Part 4. Disturbances

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Part 4. Disturbances

• Chapter 12
• Tropical Storms and Hurricanes

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Introduction

• Hurricanes are responsible for astonishing
  amounts of property damage and loss of life in
  many regions of the world
  

Hurricane Andrew
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Hurricanes Around the Globe

• Atlantic and eastern Pacific tropical cyclones
  are known as hurricanes, while over the western
  Pacific they are referred to as typhoons
  
• Over the Indian Ocean and Australia they are
  known as cyclones
  
• The eastern North Pacific has the highest
  frequency of tropical cyclones for the globe with
  an annual average of 16
  
• The South Atlantic produces none as the basin is
  too small to initiate cyclogenesis

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Tropical cyclone genesis areas
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The Tropical Setting

• A subsidence inversion, or trade wind inversion,
  forms on the eastern side of the subtropical
  anticyclones
  
• Below this, a marine layer of cool, moist air
  resides
  
• The marine layer is shallowest and the inversion
  lowest towards the eastern basin edges where cold
  water upwelling and cold ocean currents dwell
  
• Towards the western edges of the ocean basins the
  marine layer warms with corresponding surface
  temperatures and expands to greater heights
  
• Convection results in large cumulonimbus clouds
  over these regions as opposed to the eastern
  areas, where only stratus may exist

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Hurricane Characteristics

• Hurricanes, the most powerful of all storms, have
  sustained winds of 74 mph
  
• Although of lesser intensity than tornadoes, the
  much larger size and longer life span makes
  hurricanes much more devastating
  
• Average diameters approximately 350 mi. Central
  pressure averages about 950 mb. May be 870 mb
  
• Most energy attained by hurricanes stems from
  latent heat release in the cloud formation
  process
  
• Hurricanes occur where warm waters abound and
  during the times of highest SSTs
  
• August and September are the most active months

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Tropical cyclone structure
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Hurricane Characteristics

• A central eye is surrounded by large cumulonimbus
  thunderstorms occupying the adjacent eye wall
  
• Weak uplift and low precipitation regions
  separate individual cloud bands
  
• Although smaller, the pressure differences into
  the center of the storm are about twice as great
  as the average mid-latitude cyclone
  
• Unlike mid-latitude cyclones, hurricanes are
  warm-cored lows as a result of adiabatic
  expansion of in-rushing air

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Tropical cyclone structure
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Hurricane Characteristics

• Only slight horizontal temperature differences
  toward eye
  
• Latent heat release from condensation causes the
  eye to be much warmer than surrounding storm
  
• The horizontal pressure gradient with altitude
  decreases slowly
  
• Upper portion of the storm rotates
  anticyclonically while lower portions rotate
  cyclonically
  
• The upper portions of the storm are also
  blanketed by a cirrostratus cloud cap due to
  overall low temperatures

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Tropical cyclone structure
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The Eye and Eye Wall

• The eye is an area of descending air and light
  winds which is about 25 km (15 mi) in diameter on
  average
  
• A shrinking eye indicates storm intensification
• The eye wall is comprised of the strongest winds,
  the largest clouds, and the heaviest
  precipitation with rainfall rates as high as 100
  in./day
• Sinking air, warming adiabatically, causes air in
  the eye to be warmer than elsewhere
  
• Relative humidities are lower in this region due
  to higher temperatures

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Tropical cyclone structure
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Vertical temperature profile across a hurricane
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Hurricane Formation

• Tropical disturbances often begin in the eastern
  ocean basins as disorganized clusters of
  thunderstorms
  
• Some form in association with mid-latitude
  troughs migrating toward lower latitudes but most
  form from ITCZ-related convection
  
• Hurricanes are smaller than mid-latitude cyclones
  but larger than thunderstorms or tornadoes
  
• The most important condition for the formation of
  a hurricane is a deep surface layer of warm
  water

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Hurricane Formation

• Easterly waves, or undulations in the trade wind
  pattern, spawn hurricanes in the Atlantic
  
• Depicted by plotting streamlines or lines of wind
  direction
  
• Waves typically stretch between 12001800 mi
• Close-fitting streamlines east of the wave axis
  indicate zones of convergence, while west of the
  axis streamlines indicate divergence
  
• The disturbance is located upstream of the wave
  axis

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An easterly wave
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Hurricane Formation

• Atlantic hurricanes form from seedlings emanating
  from western Africa
  
• Move westward in the trade wind flow and
  intensify as waters warm beneath them
  
• Take about 1 week to traverse the Atlantic as
  average speeds are about 1020 mph
  
• Only about 10 intensify into more organized,
  rotating storms
  
• In strength, the order of development is tropical
  disturbance, tropical depression, tropical storm,
  and hurricane

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Hurricane Formation

• When at least one closed isobar is present, the
  disturbance is classified as a tropical
  depression
  
• Further intensification, to wind speeds of 60
  km/hr (37 mph), place the storm in the category
  of tropical storm
  
• Hurricane status is gained when winds reach or
  exceed 120 km/hr (74 mph)
  
• A higher percentage of depressions become
  tropical storms and an even higher percentage
  reach hurricane status

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Conditions Necessary for Hurricane Formation

• Hurricanes form over deep water layers with
  surface temperatures in excess of 81 oF
  
• Energy is derived from latent heat release
• Poleward of about 20o, water temperatures too
  cold
  
• Hurricanes most frequent in late summer and early
  autumn
  
• Coriolis force important contributor, and as
  such, hurricanes do not form equatorward of 5o
  
• An unstable atmosphere is also necessary and this
  typically occurs toward the central to western
  ocean basins

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Hurricane Movement and Dissipation

• Tropical disturbances and depressions regulated
  by trade wind flow and move westward
  
• For tropical storms and hurricanes, upper-level
  winds and ocean temperatures gain importance
  
• Movement may be highly erratic in particular
  storms sometimes backtracking previous routes
  
• In the Atlantic, storms that gain latitude
  recurve toward the north or northeast due to the
  influence of surface and upper-level
  westerlies
• Hurricanes loose energy and dissipate on reaching
  landfall or cold ocean waters

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Hurricane paths
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Destruction by Hurricanes

• Hurricane winds cause excessive damage
• Heavy rainfall responsible for large amounts of
  property damage
  
• The storm surge is responsible for largest
  percentage of damage and death along coastal
  regions
  
• The storm surge is most destructive on the right
  side of the storm, especially at high tide
  
• Winds and surge are typically most intense in the
  right quadrant of the storm where wind speeds
  combine with the speed of the storms movement
  
• This area also produces the greatest frequency of
  tornadoes within the hurricane

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Wind speed variations by quadrant
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Average tornado locations
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Hurricane Forecasts and Advisories

• The National Hurricane Center responsible for
  predicting and tracking hurricanes
  
• Data gathered through satellite observations,
  surface observations, and aircraft
  
• Statistical, dynamic, and hybrid computer models
  assist in predictions
  
• Future positions are given along six-hour
  trajectories with accuracy decreasing as lead
  time increases
  
• Climatologists are not agreed that global warming
  would lead to more severe hurricanes

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Hurricane Watches and Warnings

• A watch is administered if an approaching
  hurricane is predicted to reach land in more than
  24 hours
  
• If the time frame is less, a warning is given
• The erratic nature of the systems leads to
  difficulties in exact prediction, warning, and
  evacuation of prone areas
  
• There is substantial economic cost to an area
  that evacuates even if no hurricane strikes the
  area at all

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Hurricane Intensity Scale

• The Saffir-Simpson scale classifies hurricanes
  into five categories based on central pressures,
  maximum sustained wind speeds, and storm surge

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End of Chapter 12 Understanding Weather and
Climate