Tropical Cyclones and Hurricanes Read Chapter 11

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Tropical Cyclones and Hurricanes Read Chapter 11
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Charley
Ivan
Katrina's landfall
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The Hurricane

• A low pressure system of tropical origin that
  produces sustained surface winds 74mph
• Organized convection around center
• Tropical cyclone is generic name given
  low-pressure systems forming over warm, tropical
  seas (typhoon in western pacific, super typhoon
  if winds 150 mph)

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The Hurricane Birth Regions

• Not over land
• Not high latitudes, but not at the equator either
• (Graphic by Robert Simmon, NASA GSFC)

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A Recipe for Hurricane Formation

• 1) Sea surface temperature (SST) 80F (26.5C)
  deep layer of warm water
• 2) Conditional/absolute instability through a
  deep layer of troposphere- Important for
  thunderstorms!
• 3) Moist air in mid-troposphere
• 4) Weak vertical wind shear
• 5) Genesis region at least 5 away from equator
• 6) Source of low-level cyclonic spin

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Ingredient 1 Warm water

• Warm water ensures high evaporation rates, thus
  high low level dew pointspromotes instability
  convection!
• Tropical cyclones can rapidly strengthen if they
  move over patch of warm water

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Atlantic Hurricane Season Peaks Sept.10
around when ocean is warmest
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Why is a relatively deep layer of warm water
needed?

• Strong hurricane wind can lead to upwelling of
  colder water from below as
• high winds at surface mix the ocean water
• Cooler surface waters lead to less evaporation,
  more stabilized atmosphere

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Ingredient 2 Conditional Instability- How
convection leads to pressure falls
Animation on CD lets take a look
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Ingredient 3 Mid-Tropospheric Moisture

• Dry mid-level air promotes hurricane decay by
  leading to precipitation evaporating
  downdrafts cooler, more stable air near ground
  less convection

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4) Vertical wind shear separates low level
circulation from tall thunderstorms
Less wind shear better environment for
hurricane formation
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Ingredient 5 Cyclonic vorticity (spin)

• Earth vorticity is increased as one head toward
  poles, its zero at equator (related to Coriolis
  force)
• Earth vorticity needed for cluster of t-storms to
  obtain cyclonic spin
• Tropical cyclone formation at least 5 from
  equator

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Ingredient 6 Tropical Waves

• Emerge every 3-4 days off coast of Africa from
  June-October, about 60/year
• When waves spawn hurricanes in east-central
  Atlantic, storm is termed Cape Verde stormCape
  Verde season is from August to October

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Intertropical Convergence Zone (ITCZ) The source
for tropical waves
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Stages of Development I

• Tropical disturbance disorganized blob of
  thunderstorms, often forms within the ITCZ
• If atmosphere and ocean conditions favor
  development, we enter a positive feedback loop
• Latent heat release in thunderstorms through
  condensation
• Increased pressure aloft
• Upper-level divergence, latent heating
  compressional warming lead to lower surface
  pressure
• Increased inflow deflected to right and cyclonic
  circulation enhanced, more evaporation into air
  more fuel for t-storms
• Animation of this process on CD

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-Tropical Depression

• organized thunderstorms, circular wind flow
  around center, wind speed less than 39 mph
• Given number-letter tag (i.e. Tropical Depression
  5-A)

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Stages of Development II

• Tropical Storm sustained wind speeds from
  39-73mph with higher gusts
• Storm gets a name
• Six-year cycle (http//www.nhc.noaa.gov/aboutnames
  .shtml)
• Names reused or retired if storm is historic
• If all names used in a season, storms named with
  Greek alphabet
• If conditions remain favorable, positive feedback
  loop continues, leading to

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Stages of Development Hurricane

• Wind speed (sustained) at least 74 mph
• An eye forms
• Spiral bands
• Conservation
  of angular
  momentum
• Strength dictated by SSTs

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Strength of tropical cyclone related to number
intensity of thunderstorms around eye
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Winds fastest near surrounding calm eye of
hurricaneknown as eye wall
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Relative Velocity
Strongest winds in a tropical cyclone are in the
right front quadrant
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How does the eye form?

• As air spirals into center of storm, outward
  acting centrifugal force increaseseventually,
  centrifugal force equals PGF and air stops
  spiraling inward.leads to increased convergence
  and rising air motion
• Diameter 25 mi
• Smaller eye-stronger hurricane

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Loop of Hurricane Katrina showing cyclonic flow

• http//www.atmo.arizona.edu/products/radar/kat_lix
  _rad.gif

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Importance of anticyclonic flow aloft above a
hurricaneincreased upper-level divergence
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Hurricane Felix
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Vertical Wind Profile in a Hurricane

• Speedy ring of air at surface in eye wall expands
  outward as it rises due to upper-level divergence
• Wind speed decreases with height conserving
  angular momentum

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Hurricane Saffir-Simpson Scale (in need of
revision?)

• This graphic was created by Lynn A. Dombrowski,
  Ed. D.

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Storm Surge

• Water, not wind, is by far the biggest killer
  (includes both storm surge and fresh water
  flooding)
• Right front quadrant is most dangerous
  (N.Hemisphere)
• Strongest winds (rotation translation
  velocities)
• Winds blow on-shore
  which piles up
  water

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Demise

• Loss of fuel
• Moves into colder water
• Moves over land
• Wind shear
• Tilts the thunderstorms
• Lowest pressure not concentrated, pressure
  gradient relaxes and winds weaken
• Increased friction (landfall)

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Symbols on Weather Maps

• Tropical Storm Symbol
• Hurricane Symbol

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Hurricane Watches/Warnings

• Watch - Issued when there is a possibility that a
  part of the coastline could experience hurricane
  conditions within 36 hours
• Warning Issued when hurricane conditions are
  expected for a part of a coastline within 24 hours

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Understanding National Hurricane Center (NHC)
Forecasts

• http//www.nhc.noaa.gov/archive/2005/KATRINA_graph
  ics.shtml