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Formation of Tropical CyclonesFormation of Tropical Cyclones
Warm ocean waters (> 26.5°C) throughout a Warm ocean waters (> 26.5°C) throughout a sufficient depth (> 50 m)sufficient depth (> 50 m)
An atmosphere is potentially unstable to moist An atmosphere is potentially unstable to moist convectionconvection
Relatively moist layers near the mid-troposphere Relatively moist layers near the mid-troposphere (~5 km)(~5 km)
A minimum distance of at least 500 km from the A minimum distance of at least 500 km from the equator for non-negligible amounts of the Coriolis equator for non-negligible amounts of the Coriolis forceforce
A pre-existing near-surface disturbance with A pre-existing near-surface disturbance with sufficient vorticity and convergencesufficient vorticity and convergence
Low values (< 10 m/s) of vertical wind shear Low values (< 10 m/s) of vertical wind shear between the surface and the upper tropospherebetween the surface and the upper troposphere
Air Flowing into a Low Pressure AreaAir Flowing into a Low Pressure Area (North Hemisphere)(North Hemisphere)
Locations of Tropical Cyclone Formation for a 20-Locations of Tropical Cyclone Formation for a 20-year Periodyear Period
Nature of Tropical CyclonesNature of Tropical Cyclones
Huge whirlpools in atmosphereHuge whirlpools in atmosphere A disc-like shape with a vertical scale of tens of A disc-like shape with a vertical scale of tens of
kilometres against horizontal dimensions of kilometres against horizontal dimensions of hundreds of kilometreshundreds of kilometres
The rainbands rotate in the same sense as the storm The rainbands rotate in the same sense as the storm circulation (anti-clockwise in the Northern circulation (anti-clockwise in the Northern Hemisphere but clockwise in the Southern Hemisphere but clockwise in the Southern Hemisphere - Coriolis force)Hemisphere - Coriolis force)
A travelling heat engine (feeds on latent heat A travelling heat engine (feeds on latent heat released from condensation in moist air)released from condensation in moist air)
The "eye" is typically of the order of tens of The "eye" is typically of the order of tens of kilometres in diameterkilometres in diameter and relatively calmand relatively calm
Lifespan varying from a few days to a few weeksLifespan varying from a few days to a few weeks
Structure of Tropical CyclonesStructure of Tropical Cyclones
Warm core systemWarm core system Low level pressure is lowest at the centre Low level pressure is lowest at the centre
(convergence) (convergence) High level pressure is highest at the centre High level pressure is highest at the centre
(divergence)(divergence) The region of phenomenal winds is confined to The region of phenomenal winds is confined to
a ring around the eye wall (conservation of a ring around the eye wall (conservation of angular momentum)angular momentum)
Air is ascending around the eye wall and Air is ascending around the eye wall and descending at the centre descending at the centre
Wind Speed Record during the Passage of Wind Speed Record during the Passage of Hurricane Celia (1970)Hurricane Celia (1970) at Gregory, Texas, USA.at Gregory, Texas, USA.
Three Dimensional Air Trajectories through a Three Dimensional Air Trajectories through a
Typhoon as Simulated by the Computer.Typhoon as Simulated by the Computer.
Tracking and LocatingTracking and Locating
Satellite analysis (especially over the vast Satellite analysis (especially over the vast expanse of the oceans)expanse of the oceans)
A well-formed eye is a definite indicator of A well-formed eye is a definite indicator of where the storm iswhere the storm is
Tracing the spiral rainbands of the storm or Tracing the spiral rainbands of the storm or studying the relative motion of cloud features studying the relative motion of cloud features near its centrenear its centre
Tracking on the radar (if the storm is located Tracking on the radar (if the storm is located within 500 km of Hong Kong)within 500 km of Hong Kong)
Infra-red (left) and Visible (right) Satellite Pictures of Infra-red (left) and Visible (right) Satellite Pictures of
Tropical Storm Hope (1989) Taken at the Same TimeTropical Storm Hope (1989) Taken at the Same Time
Examples of Factors that Need to be Considered Examples of Factors that Need to be Considered in Track Forecastingin Track Forecasting
Inertial tendency (drift towards the northwest Inertial tendency (drift towards the northwest in the Northern Hemisphere)in the Northern Hemisphere)
The environmental currentThe environmental current Westward extent of the Pacific ridgeWestward extent of the Pacific ridge North-south migration of the ridge axisNorth-south migration of the ridge axis Point of weakness in the ridgePoint of weakness in the ridge Interaction with mid-latitude weather systemsInteraction with mid-latitude weather systems Interaction with other tropical cyclone(s)Interaction with other tropical cyclone(s) Interaction with terrainInteraction with terrain Weak flow region where the steering current Weak flow region where the steering current
is ill-definedis ill-defined
Typical Tracks of Tropical Cyclones over the Typical Tracks of Tropical Cyclones over the Western North PacificWestern North Pacific
The Fujiwhara EffectThe Fujiwhara Effect ((藤原效應)藤原效應)
A paper describing the motions of A paper describing the motions of "vortices" in water by Dr. Sakuhei "vortices" in water by Dr. Sakuhei Fujiwhara in 1921 Fujiwhara in 1921
Interactions between vorticesInteractions between vortices If two vortices are equal in size and If two vortices are equal in size and
strength and spin in the same direction => strength and spin in the same direction => rotate around one anotherrotate around one another
If two vortices spinning counter-clockwise If two vortices spinning counter-clockwise approach and one of the vortices is larger approach and one of the vortices is larger than the other => the smaller of the two than the other => the smaller of the two vortices will get caught in the circulation vortices will get caught in the circulation of the larger one and be gobbled up of the larger one and be gobbled up eventually eventually
Methods Used for Forecasting the Track of Methods Used for Forecasting the Track of Tropical CyclonesTropical Cyclones
Conventional weather chart analysis Conventional weather chart analysis Climatology and statisticsClimatology and statistics Statistical-dynamical methodsStatistical-dynamical methods Numerical modelsNumerical models Remark: The science of tropical cyclone Remark: The science of tropical cyclone
forecasting is by no means perfect. The forecasting is by no means perfect. The multitude of contributing factors are so multitude of contributing factors are so complex that they are as yet not entirely complex that they are as yet not entirely understood, or are only crudely represented understood, or are only crudely represented (sometimes for practical reasons) in the (sometimes for practical reasons) in the various forecast schemes. various forecast schemes.
Power of Tropical CyclonesPower of Tropical Cyclones
Generating mechanical power of about Generating mechanical power of about 1.5 1.5 x 10x 101212 W W
(~ half the world-wide electrical(~ half the world-wide electrical generating generating capacity)capacity)
Sustained winds on record: Sustained winds on record: 165 165 kt (306 km/hr)kt (306 km/hr) Minimum surface pressure on record: Minimum surface pressure on record: 870 870 mbmb Height of surge on record: Height of surge on record: 13 13 mm Rainfall on record: Rainfall on record: 1,825 1,825 mm in 24 hrs.mm in 24 hrs. Size on record: Size on record: 1,100 1,100 km in radiuskm in radius Life-span on record: Life-span on record: 31 31 daysdays Casualties on record: Casualties on record: 300,000 300,000 people deadpeople dead
Typhoons Necessitating the Hoisting of the Hurricane Typhoons Necessitating the Hoisting of the Hurricane Signal No.10Signal No.10
Typhoon Wanda (Typhoon Wanda ( 颱風溫黛 颱風溫黛 - 1962)- 1962)
Duration: August 27 to September 2, 1962Duration: August 27 to September 2, 1962 Maximum sustained winds (HKO): 72 knots Maximum sustained winds (HKO): 72 knots
[133km/h] (Record)[133km/h] (Record) Maximum gust: 140 knots [259 km/h] Maximum gust: 140 knots [259 km/h]
(HKO); 153 knots [284 km/h] (Tate's Cairn) (HKO); 153 knots [284 km/h] (Tate's Cairn) (Record)(Record)
Closest distance from HKO: 20 km SSWClosest distance from HKO: 20 km SSW Instantaneous minimum surface pressure Instantaneous minimum surface pressure
(HKO): 953.2 mb (Record)(HKO): 953.2 mb (Record) Daily amounts of rainfall (HKO): 203.0 mm Daily amounts of rainfall (HKO): 203.0 mm
(September 1)(September 1) Maximum tide height (Tai Po):Maximum tide height (Tai Po): 5.4 m 5.4 m Casualties: 130 persons dead (Record)Casualties: 130 persons dead (Record)
Typhoon Wayne (Typhoon Wayne ( 颱風韋因 颱風韋因 - 1986)- 1986)
Duration: August 18 to September Duration: August 18 to September 6, 19866, 1986
Tropical cyclone signals were Tropical cyclone signals were hoisted and lowered on three hoisted and lowered on three separate occasions for the same separate occasions for the same stormstorm
Classic example of delicate Classic example of delicate interplay and balance among interplay and balance among various meteorological factorsvarious meteorological factors
The Fujiwhara effectThe Fujiwhara effect
Interaction between Wayne and Another Typhoon Interaction between Wayne and Another Typhoon named Vera - the Fujiwhara Effectnamed Vera - the Fujiwhara Effect
Formation of TornadoesFormation of Tornadoes
Spawned in thunderstormsSpawned in thunderstorms Vertical shear of horizontal wind Vertical shear of horizontal wind (垂直風切(垂直風切變)變)
Horizontal vorticity formed (aligned with the Horizontal vorticity formed (aligned with the average wind direction - streamwise vorticity)average wind direction - streamwise vorticity)
Convection (updrafts) in thunderstorms distort Convection (updrafts) in thunderstorms distort the vortex tube from horizontal to verticalthe vortex tube from horizontal to vertical
Regions of rotating updraft air in the Regions of rotating updraft air in the thunderstorms formed (mesocyclones)thunderstorms formed (mesocyclones)
The rotations in mesocyclones concentrated to The rotations in mesocyclones concentrated to form tornadoes (similar to the water vortices - form tornadoes (similar to the water vortices - vertical stretching)vertical stretching)
Nature of TornadoesNature of Tornadoes
Vortices in atmosphereVortices in atmosphere Funnel shapes with diameters on Funnel shapes with diameters on
the scale of 100s of meters the scale of 100s of meters (Coriolis force is negligible in (Coriolis force is negligible in such small system)such small system)
Rotate anti-clockwise in many Rotate anti-clockwise in many cases (the North Hemisphere)cases (the North Hemisphere)
Low pressure centres (capricious Low pressure centres (capricious effect may be caused)effect may be caused)
Lifespan measured in minutesLifespan measured in minutes
Tornado Variations and WaterspoutsTornado Variations and Waterspouts (水龍捲)(水龍捲)
Some tornadoes may form during the Some tornadoes may form during the early stages of rapidly developing early stages of rapidly developing thunderstormsthunderstorms
Tornadoes may appear nearly Tornadoes may appear nearly transparent until dust and debris are transparent until dust and debris are picked uppicked up
Occasionally, two or more tornadoes Occasionally, two or more tornadoes may occur at the same timemay occur at the same time
Tornadoes (weak) that form over warm Tornadoes (weak) that form over warm water are called waterspoutswater are called waterspouts
Waterspouts occasionally move inland Waterspouts occasionally move inland becoming tornadoes causing damage becoming tornadoes causing damage and injuries and injuries
Tornadoes Spawned by Tropical Cyclones Tornadoes Spawned by Tropical Cyclones
((TC Tornadoes)TC Tornadoes)
Spawn when certain instability and vertical shear criteria are metSpawn when certain instability and vertical shear criteria are met The smaller and shallower storm cells => weaker tornadoes (no F5-rated The smaller and shallower storm cells => weaker tornadoes (no F5-rated
TC tornadoes in the past 50 years of reliable data)TC tornadoes in the past 50 years of reliable data) Almost all tropical cyclones making landfall in the United States spawn Almost all tropical cyclones making landfall in the United States spawn
at least one tornadoat least one tornado The right-front quadrant of a tropical cyclone (relative to TC motion) is The right-front quadrant of a tropical cyclone (relative to TC motion) is
strongly favored for tornado formation (the Northern Hemisphere)strongly favored for tornado formation (the Northern Hemisphere) Tornadoes may be spawned up to three days after landfall of tropical Tornadoes may be spawned up to three days after landfall of tropical
cyclonescyclones Hurricane Beulah spawned a reported 141 tornadoes in southeast Texas Hurricane Beulah spawned a reported 141 tornadoes in southeast Texas
during the first several days after its landfall in September 1967during the first several days after its landfall in September 1967 TC tornadoes are especially difficult to deal with because the unusually TC tornadoes are especially difficult to deal with because the unusually
small storm cells that may be overlookedsmall storm cells that may be overlooked
A Waterspout Occurred during Typhoon Durian A Waterspout Occurred during Typhoon Durian (颱風榴槤 (颱風榴槤 - 2001)- 2001)
Power of TornadoesPower of Tornadoes
In an average year, In an average year, 800 800 tornadoes are reported, tornadoes are reported, resulting in 80 deaths and over resulting in 80 deaths and over 1,500 injuries in USA 1,500 injuries in USA
Wind speeds can approach 800 Wind speeds can approach 800 km/hr.km/hr.
Damage paths can be in excess Damage paths can be in excess of one mile wide and 50 miles of one mile wide and 50 miles longlong
Lifetime of violent tornadoes Lifetime of violent tornadoes can exceed 1 hourcan exceed 1 hour
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