Natural Disasters

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Natural Disasters

• Thursday 2005 Sep 1
• Lecture 4

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Todays Disasters

• Katrina

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New Orleans Breaches in at least 2
levees Passing over floaters to rescue people
trapped on rooftops or in attics 80 of city
underwater Mayor Nagin estimates 12-14 weeks
until resident return Levee repair is
fiasco French Quarter sits 5 ft above sea level
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Mississippi Biloxi and Gulfport devastated 30
storm surge wipes of 90 of buildings along
coasts Hundred dead
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Flooding around the Superdome
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Prisoners evacuated
17th St Levee break
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Levee Break
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I-10 bridge heavily damaged
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I-10 damage
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Highway 90 into Biloxi
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New Orleans, LA

• New Orleans is going down
• 350 miles of levees and walls
• Built on flood plain delta

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What is a hurricane?

• (sub)tropical
• originates close to the equator, generally
  between the Tropic of Cancer (23.5 N) and the
  Tropic of Capricorn (23.5 S)

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What is a hurricane?

• Cyclone circular, rotating storm

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What is a hurricane?

• Forms over warm water
• minimum 25C (77F) note 30C (86F) isotherm

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What is a hurricane?

• Sustained (1 minute) winds at least 74 mph.

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What is a hurricane?

• (sub) tropical
• Cyclone circular, rotating storm
• Forms over warm, tropical oceans
• Sustained (1 minute) winds at least 74 mph.

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Hurricane, Typhoon, or Cyclone?

• Local terms
• hurricane in Atlantic and eastern Pacific
• typhoon in western Pacific
• cyclone in Indian Ocean
• The difference between hurricane, typhoon, and
  cyclone is most like whether you call Coca-Cola
  soda or pop. It is a matter of local custom.

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Cyclones

• Cyclones rotate
• from the word "cycle" related to the word
  "circle."
• Most generally, cyclone is any organized circling
  wind, including dust devils, tornadoes, tropical
  depressions, tropical storms, and hurricanes (
  typhoons).

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Cyclones
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Cyclones

• Context means a lot.
• When the Hickory (the Tin Man in Oz) tells
  Dorothys Uncle Henry that The cyclones
  comin, he means a tornado.
• When Miami evacuates in the face of a cyclone,
  it's a hurricane.

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Cyclonic Rotation

• northern hemisphere
• counter-clockwise
• southern hemisphere
• clockwise
• This really has nothing to do with whether it is
  a "hurricane" or "typhoon," except that there are
  relatively few cyclones in the southern
  hemisphere.

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Hurricane Structure
Hurricane cutaway
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Hurricane Structure
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When How Does a Hurricane Form?

• Deep warm ocean water

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Hurricane Evolution
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• To undergo tropical cyclogenesis, there are
  several favorable pre cursor environmental
  conditions that must be in place (Gray 1968,1979)
  
• Warm ocean waters (of at least 26.5C 80F)
  throughout a sufficient depth (unknown how deep,
  but at least on the order of 50 m 150 ft). Warm
  waters are necessary to fuel the heat engine of
  the tropical cyclone.
• An atmosphere which cools fast enough with height
  such that it is potentially unstable to moist
  convection. It is the thunderstorm activity which
  allows the heat stored in the ocean waters to be
  liberated for the tropical cyclone development.
• Relatively moist layers near the mid-troposphere
  (5 km 3 mi). Dry mid levels are not conducive
  for allowing the continuing development of
  widespread thunderstorm activity.
• A minimum distance of at least 500 km 300 mi
  from the equator. For tropical cyclogenesis to
  occur, there is a requirement for non-negligible
  amounts of the Coriolis force to provide for near
  gradient wind balance to occur. Without the
  Coriolis force, the low pressure of the
  disturbance cannot be maintained.
• A pre-existing near-surface disturbance with
  sufficient vorticity and convergence. Tropical
  cyclones cannot be generated spontaneously. To
  develop, they require a weakly organized system
  with sizable spin and low level inflow.
• Low values (less than about 10 m/s 20 kts 23
  mph) of vertical wind shear between the surface
  and the upper troposphere. Vertical wind shear is
  the magnitude of wind change with height. Large
  values of vertical wind shear disrupt the
  incipient tropical cyclone and can prevent
  genesis, or, if a tropical cyclone has already
  formed, large vertical shear can weaken or
  destroy the tropical cyclone by interfering with
  the organization of deep convection around the
  cyclone center.
• Having these conditions met is necessary, but not
  sufficient as many disturbances that appear to
  have favorable conditions do not develop. Recent
  work (Velasco and Fritsch 1987, Chen and Frank
  1993, Emanuel 1993) has identified that large
  thunderstorm systems (called mesoscale convective
  complexes MCC) often produce an inertially
  stable, warm core vortex in the trailing
  altostratus decks of the MCC. These mesovortices
  have a horizontal scale of approximately 100 to
  200 km 75 to 150 mi, are strongest in the
  mid-troposphere (5 km 3 mi) and have no
  appreciable signature at the surface. Zehr (1992)
  hypothesizes that genesis of the tropical
  cyclones occurs in two stages
• stage 1 occurs when the MCC produces a mesoscale
  vortex.
• stage 2 occurs when a second blow up of
  convection at the mesoscale vortex initiates the
  intensification process of lowering central
  pressure and increasing swirling winds.

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Hurricane Structure
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Hurricane Relative Energy
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Hurricane Relative Energy
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Hurricane Relative Power
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• Seven Basins
• There are seven tropical cyclone "basins" 1.
  Atlantic basin,
• 2. Northeast Pacific basin
• 3. Northwest Pacific 4. North Indian
  basin 5. Southwest Indian basin
• 6. Southeast Indian/Australian basin
• 7. Australian/Southwest Pacific basin

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Saffir-Simpson Scale
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Hurricane Damage

• Wind
• Storm surge flooding
• Rain flooding
• Erosive waves
• Thunderstorms
• lightening
• tornadoes

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Hurricane DamageStorm Surge Flooding
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Hurricane DamageErosive Waves
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Hurricane DamageWind
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Hurricane/Tropical StormRecurrence Tracks
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