NATS 101 Lecture 20 Hurricanes

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NATS 101 Lecture 20Hurricanes

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Supplemental References for Todays Lecture

• Aguado, E. and J. E. Burt, 2001 Understanding
  Weather Climate, 2nd Ed. 505 pp. Prentice Hall.
  (ISBN 0-13-027394-5)
• Danielson, E. W., J. Levin and E. Abrams, 1998
  Meteorology. 462 pp. McGraw-Hill. (ISBN
  0-697-21711-6)

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Types of Tropical Cyclones

• Cyclone Type Winds
• Tropical Depression 25-39 mph
• Tropical Storm 40-74 mph
• Hurricane/Typhoon ? 75 mph
• Most Depressions do not develop into Storms
• Majority of Storms reach Hurricane status

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Some Hurricane Extremes

• Lowest Central Pressure Pressure
• Pacific Typhoon Tip 1979 870 mb
• Atlantic Hurricane Wilma 2005 882 mb
• Costliest Hurricanes Cost-Loss
• Hurricane Andrew 1992 25 billion
• Hurricane Katrina 2005 200 billion?
• Bangladesh Cyclone 1970 300,000 dead

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Andrew 1992 Time Sequence
2005 Atlantic Hurricanes NASA Note cooler water
in wake of Dennis, Emily and Katrina
Link to Older NASA Satellite Animations
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U.S. Hurricane Deaths and Costs
Williams, The Weather Book
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Hurricane Lecture Overview

• What are the primary differences between
  hurricanes and extratropical cyclones?
• When and where do hurricanes form?
• How do hurricanes intensify?
• What is the structure of a hurricane?
• What kind damage do hurricanes inflict?
• When and where do hurricanes dissipate?

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Differences Between Tropical and Extratropical
Storms
Williams, The Weather Book

• Strong Fronts
• Cold at Storm Center Aloft
• Strongest Winds Aloft
• Forms outside Tropics
• Diameter of 500-1000 miles
• Energy Source Horizontal Temperature Contrast

• No Fronts
• Warm at Storm Center Aloft
• Strongest Winds near Surface
• Forms over Tropical Oceans
• Diameter of 200-500 miles
• Energy Source Energy Fluxes from Warm Ocean

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Where Hurricanes Form?

• Hurricanes go by different names in different
  regions of the world.
• Form over warm tropical waters, equatorward of
  20? latitude
• Not on equator (poleward of 5 ?) b/c non-zero
  Coriolis is needed.
• Occur most frequently over Western North Pacific
  Ocean.

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Atlantic Hurricane Frequency

• Occur in Warm Season
• Maximum Likelihood when Sea Surface Temperatures
  are Warmest-September
• Average of 6 Per Year
• Large Yearly Variability
• Fewer in El Nino Years
• More in La Nina Years

Danielson et al. Fig. 13.2
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Atlantic Hurricane Tracks

• Atlantic hurricanes tend to form in the Middle
  Tropical Atlantic Ocean and Caribbean Sea
• They usually propagate westward before turning
  northward
• They dissipate rapidly over land

Danielson et al. Fig. 13.12
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Hurricane Steering
Large-scale flow controls where hurricanes go.
Williams, The Weather Book
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Hurricane Necessary Ingredients

• Warm Water with T ? 82oF Deep Warmth gt 200 ft
• Converging Surface Winds Seedling Low Required
• Conditionally Unstable Air Supports Deep
  Convection
• Widespread, Deep Humid Air Supplies More Latent
  Heat
• Weak Vertical Wind Shear Shear Shreds Storm Apart
• Diverging Winds Aloft

Williams, The Weather Book
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Where do Seedling Vortices Come?Lots of Places
and Ways
Remnant mid-lat circulation
Remnant MCC circulation
Danielson et al. Fig. 13.14
Vortices along ITCZ
Easterly Waves
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3D Flow within Hurricanes
Winds aloft spiral outward clockwise
Winds inside eye spiral downward clockwise
Eyewall winds spiral upward cclockwise
Surface winds spiral inward cclockwise
Aguado and Burt, Fig. 12-3
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Thermal Structure of Hurricane
Aguado and Burt, Fig. 12-4
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Radar of Andrews Landfall

• Most intense rainfall is along the eyewall.
• Fastest surface winds are along the eyewall.
• Region inside of eye is dry with light winds

Rita TRMM "Hot Towers"
Rita TRMM Rain Rate
Danielson et al. Fig. 13.25
Floyd hourly rain loop from RSMAS
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Eye of Hurricane Luis 1995
Luis Visible Eye Animation
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Asymmetry of Hurricane Winds
Region of Maximum Storm Surge
20 kts
100 kts
80 kts
80 kts
60 kts
Aguado and Burt, Fig. 12-10
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Hurricane Intensity Scale
(gt 980 mb)
(965-980 mb)
(945-964 mb)
(920-944 mb)
(lt 920 mb)
Williams, The Weather Book
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Primary Hurricane Hazards

• Wind Damage
• Large-Scale Hurricane Circulation Itself
• Embedded Tornadoes
• Flooding
• Heavy Rains Far Inland, 5-10 Common
• Storm Surge along Shoreline

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Hurricanes Spawn Tornadoes

• Tornadoes embedded within a hurricane after
  landfall tend to be weak (category F1-F2)
• But they are embedded within an environment with
  65 kt winds.
• Causes hurricane wind damage to be localized.

Aguado and Burt, Fig. 12-11
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Inland Flooding-Agnes 1972

• Even weak hurricanes can be catastrophic,
  hundreds of miles inland.
• Agnes 1972, category 1 storm for a few hours.
• Agnes merged with a slow-moving ET cyclone.
• Up to 15 of rain in 24 h fell over Pennsylvania.
• Previous flood records exceeded by 6 ft.
• Damage gt 10B in inflation adjusted dollars.
• Costliest U.S. storm prior to Andrew and Katrina.

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Storm Surge I

• Low atmosphere pressure raises a mound of water
  inside eye.
• Water rises about 1 cm for every 1 mb decrease
  in pressure.
• Inward spiraling winds push more water toward
  hurricane eye.
• Deep hurricanes only raise about 1 meter of
  water over deep ocean.
• Water can sink downward and flow away from the
  surface.

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Storm Surge II

• In shallow water near land, water can not flow
  away under surface.
• But winds continue to push water inward towards
  storms center.
• Winds along hurricanes right flank also push
  water against shore.
• Water piles up along shoreline and rushes
  inland. The big effect!
• Effect is worse where ocean floor slopes gently
  - Gulf of Mexico!
• Link to COMET Surge Animation

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Storm Surge III

• If hurricane hits at high tide, the two effects
  superimpose.
• A 2 ft tide plus a 10 ft surge rises water 12 ft
  above mean sea level.
• Penetration of storm-whipped waves inland
  worsens damage.
• Waves cause far more destruction than the high
  water alone.

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Winds and Storm Surge
Danielson et al. Fig. 13.20
Floyd wave height forecast from RSMAS
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Surge Damage

• Richelieu Apartments before and after landfall
  of Camille 1969.
• Camille was a Category 5 hurricane.
• Sustained winds gt 180 mph!
• Storm surge was 24 feet along the coast!
• Many tired citizens took refuge in apartments.
• Sadly, many died.

http//en.wikipedia.org/
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Hurricane Decay
Andrew Central Pressure
Danielson et al. Fig. 13.26
Hurricanes weaken when they make landfall (or go
over cool water). Intense surface energy fluxes
are cut off and friction increases.
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Summary Hurricanes

• What are differences between hurricanes and
  extratropical cyclones?
• Many significant ones! See earlier slide.
• Where and when do hurricanes form?
• 5-20? latitude over oceans during warm season
• How do hurricanes intensify?
• Energy source is surface energy fluxes from the
  underlying warm ocean

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Summary Hurricanes

• What is the structure of a hurricane?
• Eyewall - strongest winds, heaviest rain
• Eye - dry with light winds
• What kind damage do hurricanes inflict?
• Can be catastrophic due to high winds,
  torrential rains, and coastal storm surges
• When and where do hurricanes dissipate?
• At landfall or when they go over cold water

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Assignment for Next Lecture

• Topic Air Pollution
• Reading - Ahrens, pg 317-324, 327-340
• Problems - 12.1, 12.5, 12.14, 12.15, 12.19, 12.23