Download Extratropical Cyclones

Lecture Presentation
Chapter 10
Hurricanes and
Extratropical
Cyclones
© 2012 Pearson Education, Inc.
Learning Objectives
 Understand the weather conditions that create,
maintain, and dissipate cyclones
 Understand the difficulties in forecasting cyclone
behavior
 Know what geographic regions are at risk for
hurricanes and extratropical cyclones
 Understand the effects of cyclones in coastal and
inland areas
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Learning Objectives, cont.
 Recognize linkages between cyclones and other
natural hazards
 Know the benefits derived from cyclones
 Understand adjustments that can minimize damage
and personal injury from coastal cyclones
 Know the prudent actions to take for hurricane or
extratropical cyclone watches and warnings
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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 extratropical
 Based on origin and core temperature
 Characterized by intensity
 Sustained wind speeds and lowest atmospheric
temperature
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Tropical and Extratropical Cyclones
 Tropical Cyclones
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Form over warm tropical or subtropical ocean water (5°–20°)
Have warm central cores
Tropical depressions, tropical storms, hurricanes
High winds, heavy rain, surges, and tornadoes
Derive energy from warm ocean water and latent heat
 Extratropical Cyclones
 Form over land or water in temperate regions (30°–70°)
 Associated with fronts and cool central cores
 Strong windstorms, heavy rains, surges, snowstorms,
blizzards
 Most do not produce severe weather
 Derive energy from temperature contrasts along fronts
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Classification
 Nor’easter
 Extratropical 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
 Cyclones
 Tropical cyclones in Indian Ocean
 Saffir-Simpson Scale classifies hurricanes based on wind
speed
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Table 10.1
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Naming
 Extratropical storms are sometimes named after their
origins
 Example: 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 (example: Katrina)
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Cyclone Development: Tropical
Disturbance
 A organized mass of thunderstorms persisting for > 24 hours
 Typically 200 to 600 km (120 to 370 mi.)
 Has a weak rotation due to Coriolis effect
 Formed by
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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 Depressions and Tropical
Storms
 Tropical Depression
 Tropical disturbance wind speeds increase and
begins to spin
 A low pressure center is formed
 Tropical Storm
 Winds increase to 63 km (39 mi.) ph
 Storm is given a name
 Wind speeds are not at hurricane strength, but
rainfall can be intense
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Hurricanes
 Not all tropical storms develop into hurricanes
 Classified when winds reach 119 km (74 mi.) per hour
 Environmental conditions
 Thick layer of warm ocean water
 Water must be warm and there must be deep
 Steep vertical temperature gradient
 Atmosphere must cool quickly with increasing altitude
 Weak vertical wind shear
 Strong winds aloft prevent hurricane development.
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Hurricane Structure
 Rain bands
 Clouds that spiral inward around center
 Counterclockwise 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, cont.
 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 10.14
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Hurricane Paths and Demise
 Movement is controlled by the Coriolis effect and
steering winds
 In Northern Hemisphere storms deflect to the right
 Track west in trade winds and curve northwest and
then northeast
 Hurricanes can make a loop
 In North Atlantic, steered by Bermuda High
 As hurricane moves over land, it loses energy
(warm water)
 Can become extratropical cyclone
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Extratropical Cyclones
 Necessary conditions
 Strong temperature
gradient at surface
usually along cold, warm
or stationary fronts
 Strong upper level winds
provided by jet stream
 Polar jet stream
 Subtropical jet stream
Figure 10.18
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Polar and Tropical Jet Stream
 Polar jet stream shifts from crossing the United States
in the winter to crossing southern Canada in the
summer
 Subtropical jet stream crosses Mexico and Florida and is
strongest in the winter
 Large high-pressure ridges and low-pressure troughs
cause jet streams to bend and producing waves or
meanders
 May also split in two around isolated high-pressures and
reunite
 Extratropical cyclones often develop in curves or
divergences in jet streams
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Polar and Tropical Jet Stream, cont.
 Bending or splitting cause the polar jet stream to dip
south and the subtropical jet stream to flow northeast
 The southern branch of a split polar jet stream in the
Pacific Ocean brings warm moist air out of the tropics
 West Coast forecasters refer to the flow of warm moist air
as the Pineapple Express, because of its origin near
Hawai’i
 Nor’easters form when bends of the polar and
subtropical jet streams begin to merge off the
southeastern coast of the United States
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Extratropical Cyclone Development
 Low-pressure center develops along frontal boundary
 Cold front on southwest, warm front on east
 Conveyor belt of cold air circulates counter-clockwise
 Warm air is wedged to the east
 Conveyor belt of warm air rises on the southeast side creating
a comma
 Conveyor belt of dry air aloft feeds the cyclone from behind
the cold front
 Cold front wraps around the warm front, causing an
occluded front develop trapping warm air aloft
 Cold air completely displaces the warm air, pressure
gradient weakens and storm dissipates
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Figure 10.20
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Figure 10.21
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Geographic Regions at Risk for
Cyclones: North America
 Hurricanes threaten
contiguous United States,
Puerto Rico, the Virgin
Islands, and U.S. territories
in the Pacific Ocean
 They are a lesser threat to
Hawai’i and Atlantic Canada
 On the Pacific coast,
hurricanes strike Baja
California and the west
coast of the Mexican
mainland
Figure 10.22
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Atlantic Hurricane Paths
 West toward East coast of
Florida, sometimes passing
over Caribbean
 Move out into the Atlantic
Ocean to the northeast
 Westward over Cuba and
into the Gulf of Mexico to
strike the Gulf Coast
 Westward to the Caribbean
and then northeastward
skirting the East Coast
 May strike the continent
from central Florida to New
York
Figure 10.17
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Figure 10.24
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Geographic Regions at Risk for
Cyclones: Worldwide
 Northwest Pacific is much more active than North
Atlantic
 Indian Ocean is also a very active hurricane zone
 South Atlantic and southeast Pacific, rarely have
hurricanes because of cold ocean water
 Hurricanes do not form close to the equator
because of the absence of the Coriolis effect
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Figure 10.25
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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
 Extratropical cyclones
 Winter windstorms in Pacific Coast
 Winter snow Sierra Nevada, Rocky Mountains and
east
 Spring and summer thunderstorms and tornadoes in
United States and Canada
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Cyclone Effects: Storm Surge
 Local rise in sea level resulting from storm winds
 Can be > 3 m (10 ft.)
 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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Figure 10.26
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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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Cyclone Effects: High Winds
 Described by Saffir-Simpson Scale
 Decrease exponentially with landfall
 Strongest recorded winds in United States
from extratropical cyclone
 Responsible for strong winds in blizzards and
tornadoes
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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:
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Storm’s 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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Natural Service Functions of Cyclones
 Source of precipitation
 Redistribute warm air from tropics
 Maintain ecosystems
 Winds carry plants, animals, and
microorganisms
 Waves stir up deeper, nutrient-rich waters
 Winds topple weak and diseased trees in forests
 Waves break apart some corals
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Human Interaction with Cyclones
 Urbanization of vulnerable coastlines increases
magnitude of the effect of cyclones
 Destruction of sand dunes makes areas more
susceptible to hurricane winds
 Construction of seawalls and bulkheads reflect waves
and contribute to beach erosion
 Poor building materials and practices can make
hurricanes more dangerous to people
 Global warming may contribute to higher intensity and
frequency of hurricanes in the future
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Minimizing the Effects of Cyclones:
Forecasting and Warnings
 Forecast includes:
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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 hours
 Hurricane warning given when hurricane is likely within
24 hours or less
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Forecasting Tools
 Weather satellites
 Detect early warning signs
 Can not 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
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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
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Figure 10.32
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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
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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,
increasing risk
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Perception of and Adjustment to
Cyclones
 Perception of hazard depends on personal
experience
 More experienced people may take hazard more
seriously
 More seasoned people may also take less precautions
 Community adjustments to cyclone hazard
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Warning systems
Evacuation plans and shelters
Insurance
Building design
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Perception of and Adjustment to
Cyclones, cont.
 Personal adjustments to cyclone hazard
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Be aware of hurricane season
Prepare homes and property for hazard
Obtain flood insurance
Install heavy shutters that can be latched
Learn evacuation route
Make a family emergency plan
Collect emergency supplies
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End
Hurricanes and Extratropical Cyclones
Chapter 10
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