Tropical Cyclones Tropical Cyclones Part 10 Part 10 W hat is a - - PowerPoint PPT Presentation

Tropical Cyclones Tropical Cyclones Part 10 Part 10

W hat is a Tropical Cyclone?

A tropical cyclone is the general term for all rotating weather systems that originate over warm tropical waters. Tropical cyclones are classified as follows:

• Tropical Depression: Maximum sustained

winds of 38 mph (33 knots) or less.

• Tropical Storm : Maximum sustained winds of

39 to 73 mph (34-63 knots).

• Hurricane: Maximum sustained winds of 74 mph

(64 knots) or higher. In the northwestern Pacific, hurricanes are called "typhoons," and similar storms in the Indian Ocean and southwestern Pacific are called "cyclones."

Stages of Hurricane Growth Stages of Hurricane Growth

General Characteristics General Characteristics

• Approximate average diameter of hurricanes is 500 km –

circulation of largest systems extend for thousands of kilometers

• The overall lifecycle of tropical cyclone growth from

tropical depression to hurricane and subsequent dissipation is on the order of days

• Generally classified as very large mesoscale systems

(meso-α) that are inertially stable vortices at maturity

• Most tropical cyclones form between 5° and 20° latitude

(more form poleward in NH)

• The majority of tropical storms form in the summer

hemisphere

– North Atlantic hurricane season is June through November with a peak around September 10

General Characteristics General Characteristics

• Hurricanes are warm core

lows

– Low pressure at the surface – High pressure aloft – Decreasing intensity with height

• Weak vertical wind shear

exists

• Form over warm ocean

waters

• Latent heat of condensation

very important to the formation and maintenance

• f tropical storms

Tropical Storm Circulation Tropical Storm Circulation

High Low warm

Vertical Cross Sections Vertical Cross Sections

Infrared Satellite Pictures of Hurricane Mitch Infrared Satellite Pictures of Hurricane Mitch October 26 October 26 -

• 27, 1998

27, 1998

Differences between Hurricanes and Extratropical Cyclones:

Energy Source Vertical Structure Horizontal Structure Hurricane Hurricane

warm ocean/latent heat release warm core/decays with height smaller size/just maritime Tropical air mass Extratropical Cyclone Extratropical Cyclone temp difference between air masses cold core/strengthens & slopes with height larger size/fronts (boundaries between 2 different air masses)

Parameters Important for Parameters Important for Tropical Cyclone Formation Tropical Cyclone Formation

• Large values of low-level vorticity (ζ) –

likely from a pre-existing tropical disturbance

• Large values of Coriolis parameter (f)
• Minimum of vertical wind shear
• Sea surface temperatures >26°C to a depth of

50 m

• Large values of moist lapse rate instability
• Large values of mid-troposphere (500-700

mb) relative humidity

Organized Convection Theory of Organized Convection Theory of Tropical Cyclone Development Tropical Cyclone Development

• 1 - surface convergence leads to rising motion / increase in

moisture content.

• 2 - rising air condenses, releases large amount of latent

heat.

• 3 - latent heating aloft leads to high pressure and

divergence aloft (organizes the large scale circulation)

• 4 - divergence aloft leads to lower surface pressure.
• 5 - lower surface pressure increases surface convergence.
• 6- stronger surface winds increase wave height; increase

friction and convergence; also increase ocean-air moisture flux.

• 7 - rotation serves to organize flow.

Easterly Wave Easterly Wave

Atlantic hurricane development initiated by the passage

• f a wave disturbance in the tropics/subtropics (e.g.,

easterly waves).

Easterly Wave Easterly Wave

[From Gray (2000)]

• Top panel (a): Conditions favorable for an easterly wave

to develop into a hurricane

• Bottom panel (b): Conditions unfavorable for hurricane

development

Typical Tropical Cyclone Tracks

World World-

• Wide Tropical Cyclone

Wide Tropical Cyclone Frequency Frequency

Tropical Cyclones by Basin Tropical Cyclones by Basin

Tropical Storms Hurricanes Average % of Total Average % of Total Atlantic 9.8 11.4 5.7 12.1 NE Pacific 17.0 19.7 9.8 20.7 NW Pacific 26.9 32.1 16.8 35.5 N Indian 5.4 6.3 2.2 4.6 SW Indian 10.3 12.0 4.9 10.4 SE Indian 6.5 7.5 3.3 7.0 SW Pacific 10.2 11.8 4.6 9.7 Global 86.1 47.3

NHC Track Forecast Error 1954 NHC Track Forecast Error 1954-

• 2001

2001

100 200 300 400 500 600 700 1 9 5 4 1 9 5 7 1 9 6 1 9 6 3 1 9 6 6 1 9 6 9 1 9 7 3 1 9 7 6 1 9 7 9 1 9 8 2 1 9 8 5 1 9 8 8 1 9 9 1 1 9 9 4 1 9 9 7 2

Year Track Error (nm)

24 h OFCL 48 h OFCL 72 h OFCL

Hazards from Tropical Cyclones Hazards from Tropical Cyclones

• Storm Surge

– Ocean can rise up to 25 feet

• High Wind

– Gusts up to 200 mph

• Flooding from rainfall

– Up to 40 inches of rain

• Tornadoes

– Spawned by the hurricane up to 1000 miles inland

U.S. U.S. Landfalling Landfalling Hurricanes Hurricanes

Atlantic Hurricane Names Atlantic Hurricane Names

2000 2001 2002 2003 2004 2005 Alberto Allison Arthur Ana Alex Arlene Beryl Barry Bertha Bill Bonnie Bret Chris Chantal Cristobal Claudette Charley Cindy Debby Dean Dolly Danny Danielle Dennis Ernesto Erin Edouard Erika Earl Emily Florence Felix Fay Fabian Frances Franklin Gordon Gabrielle Gustav Grace Gaston Gert Helene Humberto Hanna Henri Hermine Harvey Isaac Iris Isidore Isabel Ivan Irene Joyce Jerry Josephine Juan Jeanne Jose Keith Karen Kyle Kate Karl Katrina Leslie Lorenzo Lili Larry Lisa Lee Michael Michelle Marco Mindy Matthew Maria Nadine Noel Nana Nicholas Nicole Nate Oscar Olga Omar Odette Otto Ophelia Patty Pablo Paloma Peter Paula Philippe Rafael Rebekah Rene Rose Richard Rita Sandy Sebastien Sally Sam Shary Stan Tony Tanya Teddy Teresa Tomas Tammy Valerie Van Vicky Victor Virginie Vince William Wendy Wilfred Wanda Walter Wilma

Typical Year: 10 Named Storms 6 Hurricanes 2 Major Hurricanes

Where are the winds likely to be strongest?

Prevailing wind

The Coriolis Force The Coriolis Force Affects Ocean Currents Too Affects Ocean Currents Too

Rotation of the Earth deflects ocean currents to the right of the wind in the Northern Hemisphere wind Current (N.H.)

Storm Surge Storm Surge

• Surface wind converges
• Surface water diverges
• Makes a huge pile of water
• Causes terrible damage

and loss of life!

Hurricane Storm Surge Makes the Ocean Level Rise

DUNE STORM SURGE STORM SURGE DUNE

HIGH TIDE

STORM SURGE

MEAN TIDE (MSL) LOW TIDE

Folly Beach, South Carolina (Before Hugo)

Folly Beach, South Carolina (After Hugo)

x

Hurricane Andrew 1992 Wind Damage Hurricane Andrew 1992 Wind Damage

Hurricane Andrew 1992 Wind Damage Hurricane Andrew 1992 Wind Damage

Hurricane Floyd 1999 Rainfall Damage Hurricane Floyd 1999 Rainfall Damage

North Carolina Rainfall Damage North Carolina Rainfall Damage from Hurricane Floyd 1999 from Hurricane Floyd 1999

U.S. Loss of Life from Tropical Cyclones 1970-98 (517 Total)

53% 14% 13% 13% 4% 1% 2% Freshwater Shoreline Offshore Wind Tornado Stormsurge Other

(Rappaport, Fuchs and Lorentson, 1999)

Summary Summary

• Tropical Cyclones are “warm core” rotating weather

systems that derive their energy from the warm ocean

– Counter-clockwise in N. hemisphere, clockwise in S. hemisphere – Tropical storm winds > 39 mph, Hurricane winds > 74 mph

• Formation requires warm ocean, weak upper level winds,

moist mid-troposphere, moist lapse rate instability, earth’s rotation, and pre-existing disturbance

• They occur in most tropical and sub-tropical ocean basins
• Motion is usually east to west in tropics, west to east in

higher latitudes

• Track forecasting has greatly improved in last few decades
• Hazards include storm surge, wind, rainfall and tornadoes