Tropical CyclonesTropical Cyclones

Part 10Part 10

What 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 GrowthStages of Hurricane Growth

General CharacteristicsGeneral 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 CharacteristicsGeneral 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 of tropical storms

Tropical Storm CirculationTropical Storm CirculationHigh

Low

warm

Vertical Cross SectionsVertical Cross Sections

Infrared Satellite Pictures of Hurricane MitchInfrared Satellite Pictures of Hurricane MitchOctober 26 October 26 -- 27, 199827, 1998

Differences between Hurricanes and Extratropical Cyclones:

Energy Source

Vertical Structure

Horizontal Structure

HurricaneHurricanewarm ocean/latent heat release

warm core/decays with height

smaller size/just maritime Tropical air mass

Extratropical CycloneExtratropical Cyclonetemp 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 FormationTropical 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 DevelopmentTropical 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 WaveEasterly Wave

Atlantic hurricane development initiated by the passage of a wave disturbance in the tropics/subtropics (e.g., easterly waves).

Easterly WaveEasterly 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

WorldWorld--Wide Tropical Cyclone Wide Tropical Cyclone FrequencyFrequency

Tropical Cyclones by BasinTropical Cyclones by Basin

Tropical Storms HurricanesAverage % of Total Average % of Total

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

NHC Track Forecast Error 1954NHC Track Forecast Error 1954--20012001

0

100

200

300

400

500

600

700

1954195719601963196619691973197619791982198519881991199419972000

Year

Trac

k E

rror

(nm

)

24 h OFCL48 h OFCL72 h OFCL

Hazards from Tropical CyclonesHazards 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. LandfallingLandfalling HurricanesHurricanes

Atlantic Hurricane NamesAtlantic Hurricane Names2000 2001 2002 2003 2004 2005Alberto 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 GertHelene Humberto Hanna Henri Hermine Harvey Isaac Iris Isidore Isabel Ivan Irene Joyce Jerry Josephine Juan Jeanne Jose Keith Karen Kyle Kate Karl KatrinaLeslie Lorenzo Lili Larry Lisa Lee Michael Michelle Marco Mindy Matthew Maria Nadine Noel Nana Nicholas Nicole NateOscar Olga Omar Odette Otto Ophelia Patty Pablo Paloma Peter Paula Philippe Rafael Rebekah Rene Rose Richard Rita Sandy Sebastien Sally Sam Shary StanTony Tanya Teddy Teresa Tomas Tammy Valerie Van Vicky Victor Virginie Vince William Wendy Wilfred Wanda Walter Wilma

Typical Year:10 Named Storms

6 Hurricanes2 Major Hurricanes

Where are the winds likely to be strongest?

Prevailing wind

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

Rotation of the Earth deflects ocean currentsto the right of the wind in the Northern Hemisphere

wind

Current(N.H.)

Storm SurgeStorm Surge

• Surface wind converges

• Surface water diverges

• Makes a huge pile of water

• Causes terrible damageand loss of life!

Hurricane Storm Surge Makes the Ocean Level Rise

DUNE

STORM SURGESTORM 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 DamageHurricane Andrew 1992 Wind Damage

Hurricane Andrew 1992 Wind DamageHurricane Andrew 1992 Wind Damage

Hurricane Floyd 1999 Rainfall DamageHurricane Floyd 1999 Rainfall Damage

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

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

53%

14%

13%

13%

4%

1%

2%

FreshwaterShoreline OffshoreWindTornadoStormsurgeOther

(Rappaport, Fuchs and Lorentson, 1999)

SummarySummary• 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