Tropical CyclonesTropical Cyclones&&

TornadoesTornadoes

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

A Tropical Cyclone in the Northern HemisphereA Tropical Cyclone in the Northern Hemisphere

A Tropical Cyclone in the Southern HemisphereA Tropical Cyclone in the Southern Hemisphere

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

Model Vertical Profile of a Mature TyphoonModel Vertical Profile of a Mature Typhoon

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)

Typhoon York (Typhoon York ( 颱風約克 颱風約克 - 1999) - 1999) as Seen from as Seen from the Radarthe Radar

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.

A Numerical Weather Forecast Map (NGP)A Numerical Weather Forecast Map (NGP)

Typhoon Season of Hong Kong in 1999Typhoon Season of Hong Kong in 1999

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)

Record of Wind Speed of Typhoon WandaRecord of Wind Speed of Typhoon Wanda

Record of Surface Pressure of Typhoon WandaRecord of Surface Pressure of Typhoon Wanda

Damage due to Typhoon WandaDamage due to Typhoon Wanda

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

Typhoon Typhoon WayneWayne as Seen from the Radar as Seen from the Radar

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