Tropical Cyclones
Types of Cyclones
• Tropical Cyclone
• Extratropical Cyclone
• Subtropical Cyclone
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.
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.
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.
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.”
Possible Sources of Tropical
Cyclones
1. Tropical Disturbances
Tropical Waves
ITCZ Disturbances
Other thunderstorm clusters
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
N
Subtropical High
1032 mb
E
Tropical Wave
1024 mb
1016 mb
Intertropical Convergence Zone (ITCZ)
1008 mb
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.
N
Subtropical High
1032 mb
E
The northeast trade
winds produce
divergence to the west Tropical Wave
of the wave and
suppress cloud
formation
1024 mb
The northeast trade winds
produce convergence to the
1016 mb east of the wave and support
the formation of clouds and
thunderstorms.
Intertropical Convergence Zone (ITCZ)
1008 mb
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.
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
N
Subtropical High
1032 mb
E
Tropical Wave
1024 mb
A cluster of
1016 mb thunderstorms
separates from the ITCZ
forming an ITCZ
1008 mb disturbance
The ITCZ bulges toward the north in response to
the passage of the tropical wave
Possible Sources of Tropical
Cyclones (continued)
2. Monsoon Disturbances
Monsoon Troughs
Monsoon Gyres
Monsoon Depressions
Possible Sources of Tropical
Cyclones (continued)
3. Subtropical Cyclones
4. Extratropical systems
Old stationary frontal boundaries
Upper lows
5. Mesoscale Convective Systems (MCSs)
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.
Conditions necessary for formation
of a tropical cyclone
• Warm Sea Surface Temperatures (SSTs
greater than 26°C)
• Weak winds in the upper levels
• A disturbance to provide low level
convergence and rotation
Warming creates upper level divergence
Air Warms
Latent energy A warm
is converted to core forms
internal energy.
Water Vapor Condenses
Latent energy
Water evaporates increases
Low level convergence
Warm Water at least 26°C
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
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.
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.
Subtropical High 1028 mb
1020 mb
Tropical depression
1012 mb
L
1012 mb
ITCZ 1008 mb
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
Classification of Tropical Cyclones
(Cont.)
Hurricanes are typhoons over the western
Pacific and are called cyclones over the
Indian Ocean.
Eye of a hurricane is a clear
area at the center of the storm
EYE
LOW
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.
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.
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
Categories 3, 4 and 5 are
considered to be Major
Hurricanes
Tropical depression (Katrina)
Hurricane Katrina
Cat. 5 Hurricane Katrina
Hurricanes come in all sizes
Primary Threats from Hurricanes
1. High winds
2. Storm surge
3. Fresh water flooding due to heavy
rainfall
4. Tornadoes
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.
Storm Surge (Cont.)
N
Land
E
Winds blow water
away from the Storm surge
shore
floods land
Low Winds blow water
toward the land
pushing water
toward the shore
Ocean
Global tropical cyclogenesis
1. Western North Pacific 31.8
2. Southern Indian/SW Pacific 28.2
3. Eastern North Pacific 15.0
4. North Atlantic 9.6
5. Northern Indian 5.0
6. Central Pacific 1.1
Total 90.7
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.
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.
Atlantic Hurricane Season (Cont.)
In an average year over the Atlantic Ocean
there will be:
10 named tropical cyclones
6 hurricanes
2 major hurricanes