Hurricanes and Cyclones

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Hurricanes and Cyclones
Hurricane Katrina (Aug. 28, 2005)
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Introduction to Cyclones

• An area or center of low pressure with rotating
  winds
• Counter-clockwise in Northern Hemisphere
• Clockwise in Southern Hemisphere
• Tropical or extra-tropical (outside the tropics)
• Based on origin and core temperature
• Characterized by intensity
• Wind speeds and lowest atmospheric temperature

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Tropical Disturbance

• An organized mass of thunderstorms persisting for
  24 hrs
• Typically 200600 km (120370 mi)
• Formed by
• Lines of convection
• Upper-level low pressure troughs
• Cold front remnants
• Easterly waves of converging and diverging winds
• Atlantic Ocean hurricanes

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Tropical and Extra-tropical Cyclones

• Tropical cyclones
• Form over warm tropical or subtropical ocean
  water (50200)
• Have warm central cores
• Tropical depressions, tropical storms, hurricanes
• High winds, heavy rain, surges, and tornadoes
• Extra-tropical cyclones
• Form over land or water in temperate regions
  (30o700)
• Associated with fronts and cool central cores
• Strong windstorms, heavy rains, surges,
  snowstorms, blizzards

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Classification

• Noreaster
• Extra-tropical cyclone that moves along northward
  along East Coast U.S.
• Hurricanes
• Tropical cyclones in Atlantic and eastern Pacific
  Oceans
• Typhoons
• Tropical cyclones in Pacific Ocean west of
  International Dateline and north of the equator
• Saffir-Simpson Scale classifies hurricanes based
  on wind speed

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Naming

• Extra-tropical storms are sometimes named after
  their origins.
• i.e., Alberta Clipper
• Hurricanes named by international agreement
  through World Meteorological Organization
• Named once winds exceed 63 km (39 mi) per hour.
• Names assigned sequentially each year from list
  for each origin.
• Male/female names alternated.
• Names are reused every 6 years.
• Names of big storms are retired.

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Tropical Depressions and Tropical Storms

• Tropical depressions
• Tropical disturbance wind speeds increase and
  begin to spin.
• A low pressure center is formed.
• Tropical storm
• Winds increase to 63 km (39 mi) ph.
• Storm is given a name.

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Hurricanes, cont.

• Rain bands
• Clouds that spiral inward around center
• Counter-clockwise in Northern Hemisphere
• Increase in intensity towards the center of the
  hurricane
• Eyewall
• Innermost band of clouds
• Contain the greatest winds and rainfall
• Eye
• Area of calm at center of the hurricane
• Narrow at surface and wider at top

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Hurricane Structure

• Warm, moist air spirals upward around eyewall.
• Air rises, it loses moisture.
• Upward rotation draws air from eye, causing dry
  air to sink back into center.
• Upward rotation also causes air to flow out the
  top of the storm concentrated in exhaust jets.
• Allows additional warm air to feed bottom of the
  storm

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Figure 9.13
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Hurricanes

• Necessary conditions
• Thick layer of warm water at the ocean surface
• Atmosphere that allows warm, moist air to rise
  upward to top of troposphere
• Weak upper level winds
• Hurricane description
• Winds 119 km (74 mi) ph
• Average diameter 500 km (310 mi)

Hurricane eyewall (Katrina)
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Hurricane Demise

• Hurricane energy comes from warm ocean water.
• As hurricane moves over land, it loses energy.
• Can become extra-tropical cyclone

http//coastal.er.usgs.gov/hurricanes/index.html
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Extra-tropical Cyclones

• Necessary conditions
• Strong temperature gradient at surface
• Along cold, warm, or stationary fronts
• Strong upper-level winds
• Jet stream
• Polar jet stream
• Subtropical jet stream

Figure 9.16
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Extratropical Cyclone Development
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Extratropical Cyclone Structure
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Extratropical cyclone in Pacific
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Hurricanes in East and Gulf Coasts

• Usually, take one of three paths
• Season starts June 1 and ends Nov 30

Figure 9.21
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Geographic Regions at Risk for Cyclones
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Geographic Regions at Risk for Cyclones, cont.
Figure 9.24
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Geographic Regions at Risk from Cyclones, Summary

• Tropical cyclones
• East and Gulf Coasts
• Hawaii and Atlantic Canada
• Baja California and West Coast Mexico
• Extra-tropical cyclones
• Winter windstorms in Pacific Coast
• Winter snow Sierra Nevada, Rocky Mountains, and
  east
• Spring and summer thunderstorms and tornadoes in
  U.S. and Canada

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Cyclone Effects Storm Surge

• Local rise in sea level, resulting from storm
  winds.
• Can be 3m (10ft)
• Because of spinning, surge is greatest in right
  quadrant of storm as it makes landfall.
• Height is greatest near time of maximum winds.
• Height is also greater if landfall coincides with
  high tide.

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Where are the strongest winds?
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Effects on Storm Surge Magnitude

• Largest effect from stress exerted by wind on
  water
• Fetch refers to the area over which the wind
  blows
• Larger fetch results in larger storm surge
• Smaller effect from low atmospheric pressure in
  storm pulling up on water surface.
• Also depends on shape of coastline.
• Water level tends to increase continually as
  storm approaches.

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Case Study New OrleansWhy so much flooding?
Subsidence map mean subsidence is 6 mm/yr with
sea level rise, its 8 mm/yr
(Nature 6/1/06)
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Cyclone Effects High Winds

• Described by Saffir-Simpson Scale
• Decrease exponentially with landfall
• Strongest recorded winds in U.S. from
  extra-tropical cyclone
• Responsible for strong winds in blizzards and
  tornadoes

Hurricane Andrew, FL (1992)
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Cyclone Effects Heavy Rains

• Average hurricane produces trillion gallons of
  water
• Rainfall from cyclones can cause inland flooding
• Flooding affected by
• Storms speed
• Land elevation over which the storm moves
• Interaction with other weather systems
• Amount of water in soil, streams, and lakes prior
  to storm

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Links to Other Natural Disasters

• Coastal erosion
• Flooding
• Mass wasting
• Other types of severe weather
• Tornadoes, severe thunderstorms, snowstorms, and
  blizzards

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Human Interaction with Cyclones

• Urbanization of vulnerable coastlines
• Destruction of sand dunes
• Construction of seawalls and bulkheads
• Poor building materials and practices
• Global warming

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Forecasting and Warnings

• Forecast includes
• If it will make landfall
• Where and when
• Wind strength
• Rainfall amount
• Storm surge
• Monitored by U.S. Hurricane Center, Canadian
  Hurricane Center.
• Hurricane watch means likely hurricane in 36 hrs.
• Hurricane warning given when hurricane is likely
  within 24 hrs or less.

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Forecasting Tools

• Weather satellites
• Detect early warning signs
• Cannot show wind speed
• Aircraft
• U.S. Air Force, NOAA airplanes fly into the storm
  to collect data
• Doppler radar
• Give information on rainfall, wind speed, and
  direction of the storm

Hurricane Charley, 2004
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Forecasting Tools, cont.

• Weather buoys
• Automated weather stations that give information
  at their locations
• Computer models
• Predict and track hurricane progress
• Have vastly improved hurricane information
• Still lacking in predicting storm intensity

Strom track Hurricane Charley (2004)
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Storm Surge Predictions

• Predict the time and elevation of surge.
• Forecasters use wind speed, fetch, and average
  water depth.
• Need detailed information on topography.
• Different elevations on land affect the storm
  surge.
• Computer models use central pressure, size,
  forward speed, track, wind speed, and seafloor
  topography.

Simulation of Category 5 hurricane hitting
Galveston, TX with 19 ft. storm surge
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Hurricane Prediction and the Future

• Deaths have decreased dramatically because of
  better forecasting, improved education, and
  greater public awareness.
• However, coastal populations are increasing,
  causing an increasing risk.

Hurricane-resistant building, Florida Keys