Hurricane Winds

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Hurricane Winds

• Mark Powell NOAA-AOML Hurricane Research
  Division

M. POWELL HURRICANE IMPACTS 2012
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Measuring
A VARIETY OF SENSORS EXAMINE THE STORMS WIND
FIELD
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Wind Field

• Peak wind determines intensity
• Extent of damaging winds radii
• Projection of tropical storm winds relates to
  evacuation

KATRINA 24 HOURS BEFORE LANDFALL
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Landfall
KATRINA 4 AM CDT

• Moving northward closer to land, storms
• Encounter cooler water on the continental shelf
• Interact with shear from approaching cold fronts
• Receive inflow of cooler, drier air flowing
  offshore

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Landfall
KATRINA 7 AM CDT

• Pressure begins to rise
• Pressure gradient weakens
• Peak winds begin to decrease
• wind field expands conservation of angular
  momentum

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Landfall Winds
KATRINA 7 AM CDT

• Structures and vegetation (roughness) slow down
  the wind
• Frictional Inflow increases
• Convergence zones can increase
• Gustiness (turbulence) increases
• Large wind differences between marine, open,
  suburban areas

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Post Landfall
KATRINA 10AM CDT

• Pressure continues rising
• Pressure gradient continues weakening
• Enhanced friction diminishes the wind field

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Storm Decay
ANDREW 6 HOURS AFTER LANDFALL

• Cooler, drier air over land less fuel for the
  heat engine
• Stronger storms decay faster
• By 6 h after landfall, Andrew reduced from 140
  kts to 95 kts

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Wind Damage

• Turbulence lofts missiles
• Complex terrain
• Local hilltops can double the wind
• Valleys and gorges can funnel the wind
• Wind tunnel modeling and flow models can help
  identify risk areas

MISSILES LOFTED DURING HURRICANE INIKI ON LIHUE
(BRUCE ASATO, HONOLULU ADVERTISER)
HURRICANE MARILYN 1995 HILLTOP DAMAGE ON ST.
THOMAS
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Hurricane Tornadoes

• Mostly in outer bands
• Mostly front right
• Weaker than great plains types
• Difficult to detect
• Short life times, move quickly
• Additional damage microbursts

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Hurricane Andrew 1992
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Wind Damage

• Force goes with wind2
• Turbulence Cycling between gusts and lulls
  causes fatigue
• Roof suction pressures related more to turbulent
  wind structure than peak wind
• Wind direction houses experiencing winds from
  many directions compared to one general direction
  (eye vs eyewall)
• Upstream terrain

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Eyewall Damage Hurricane Andrew

• Importance of construction details
• Strong building code
• Code enforcement and product approval

PHOTO POWELL, NARANJA LAKES AFTER HURRICANE
ANDREW
M. POWELL HURRICANE IMPACTS 2012