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Tropical Cyclones

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Atlantic Hurricane Season ... The official hurricane season for the eastern North Pacific runs from May 15 ... Atlantic Hurricane Season (Cont.) In an average ... – PowerPoint PPT presentation

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Title: Tropical Cyclones


1
Tropical Cyclones
2
Types of Cyclones
  • Tropical Cyclone
  • Extratropical Cyclone
  • Subtropical Cyclone

3
Types of Cyclones (continued)
  • Tropical cyclone is a warm-core, non-frontal
    low pressure system of synoptic scale that
    develops over the tropical or subtropical oceans.

4
Types of Cyclones (continued)
  • Extratropical (midlatitude) cyclone is a
    cold-core synoptic scale low pressure system
    whose primary source of energy is baroclinic. It
    is often associated with frontal systems.

5
Types of Cyclones (continued)
  • Subtropical cyclone is a low pressure system
    that develops over subtropical oceans and
    initially has a non-tropical circulation but
    exhibits some elements of the cloud patterns
    associated with tropical cyclones. It is often
    considered a hybrid system with some
    characteristics of tropical and extratropical
    cyclones.

6
Basic Definitions
  • Tropical Disturbance A discrete tropical
    weather system of apparently organized convection
    generally 100 to 300 mi in diameter
    originating in the tropics or subtropics, having
    a migratory character, and maintaining its
    identity for 24 hours. It may or may not be
    associated with a detectable perturbation of the
    wind field.

7
Possible Sources of Tropical Cyclones
  • Tropical Disturbances
  • Tropical Waves
  • ITCZ Disturbances
  • Other thunderstorm clusters

8
Tropical Disturbances
  • Tropical Wave an elongated trough of low
    pressure that moves from east to west between the
    subtropical high pressure system and the
    Intertropical Convergence Zone (ITCZ)
  • ITCZ Disturbance an area of thunderstorms that
    separates from the ITCZ

9
N
Subtropical High 1032 mb
E
Tropical Wave
1024 mb
1016 mb
Intertropical Convergence Zone (ITCZ) 1008 mb
10
Tropical Waves
  • The isobars make an inverted (i.e. upside down) V
    pattern at the tropical wave.
  • Surface convergence produces rising motion and
    thunderstorms to the east of the wave.
  • Surface divergence produces sinking motion and
    clearer skies to the west of the wave.
  • Tropical waves form over the interior of Africa
    and move westward over the Atlantic Ocean.

11
N
Subtropical High 1032 mb
E
The northeast trade winds produce divergence to
the west of the wave and suppress cloud formation
Tropical Wave
1024 mb
The northeast trade winds produce convergence to
the east of the wave and support the formation of
clouds and thunderstorms.
1016 mb
Intertropical Convergence Zone (ITCZ) 1008 mb
12
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15
ITCZ Disturbance
  • Sometimes as a tropical wave passes a cluster of
    thunderstorms develops and separates from the
    ITCZ.
  • If the cluster of thunderstorms persists for 24
    hours it is called an ITCZ disturbance.

16
N
Subtropical High 1032 mb
E
Tropical Wave
1024 mb
1016 mb
1008 mb
The ITCZ bulges toward the north in response to
the passage of the tropical wave
17
N
Subtropical High 1032 mb
E
Tropical Wave
1024 mb
A cluster of thunderstorms separates from the
ITCZ forming an ITCZ disturbance
1016 mb
1008 mb
The ITCZ bulges toward the north in response to
the passage of the tropical wave
18
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19
Possible Sources of Tropical Cyclones (continued)
  • Monsoon Disturbances
  • Monsoon Troughs
  • Monsoon Gyres
  • Monsoon Depressions

20
Possible Sources of Tropical Cyclones (continued)
  • Subtropical Cyclones
  • Extratropical systems
  • Old stationary frontal boundaries
  • Upper lows
  • 5. Mesoscale Convective Systems (MCSs)

21
Tropical Disturbances (Cont.)
  • Most tropical disturbances produce thunderstorms
    and heavy precipitation for a few days before
    dissipating.
  • However, if the conditions are favorable a
    tropical disturbance may develop into a tropical
    cyclone.

22
Conditions necessary for formation of a tropical
cyclone
  • Warm Sea Surface Temperatures (SSTs greater than
    26C)
  • Weak winds in the upper levels
  • A disturbance to provide low level convergence
    and rotation

23
Warming creates upper level divergence
Air Warms
Latent energy is converted to internal energy.
A warm core forms
Condenses
Water Vapor
Latent energy increases
Water evaporates
Low level convergence
Warm Water at least 26C
24
Tropical Cyclone Formation
  • Low Level Convergence collects latent energy and
    causes air to rise
  • Water Vapor Condenses and latent energy is
    converted to internal energy and the air warms
  • Warming generates upper level divergence
  • Upper level divergence removes mass and surface
    pressure decreases
  • Tropical Cyclone forms

25
Tropical Cyclone Formation (Cont.)
  • Warm Sea Surface Temperatures are required to
    supply enough energy to power the low pressure
    system
  • Weak winds in the upper levels are necessary to
    keep the tops of the thunderstorms from being
    blown away and preventing the development of the
    warm core.

26
Tropical Depression
  • When it is possible to draw at least one isobar
    that goes completely around the low pressure
    system and/or aerial reconnaissance determines
    that westerly winds exist on the western side of
    the system, then it is classified as a tropical
    depression.
  • A tropical depression is the weakest class of
    tropical cyclones.

27
Subtropical High 1028 mb
1020 mb
Tropical depression
1012 mb
L
1012 mb
ITCZ 1008 mb
28
Classification of Tropical Cyclones
  • Tropical depression maximum sustained winds
    less than 40 mph (identified by a number)
  • Tropical storm maximum sustained winds of 40-74
    mph (identified by a name)
  • Hurricane maximum sustained winds of greater
    than 74 mph

29
Classification of Tropical Cyclones (Cont.)
  • Hurricanes are typhoons over the western Pacific
    and are called cyclones over the Indian Ocean.

30
Eye of a hurricane is a clear area at the center
of the storm
31
EYE

LOW
32
Hurricane Structure
  • Hurricanes have a distinctive structure.
  • They are surrounded by spiral bands of
    thunderstorms.
  • At their center sinking motion creates a clear
    region called the eye of the hurricane.

33
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34
Hurricane Structure (Cont.)
  • The strongest winds in the hurricane are usually
    found in the eyewall just outside the eye of the
    hurricane.
  • Winds inside the eye of the hurricane are
    relatively calm.

35
Saffir-Simpson Scale
  • Cat. 1 75-95 mph
  • Cat. 2 96-110 mph
  • Cat. 3 111-130 mph
  • Cat. 4 131-155 mph
  • Cat. 5 greater than 155 mph

36
Categories 3, 4 and 5 are considered to be Major
Hurricanes
37
Tropical depression (Katrina)
38
Tropical Storm Katrina
39
Hurricane Katrina
40
Cat. 5 Hurricane Katrina
41
Hurricanes come in all sizes
42
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45
Primary Threats from Hurricanes
  • High winds
  • Storm surge
  • Fresh water flooding due to heavy rainfall
  • Tornadoes

46
Storm Surge
  • The storm surge is caused by the winds around a
    hurricane blowing water toward the shore and
    increasing the water level at the coast.
  • Bays and other coastal features may act as
    funnels to collect the storm surge and make the
    water levels even higher.

47
Storm Surge (Cont.)
N
Land
E
Winds blow water away from the shore
Storm surge floods land
Low
Winds blow water toward the land pushing water
toward the shore
Ocean
48
Global tropical cyclogenesis
  • Western North Pacific 31.8
  • Southern Indian/SW Pacific 28.2
  • Eastern North Pacific 15.0
  • North Atlantic 9.6
  • Northern Indian 5.0
  • Central Pacific 1.1
  • Total 90.7

49
Global Tropical Cyclogenesis (continued)
  • When normalized for area the Eastern North
    Pacific has the greatest annual rate of genesis
    of tropical cyclones of any region.

50
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51
Atlantic Hurricane Season
  • The official Atlantic hurricane season runs from
    June 1 through November 30.
  • Named tropical cyclones have been observed in
    every month of the year.
  • The official hurricane season for the eastern
    North Pacific runs from May 15 through November
    30.

52
Atlantic Hurricane Season (Cont.)
  • In an average year over the Atlantic Ocean there
    will be
  • 10 named tropical cyclones
  • 6 hurricanes
  • 2 major hurricanes
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