Hurricanes

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Hurricanes

• Hurricanes are large, tropical storm systems that
  form and develop over the warm waters near the
  equator.
• They are responsible for weather that can
  devastate entire communities
• Heavy rain -- Flooding
• Strong Winds
• Very Large Waves and Storm Surge
• Possibly Tornadoes

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Hurricanes

• To begin our study of hurricanes we must first
  look at the typical weather in the tropics.
• The sun typically remains high in the sky all
  year -- little variation in temperature with
  seasons.
• The strong sun heats the water and enhances the
  evaporation of the water.
• The general flow of air near the equator is out
  of the east -- Trade Winds.

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General Circulation
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Hurricanes

• The trade winds blow from the northeast in the
  Northern Hemisphere and from the southeast in the
  Southern Hemisphere.
• A region of convergence (Intertropical
  Convergence Zone -- ITCZ) creates a band of
  thunderstorms near the equator.

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General Circulation
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Hurricanes

• Since pressure gradients are small near the
  equator, we look at the flow of wind to find
  trough regions.
• A trough or ripple in the easterly flow is known
  as a tropical wave.
• Surface convergence occurs on the east side of
  this wave and surface divergence occurs on the
  west side.
• Thunderstorms tend to form on the east side of
  the tropical wave.

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Tropical Wave in the Easterlies
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Conditions for Tropical Wave Development

• The wave must be north or south of the equator.
• Coriolis force is zero at the equator.
• Conditional Instability
• Weak vertical shear
• This is different from what we want in the
  mid-latitudes.
• Warm sea-surface temperatures (SST)
• Typically SSTs are greater than 26oC

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Hurricane Development

• Convergence at the surface leads to convergence
  of moisture.
• The convection or thunderstorms that form release
  latent heat which intensifies the surface low.
• The warm core column of air will create an upper
  level high pressure center.
• The weak shear allows the storm to remain
  vertical and allows for the latent heat release
  to enhance the surface low.

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Hurricane Development
H
The energy for the growth of the storm comes
from the ocean (evaporation). Low shear will
keep the heating in the core to remain above the
surface convergence. Strong shear would rip
the storm apart.
Heating
L
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Hurricane Development
H
L
Strong Heating
As the winds increase, the ocean surface becomes
rougher and friction is added to the balance of
forces. This friction enhances
surface convergence.
Strong Convergence
L
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Hurricane Structure
This feedback mechanism continues as long as the
favorable conditions for hurricane growth
continue to exist.
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Hurricane Development

• This feedback mechanism is called CISK
• Conditional Instability of the Second Kind
• The Eye is the central region of the hurricane.
• It is often cloud free and has relatively calm
  winds.
• The eye is associated with subsidence that, in
  the strongest storms, keeps the eye cloud-free.

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Eye of the Storm
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Birthplaces of Hurricanes
Hurricane birthplaces are near the equator but
between 5-15 degrees latitude away from the
equator. The paths tend to be easterly as
the hurricanes develop and grow.
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Tropical Wave Development

• Tropical Disturbance
• A collection of thunderstorms with a slight
  circulation.
• Tropical Depression
• Closed circulation
• Wind speeds between 20-34 knots.
• Tropical Storm
• Closed circulation
• Wind speeds between 35-64 knots.
• The storm is named.

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Tropical Wave Development

• Tropical Disturbance
• A collection of thunderstorms with a slight
  circulation.
• Tropical Depression
• Closed circulation
• Wind speeds between 20-34 knots.
• Tropical Storm
• Closed circulation
• Wind speeds between 35-64 knots.
• The storm is named.

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Tropical Wave Development

• Hurricane
• Closed circulation
• Sustained wind speeds in excess of 64 kts (74
  mph).
• Saffir-Simpson Scale
• A numerical scale (1-5) that describes the damage
  potential of a hurricane.
• A quick and easy description of the strength of a
  hurricane.

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Tropical Wave Development
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Hurricane Hazards

• Strong Winds
• Hurricane sustained wind speeds are in excess of
  74 mph.
• Occasionally can get as high as 155 kts.
• Winds are typically stronger in Pacific storms --
  storms have more time grow in the larger Pacific
  ocean.
• Wind damage can be significant especially to
  weakly built houses.
• Spin-up vortices can cause very strong and
  damaging wind gusts.

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Hurricane Winds
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Hurricane Winds

• Hurricane winds are typically the strongest on
  the right side of the storm.
• The forward motion of the storm is added to the
  wind speeds on the right side of the storm to
  enhance the surface winds.

Strongest winds on the right side of the storm.
Weaker winds
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Hurricane Wind Damage
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Hurricane Hazards

• Inland Flooding
• Torrential rains can, especially if the hurricane
  moves slowly inland, can cause substantial
  flooding.
• Camille (1969)
• Inland in Mississippi
• Flooding in Virginia
• Agnes (1972)
• Inland in Florida
• 6.3 Billion in damage along the East Coast
• Flooding in Pennsylvania
• Alberto (1994)
• Never a hurricane! (Tropical Storm)
• Stalled over Georgia -- Americus, GA received 21

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Flooding in Richmond, VA
Hurricane Camille -- 19 August 1969
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Flooding in Wilkes-Barre, PA
Hurricane Agnes -- 19-20 June 1972
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Storm Surge

• The abnormal rise in the ocean level associated
  with the hurricane landfall.
• As the hurricane approaches the coast, it
  pushes a large mass of water in front of it.
• When this pile of water reaches the coast, the
  water levels can rise as much as 7 meters (22
  feet).

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Storm Surge
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Storm Surge

• Usually 80 - 160 km wide.
• Not a tidal wave or tsunami.
• The dramatic rise in the sea level can cause
  catastrophic damage.
• The greatest storm surge is associated with
• Stronger hurricanes
• High tide
• Shape of the shore

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Before the Hurricane
Richelieu Apartments -- Pass Christian,
MS Hurricane Camille -- 17 Aug 1969
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After the Hurricane
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Camille Storm Surge
Pass Christian, MS Storm Surge 7.4 m Hurricane
Camille -- 17 Aug 1969
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Before Hugo
Folly Beach, SC -- 22 September 1989
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After Hugo
Folly Beach, SC -- 22 September 1989
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Death of a Hurricane

• Moves out of the warm, moist tropical air.
• Moves over land.
• Loss of moisture source
• Increased surface friction
• Temperature of the land is cooler than the warm
  ocean
• Moved under unfavorable large scale flow.
• High shear can rip a storm apart
• Large scale subsidence can inhibit convection

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Hurricane Warnings

• Hurricane warnings are designed to protect human
  life and their property.
• Possible Evacuations
• Detection techniques
• Satellites
• Radar
• Aircraft Reconnaissance
• Data Buoys
• Weather Channel Reporters!

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Satellite Detection of Camille
NIMBUS III Satellite -- 21 August 1969
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Satellite Tracking of Allen
Hurricane Allen -- August 1980
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Satellite Tracking of Allen
Hurricane Allen -- August 1980
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Satellite Tracking of Allen
Hurricane Allen -- August 1980
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Radar Detection of Hurricanes
Hurricane Andrew -- August 24, 1992
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Aircraft Detection of Hurricanes
WP-3 Aircraft
Dropsondes
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Tropical Storm Warnings

• Tropical Storm Watch
• Possible tropical storm conditions expected in
  the next 36 hours.
• Prepare to take appropriate action.
• Tropical Storm Warning
• Tropical storm conditions are expected in the
  next 24 hours.
• Take action!

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Hurricane Warnings

• Hurricane Watch
• Possible hurricane conditions expected in the
  next 36 hours.
• Prepare to take appropriate action.
• Hurricane Warning
• Hurricane conditions are expected in the next 24
  hours.
• Get out of Dodge!

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Hurricane Warning!
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Building for Disaster?

• 45 million people now live along hurricane-prone
  regions in the U.S.
• Gulf Coast population
• 1960 -- 5.2 Million
• 1990 -- 10.1 Million
• Doubled in only 30 years.
• Florida to Virginia
• 1960 -- 4.4 Million
• 1990 -- 9.2 Million

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Can We Evacuate?

• Hurricane evacuation times currently range from
  15 - 30 hours depending on the locale.
• Current warnings are only valid for 24 hours.
• Can the transportation infrastructure handle such
  a mass exodus quickly enough?

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Building for Disaster?

• We are constructing more and larger dwellings
  along our hurricane-prone coastlines.

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Building for Disaster?
Miami, FL
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Better Construction

• Improved construction techniques may help
  alleviate some storm damage.
• Reinforcing seawalls
• Creating larger sand dunes and beaches
• Building housing on stilts
• Better tie-downs between the foundation and the
  walls and between the walls and the roof.
• Not much will stop a 7 m storm surge.

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Is This Better?
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Prediction Difficulty

• We still have difficulty predicting the precise
  landfall of most hurricanes.
• Some hurricanes can loop.
• What if we evacuate and the hurricane goes
  elsewhere -- the Cry Wolf problem.
• False alarms are still high and many people
  become apathetic.
• We see that here with tornado watches and
  warnings!

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Prediction Difficulty
Hurricane Elena -- 28 August - 4 September 1985
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Possible Solutions

• Increased Research on Hurricane Prediction
• Evacuation Studies
• Emergency procedures in the event evacuation is
  not feasible
• Population growth management
• Hurricane education
• Improved construction building codes
• Wetland management
• Hurricane Modification ????