly
taught to mariners for
hurricane and tropical
storm tracking and
prediction. It refers to the
rounded long-term
NHC/TPC forecast errors
of 100-200-300 nautical
miles at 24-48-72 hours,
respectively.
Forecasting
 Because of the inherit
uncertainty in the exact
track forecast, the
national Hurricane
Center issues forecasts
that include an ever
expanding envelope of
threat area.
Forecasting
 Some forecasts however,
have less confidence
than others.
 Consider the spread in
the various track
forecasts from different
models for Hurricane
Katrina when Katrina
was crossing Florida.
Forecasting
 However, once the storm
moved into the Gulf of
Mexico and intensified,
forecast models came
into better agreement
concerning the track of
Katrina.
Forecasting
 Forecasters are less skillful
at predicting the intensity
of tropical cyclones than
cyclone track.
 The lack of improvement
in intensity forecasting is
due to the complexity of
tropical systems and an
incomplete understanding
of their internal dynamics.
Forecasting
 Hurricane Hunters are
aircraft that fly into
tropical cyclones in the
North Atlantic Ocean
and Northeastern Pacific
Ocean for the specific
purpose of directly
measuring weather data
in and around those
storms.
 1) Need to know how
intense the storm is.
 2) Need adequate data
to determine where the
storm will track.
 3) Need to know the
details of the hurricane
to verify computer
models of hurricanes.
Forecasting
 Naming
 Storms reaching tropical storm strength were initially given names to
eliminate confusion when there are multiple systems in any individual
basin at the same time, which assists in warning people of the coming
storm.
 Naming of Atlantic tropical storms has occurred since 1953
 Lists included only women’s names until 1979
 Since 1979, men’s and women’s names have been alternated
 Six lists are used
 The 2005 list will be used again in 2011 (minus Dennis,
Katrina, Rita, Stan, and Wilma)
Forecasting
 Naming
 Storms reaching tropical storm strength were initially given names to
eliminate confusion when there are multiple systems in any individual
basin at the same time, which assists in warning people of the coming
storm.
 Naming of Atlantic tropical storms has occurred since 1953
 Lists included only women’s names until 1979
 Since 1979, men’s and women’s names have been alternated
 Six lists are used
 The 2005 list will be used again in 2011 (minus Dennis,
Katrina, Rita, Stan, and Wilma)
Long Term Trends
 While the number of
storms in the Atlantic has
increased since 1995, there
is no obvious global trend;
the annual number of
tropical cyclones
worldwide remains about
87 ± 10.
 In spite of that, there is
some evidence that the
intensity of hurricanes is
increasing.
References

Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1994: Predicting Atlantic Basin
seasonal tropical storm activity by June 1. Weather Forecasting, 9, 103-115.

Larson, E., 2000: Isaac’s storm: A man, a time, and the deadliest hurricane in history.
Vintage.

Organization of American States: Primer on Natural Hazard Management in Integrated
Regional
Development
Planning.
Available
at
http://www.oas.org/osde/publications/Unit/oea66e/begin.htm#Contents

Toomey, D., 2002: Storm chasers: The hurricane hunters and their fateful flight into
Hurricane Janet. W. W. Norton and Co.
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Web Sites
http://www.srh.noaa.gov/jetstream//tropics/
http://en.wikipedia.org/wiki/Hurricane
http://en.wikipedia.org/wiki/Hurricane_Floyd
http://en.wikipedia.org/wiki/Hurricane_Andrew
 www.nhc.noaa.gov
 www.aoml.noaa.gov/hrd