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Overview of Hurricanes

Carl Schreck

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Tropical Cyclone Lifecycle

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Tropical Depression Winds < 35 kts Numbered, but not

named Semi-organized group of

thunderstorms

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Tropical Storm Winds 35–63 kts ≈ 11 per year in the

Atlantic Named More circular/spiral, but

no eye

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Hurricane Winds ≥ 64 kts ≈ 6 per year in the Atlantic Eye develops Different names around the

world: Hurricane in the Atlantic and

East Pacific Typhoon in West Pacific Cyclone in Indian Ocean and

South Pacific “Tropical Cyclone” is the

generic/scientific term

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Major Hurricane Winds ≥ 96 kts ≈ 2-3 per year in the

Atlantic Category 3–5 on the

Saffir–Simpson Scale Strong and dangerous

storms Well-defined, circular eye

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Hurricane Ingredients: Thermodynamic Deep warm ocean layer

Warm oceans are the energy source for hurricanes

But hurricanes can mix cold water upward if the warm water is too shallow

Conditionally unstable atmosphere Warm air rises, but only if it’s

warmer than its surroundings

Moist mid-troposphere Dry air 2-3 km from the

surface can cause cold downdrafts ww2010.atmos.uiuc.edu

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Hurricane Ingredients: Dynamic Pre-existing convection

Needs organized thunderstorms to get things going

Cyclonic low-level vorticity Counter-clockwise spinning

winds help organize the thunderstorms

Associated with low pressure

Weak vertical wind shear Tilting winds can knock the

storm overLifecycle Ingredients Climatology Data

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Vertical Wind Shear

Wind shear is defined as the wind vector difference between the 850 and 200 mb level (arbitrary)

High westerly shear Low easterly shear

In general, low values (< 20 kt) of vertical windshear are desired.

Bad – convectiontorn apart

Good – latentheat can concentrate inone area

Courtesy of C.C. Hennon, UNC Asheville

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Tropical Waves

Thunderstorm systems with wave-like properties

Move westward with a period of 3–6 days

Common over West Africa

Provides storms with cyclonic vorticity and convection

Berry et al. (2007, Mon. Wea. Rev.)

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Main Development Region

Warm Ocean Temperatures

Supply of Tropical Waves from Africa

10°N to 20°N

20°W to 60°W or 80°W

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Saharan Air Layer

Dry air from Sahara can weaken storms

Dust blocks the sun and cools the ocean

Zipser et al. (2009, BAMS)

climate.nasa.gov

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Global Tropical Cyclones

Knapp et al. (2010, BAMS)

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Storms Per Year Bars depict

number of named systems (open/yellow), hurricanes (hatched/green), and category 3 or greater (solid/red), 1886-2004

nhc.noaa.gov/climo

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Atlantic Hurricane Climatology

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June

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July

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August

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September

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October

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November

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Best Track Data Historical record of

tropical cyclone locations and intensities

Constructed after each season by the warning agencies

Best estimate using all available data

nhc.noaa.gov

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Tropical Cyclone Observations Weather stations

Buoys

Ships

Radar

Aircraft Reconnaissance

Visible/Infrared Satellites

Microwave satellites

Satellite estimates account for vast majority of the record Courtesy of Jack Beven (NOAA/NHC)

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Tropical Cyclone Observations Weather stations

Buoys

Ships

Radar

Aircraft Reconnaissance

Visible/Infrared Satellites

Microwave satellites

Satellite estimates account for vast majority of the record Courtesy of Jack Beven (NOAA/NHC)

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Which image on the right is the closest match for this image of

Hurricane Katrina?