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Lecture 21: Hurricanes. Typhoons (Western Pacific) Tropical Cyclones (anywhere) Cyclones (Indian Ocean + others) Hurricanes (near N. America) . 4/22/03. Cool Video . http://www.open-video.org/results.php?VideoTitle=Anatomy+of+a+Hurricane (50 MB!!!) Cool site:

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lecture 21 hurricanes
Lecture 21: Hurricanes
  • Typhoons (Western Pacific)
  • Tropical Cyclones (anywhere)
  • Cyclones (Indian Ocean + others)
  • Hurricanes (near N. America)

4/22/03

cool video
Cool Video
  • http://www.open-video.org/results.php?VideoTitle=Anatomy+of+a+Hurricane
  • (50 MB!!!)

Cool site:

  • http://www.nhc.noaa.gov/index.shtml
anatomy spiral bands and eye

Divergence aloft

Subsidence

Anatomy—Spiral bands and eye

Surface flow

(convergence)

Eye

wall

Eye

Spiral bands

anatomy spiral bands and eye1

Divergence aloft

Anatomy—Spiral bands and eye

Subsidence

Surface flow

(convergence)

Eye

wall

Eye

Spiral bands

anatomy
Anatomy
  • Outflow aloft
  • Feeder bands (spiral bands)
  • Eyewall
  • Eye
slide8
Eye
  • Hurricanes only
  • Strongest ones have very tight eyes (10 mi?)
  • Centrifuge effect of sorts keeps stuff out of eye
  • Slack winds
  • Lowest pressure
  • Subsidence
  • Birds!!
  • Surrounded by eye wall
eye wall1
Eye Wall
  • Strongest winds
  • Heaviest rains
  • Strongest lift
  • Very little lightning! (why?)
spiral bands
Spiral Bands
  • Outer part of the storm
  • Many tornadoes, thunderstorms
  • Winds may not be hurricane strength
  • Not very well understood—why bands of something and then nothing?
  • Lift in bands, subsidence in gaps
slide22

L

H

hurricane frequency
Hurricane Frequency
  • 90% between August-October (N. Atlantic)
  • “Season” is June 1 – November 30
life cycle of a hurricane
Life cycle of a hurricane
  • Required for hurricane formation:
    • Surface temperature > 78ºF (25.5ºC) to at least 60 m depth
    • At least 5º away from the equator to get started, more (8-9º) to become a hurricane
    • Weak, uniform winds, no wind shear
    • Upper level support
warm water
Warm water
  • Supplies moisture
  • More importantly, it supplies LATENT HEAT
  • Latent heat of condensation is THE source of energy for a hurricane.
  • Must be deep (60 m) or the hurricane will stir up some colder water
coriolis force
Coriolis force
  • Must be outside of 5º to get started or 8º -9º to really get going.
  • Otherwise, there’s not enough spin, and air travels directly from high to low pressure
weak uniform winds
Weak, uniform winds
  • Wind shear will tear apart a hurricane
  • To get good vertical development, it all needs to occur in the same spot—can’t have it getting all slanted on us!
  • This is why El Nino means few hurricanes (strong shear—Westerlies aloft) while La Nina means many hurricanes in the N. Atlantic (weak shear)
upper level support
Upper level support
  • Must have divergence aloft or the hurricane fountain can’t get going nor will it keep going.
beginnings
Beginnings
  • Easterly wave
    • Tropics are RIPE!!!
      • No thermal differences
      • No pressure differences
    • Look for convergence in winds!!!
  • Or a castoff from the ITCZ

Streamlines

tropical disturbances
Tropical Disturbances
  • 90% of these die out
  • If, at any time, wind shear, cold water, or land are encountered, the whole thing stops.
  • Some form a closed circulation, and are able to continue to strengthen—a tropical depression
spinning up
Spinning up
  • Circulation begins in a weak tropical disturbance
  • A WARM CORE develops from all the latent heat.
  • Winds converge toward the center…
    • Accumulating more moisture
    • Which creates more storms & more intense storms
    • Pressure near tropopause starts to increase—divergence aloft—in response to latent heat release.
hurricane transition
Hurricane transition
  • Some tropical depressions intensify to become tropical storms
  • About half of those go on to become hurricanes
fates
Fates
  • Hurricane can:
    • Go over land (fizzle)
    • Lose upper level support (fizzle)
    • Encounter wind shear (fizzle)
    • Wander over cooler waters (fizzle)
    • Turn into an extratropical cyclone w/ fronts
    • Get sucked up into an extratropical cyclone
what the heck is that
What the heck is that?
  • A subtropical storm
  • Has some features of tropical, some features of extratropical storms