Tropical Cyclones
&
Tornadoes
Formation of Tropical Cyclones
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Warm ocean waters (> 26.5°C) throughout a
sufficient depth (> 50 m)
An atmosphere is potentially unstable to moist
convection
Relatively moist layers near the mid-troposphere
(~5 km)
A minimum distance of at least 500 km from the
equator for non-negligible amounts of the Coriolis
force
A pre-existing near-surface disturbance with
sufficient vorticity and convergence
Low values (< 10 m/s) of vertical wind shear
between the surface and the upper troposphere
Air Flowing into a Low Pressure Area
(North Hemisphere)
Locations of Tropical Cyclone Formation for a 20year Period
Nature of Tropical Cyclones
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Huge whirlpools in atmosphere
A disc-like shape with a vertical scale of tens of
kilometres against horizontal dimensions of
hundreds of kilometres
The rainbands rotate in the same sense as the storm
circulation (anti-clockwise in the Northern
Hemisphere but clockwise in the Southern
Hemisphere - Coriolis force)
A travelling heat engine (feeds on latent heat
released from condensation in moist air)
The "eye" is typically of the order of tens of
kilometres in diameter and relatively calm
Lifespan varying from a few days to a few weeks
A Tropical Cyclone in the Northern Hemisphere
A Tropical Cyclone in the Southern Hemisphere
Structure of Tropical Cyclones
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Warm core system
Low level pressure is lowest at the centre
(convergence)
High level pressure is highest at the centre
(divergence)
The region of phenomenal winds is confined to
a ring around the eye wall (conservation of
angular momentum)
Air is ascending around the eye wall and
descending at the centre
Model Vertical Profile of a Mature Typhoon
Wind Speed Record during the Passage of
Hurricane Celia (1970) at Gregory, Texas, USA.
Three Dimensional Air Trajectories through a
Typhoon as Simulated by the Computer.
Tracking and Locating
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Satellite analysis (especially over the vast
expanse of the oceans)
A well-formed eye is a definite indicator of
where the storm is
Tracing the spiral rainbands of the storm or
studying the relative motion of cloud features
near its centre
Tracking on the radar (if the storm is located
within 500 km of Hong Kong)
Typhoon York (颱風約克 - 1999) as Seen from the
Radar
Infra-red (left) and Visible (right) Satellite Pictures of
Tropical Storm Hope (1989) Taken at the Same Time
Examples of Factors that Need to be Considered
in Track Forecasting
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Inertial tendency (drift towards the northwest
in the Northern Hemisphere)
The environmental current
Westward extent of the Pacific ridge
North-south migration of the ridge axis
Point of weakness in the ridge
Interaction with mid-latitude weather systems
Interaction with other tropical cyclone(s)
Interaction with terrain
Weak flow region where the steering current
is ill-defined
Typical Tracks of Tropical Cyclones over the
Western North Pacific
The Fujiwhara Effect （藤原效應）
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A paper describing the motions of
"vortices" in water by Dr. Sakuhei
Fujiwhara in 1921
Interactions between vortices
If two vortices are equal in size and
strength and spin in the same direction =>
rotate around one another
If two vortices spinning counter-clockwise
approach and one of the vortices is larger
than the other => the smaller of the two
vortices will get caught in the circulation of
the larger one and be gobbled up
eventually
Methods Used for Forecasting the Track of
Tropical Cyclones
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Conventional weather chart analysis
Climatology and statistics
Statistical-dynamical methods
Numerical models
Remark: The science of tropical cyclone
forecasting is by no means perfect. The
multitude of contributing factors are so
complex that they are as yet not entirely
understood, or are only crudely represented
(sometimes for practical reasons) in the
various forecast schemes.
A Numerical Weather Forecast Map (NGP)
Typhoon Season of Hong Kong in 1999
Power of Tropical Cyclones
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Generating mechanical power of about 1.5 x 1012 W
(~ half the world-wide electrical generating capacity)
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Sustained winds on record: 165 kt (306 km/hr)
Minimum surface pressure on record: 870 mb
Height of surge on record: 13 m
Rainfall on record: 1,825 mm in 24 hrs.
Size on record: 1,100 km in radius
Life-span on record: 31 days
Casualties on record: 300,000 people dead
Typhoons Necessitating the Hoisting of the Hurricane
Signal No.10
Typhoon Wanda (颱風溫黛 - 1962)
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Duration: August 27 to September 2, 1962
Maximum sustained winds (HKO): 72
knots [133km/h] (Record)
Maximum gust: 140 knots [259 km/h]
(HKO); 153 knots [284 km/h] (Tate's
Cairn) (Record)
Closest distance from HKO: 20 km SSW
Instantaneous minimum surface pressure
(HKO): 953.2 mb (Record)
Daily amounts of rainfall (HKO): 203.0
mm (September 1)
Maximum tide height (Tai Po): 5.4 m
Casualties: 130 persons dead (Record)
Record of Wind Speed of Typhoon Wanda
Record of Surface Pressure of Typhoon Wanda
Damage due to Typhoon Wanda
Typhoon Wayne (颱風韋因 - 1986)
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Duration: August 18 to September
6, 1986
Tropical cyclone signals were
hoisted and lowered on three
separate occasions for the same
storm
Classic example of delicate
interplay and balance among
various meteorological factors
The Fujiwhara effect
Typhoon Wayne as Seen from the Radar
Interaction between Wayne and Another Typhoon
named Vera - the Fujiwhara Effect
Formation of Tornadoes
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Spawned in thunderstorms
Vertical shear of horizontal wind （垂直風
切變）
Horizontal vorticity formed (aligned with the
average wind direction - streamwise vorticity)
Convection (updrafts) in thunderstorms
distort the vortex tube from horizontal to
vertical
Regions of rotating updraft air in the
thunderstorms formed (mesocyclones)
The rotations in mesocyclones concentrated
to form tornadoes (similar to the water
vortices - vertical stretching)
Nature of Tornadoes
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Vortices in atmosphere
Funnel shapes with diameters on
the scale of 100s of meters
(Coriolis force is negligible in
such small system)
Rotate anti-clockwise in many
cases (the North Hemisphere)
Low pressure centres (capricious
effect may be caused)
Lifespan measured in minutes
Tornado Variations and Waterspouts（水龍捲）
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Some tornadoes may form during the
early stages of rapidly developing
thunderstorms
Tornadoes may appear nearly
transparent until dust and debris are
picked up
Occasionally, two or more tornadoes
may occur at the same time
Tornadoes (weak) that form over
warm water are called waterspouts
Waterspouts occasionally move inland
becoming tornadoes causing damage
and injuries
Tornadoes Spawned by Tropical Cyclones
(TC Tornadoes)
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Spawn when certain instability and vertical shear criteria are met
The smaller and shallower storm cells => weaker tornadoes (no F5rated TC tornadoes in the past 50 years of reliable data)
Almost all tropical cyclones making landfall in the United States
spawn at least one tornado
The right-front quadrant of a tropical cyclone (relative to TC motion)
is strongly favored for tornado formation (the Northern Hemisphere)
Tornadoes may be spawned up to three days after landfall of tropical
cyclones
Hurricane Beulah spawned a reported 141 tornadoes in southeast Texas
during the first several days after its landfall in September 1967
TC tornadoes are especially difficult to deal with because the
unusually small storm cells that may be overlooked
A Waterspout Occurred during Typhoon Durian
（颱風榴槤 - 2001)
Power of Tornadoes
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In an average year, 800
tornadoes are reported, resulting
in 80 deaths and over 1,500
injuries in USA
Wind speeds can approach 800
km/hr.
Damage paths can be in excess
of one mile wide and 50 miles
long
Lifetime of violent tornadoes
can exceed 1 hour