Tropical Cyclones

1
Tropical CyclonesTornadoes
2
Formation of Tropical Cyclones

• Warm ocean waters (gt 26.5C) throughout a
  sufficient depth (gt 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 (lt 10 m/s) of vertical wind shear
  between the surface and the upper troposphere

3
Air Flowing into a Low Pressure Area (North
Hemisphere)
4
Locations of Tropical Cyclone Formation for a
20-year Period
5
Nature of Tropical Cyclones

• 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

6
A Tropical Cyclone in the Northern Hemisphere
7
A Tropical Cyclone in the Southern Hemisphere
8
Structure of Tropical Cyclones

• 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

9
Model Vertical Profile of a Mature Typhoon
10
Wind Speed Record during the Passage of Hurricane
Celia (1970) at Gregory, Texas, USA.
11
Three Dimensional Air Trajectories through a
Typhoon as Simulated by the Computer.
12
(No Transcript)
13
(No Transcript)
14
Tracking and Locating

• 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)

15
Typhoon York (???? - 1999) as Seen from the Radar
16
Infra-red (left) and Visible (right) Satellite
Pictures of Tropical Storm Hope (1989) Taken at
the Same Time
17
Examples of Factors that Need to be Considered in
Track Forecasting

• 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

18
Typical Tracks of Tropical Cyclones over the
Western North Pacific
19
The Fujiwhara Effect (????)

• 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 gt rotate around
  one another
• If two vortices spinning counter-clockwise
  approach and one of the vortices is larger than
  the other gt the smaller of the two vortices will
  get caught in the circulation of the larger one
  and be gobbled up eventually

20
Methods Used for Forecasting the Track of
Tropical Cyclones

• 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.

21
A Numerical Weather Forecast Map (NGP)
22
Typhoon Season of Hong Kong in 1999
23
Power of Tropical Cyclones

• Generating mechanical power of about 1.5 x 1012 W
  ( half the world-wide electrical generating
  capacity)
• 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

24
Typhoons Necessitating the Hoisting of the
Hurricane Signal No.10
25
Typhoon Wanda (???? - 1962)

• 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)

26
Record of Wind Speed of Typhoon Wanda
27
Record of Surface Pressure of Typhoon Wanda
28
Damage due to Typhoon Wanda
29
Typhoon Wayne (???? - 1986)

• 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

30
Typhoon Wayne as Seen from the Radar
31
Interaction between Wayne and Another Typhoon
named Vera - the Fujiwhara Effect
32
Formation of Tornadoes

• Spawned in thunderstorms
• Vertical shear of horizontal wind (?????)
• 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)

33
(No Transcript)
34
Nature of Tornadoes

• 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

35
Tornado Variations and Waterspouts(???)

• 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

36
Tornadoes Spawned by Tropical Cyclones (TC
Tornadoes)

• Spawn when certain instability and vertical shear
  criteria are met
• The smaller and shallower storm cells gt weaker
  tornadoes (no F5-rated 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

37
A Waterspout Occurred during Typhoon Durian (????
- 2001)
38
Power of Tornadoes

• 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

39
(No Transcript)
40
(No Transcript)