Title: Tropical Cyclones
1Tropical Cyclones
2Types of Cyclones
- Tropical Cyclone
- Extratropical Cyclone
- Subtropical Cyclone
3Types 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.
4Types 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.
5Types 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.
6Basic 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.
7Possible Sources of Tropical Cyclones
- Tropical Disturbances
- Tropical Waves
- ITCZ Disturbances
- Other thunderstorm clusters
8Tropical 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
9N
Subtropical High 1032 mb
E
Tropical Wave
1024 mb
1016 mb
Intertropical Convergence Zone (ITCZ) 1008 mb
10Tropical 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.
11N
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
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15ITCZ 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.
16N
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
17N
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
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19Possible Sources of Tropical Cyclones (continued)
- Monsoon Disturbances
- Monsoon Troughs
- Monsoon Gyres
- Monsoon Depressions
20Possible Sources of Tropical Cyclones (continued)
- Subtropical Cyclones
- Extratropical systems
- Old stationary frontal boundaries
- Upper lows
- 5. Mesoscale Convective Systems (MCSs)
21Tropical 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.
22Conditions 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
23Warming 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
24Tropical 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
25Tropical 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.
26Tropical 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
28Classification 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 -
29Classification of Tropical Cyclones (Cont.)
- Hurricanes are typhoons over the western Pacific
and are called cyclones over the Indian Ocean.
30Eye of a hurricane is a clear area at the center
of the storm
31EYE
LOW
32Hurricane 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.
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34Hurricane 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.
35Saffir-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
36Categories 3, 4 and 5 are considered to be Major
Hurricanes
37Tropical depression (Katrina)
38Tropical Storm Katrina
39Hurricane Katrina
40Cat. 5 Hurricane Katrina
41Hurricanes come in all sizes
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45Primary Threats from Hurricanes
- High winds
- Storm surge
- Fresh water flooding due to heavy rainfall
- Tornadoes
46Storm 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.
47Storm 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
48Global 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
49Global Tropical Cyclogenesis (continued)
- When normalized for area the Eastern North
Pacific has the greatest annual rate of genesis
of tropical cyclones of any region.
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51Atlantic 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.
52Atlantic Hurricane Season (Cont.)
- In an average year over the Atlantic Ocean there
will be - 10 named tropical cyclones
- 6 hurricanes
- 2 major hurricanes