Hurricane Principles Hurricane Isabel, 2003. Outline Definitions Formation and Conditions Needed...

Hurricane Principles

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Hurricane Isabel, 2003

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Outline

• Definitions

• Formation and Conditions Needed

• Growth and Structure of a Hurricane

• Where do They Form?

• Measurement

• Damage

• Forecasting

What are hurricanes?

• Intense storm of tropical origin– Typhoon in North Pacific– Cyclone in India, Australia

• Winds sustained at 75+ mph

• ~80 form every year– Usually only few hit land/U.S.– Average ~30-40 fatalities/year

Formation Requirements

• Shallow ocean water 80°F or more

• Warm humid air above water

• Weak upper level winds, blowing in direction of developing storm

• Timing: Summer, early Fall – Season in U.S. is June-November



Stages of hurricane development

Beginnings

• Low pressure with cluster of thunderstorms– Important for circulation– Tropical disturbance stage

• Thunderstorms grow, winds strengthen

Rotation

• Surface winds converge, thunderstorms become more organized– Converge in CCW fashion due to Coriolis force– Only form ~5-20° latitude, not at equator

(Coriolis force = 0)

Strengthening

• Organization - means more warm moist air rising– Cools, releases massive amount of latent heat

during condensation– Warms surrounding air, causes more updrafts,

brings more air from below• FUEL for more t-storms

Into a Hurricane

• More fuel, more rotation, more speed

• Tropical storm: surface wind speeds between 39-74 mph

• Hurricane: surface winds sustained at 74+ mph– Also development of eye

Eye of Hurricane

• Develops at ~74 mph wind speed– Why? Harder for rotating winds to reach

surface

• Calm area, clear and cloud free– Why? Cold air sinking in eye, as descends,

warms and absorbs moisture

Eye Wall

• Cylinder of upward winds

• Strongest winds

• Heaviest precipitation





Eye and Eyewall structure

Conditions in Storm from West to East

• Approach: overcast sky, pressure begins to drop• Towards eye: increasing wind speeds, huge waves

(up to 30 ft), heavy rain• In eye: air temperature increases, low wind, no

rain, bright sky, lowest pressure• East of eye: heavy rain, strong winds• Move away from eye: pressure rises, winds and

rain decrease



Intensity

• Controlling factors: – Temperature of water– Release of latent heat– Why? Higher temperature water can drive more

t-storms

Duration of Storm

• Most last ~1 week

• Longer if they stay over warm water

• Shorter if they move over cold water or land

• Why?– Energy source gone– More friction for winds

Where do they form?

• Not at equator (no Coriolis force)

• Subtropics ~5-20° latitude– Then move to higher latitudes– Path can vary based on details of high/low

pressure systems it encounters

• Majority form SE Asia, India, Australia

General origin points and paths of hurricanes/cyclones

Origins of Atlantic Hurricanes



Measuring Size

• Saffir-Simpson scale

• Based on wind speed, pressure– Expected storm surge, possible damage– Category 1-5

Saffir-Simpson Scale

Identification

• Names assigned at tropical storm strength

• Currently alternate male and female names alphabetically– System started in 1979

– 6 lists are repeated

– Names retired if storm is very costly/deadly

Atlantic List 2005 Arlene Bret Cindy Dennis Emily Franklin Gert Harvey Irene Jose Katrina Lee Maria Nate Ophelia Philippe Rita Stan Tammy Vince Wilma

Damage from Hurricanes

• High winds

• Storm surge

• Heavy rain

• Mudslides

Winds• Can be over 155 mph

(Category 5)• Impact can depend on which

side of storm hits– Wind speed can be additive in

direction of storm

• Can also generate large (10-15 m) waves that hit shorelines in advance of hurricane


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Wind damage in Florida - Andrew 1992

Storm Surge

• Produce much of damage, fatalities

• Abnormal rise in water level of few meters

Storm surge is a big problem for low-lying areas

Deep water coastlines not as much of a problem





Effects of storm surge on Florida coastline

Storm Surge

• Why? – Eye is low pressure zone, local sea level rises– Winds near eye push water into mound, leads to

big surge of water hit shore, move far inland

• Surge can be 20-30 ft– Think about New Orleans, much below sea

level

Heavy Rain

• After moving inland, no more water vapor to add fuel

• But there is a lot in the cloud already!– Will fall as rain– Can be significant (measured in ft)– Leads to flooding



Downtown Houston after Tropical Storm Allison, 2001



Mudslides

• Many examples of rain soaked hillslopes failing after hurricanes

Forecasting: Seasonal

• General observations for Atlantic hurricanes

• More frequent if:– Wet summer in west Africa– Warmer sea temperatures– Low atmospheric pressure in Caribbean– No El Nino

• Weather phenomena of high level east-blowing winds in Pacific

Why?

West Africa wet summer: more t-storms, more hurricane potential

Warm sea temps: more energy for storms

Low pressure in Caribbean: requirement for storms

No El Nino: weaker upper level winds (also condition for formation)

General Path of Storms

Probabilities of hurricanes hitting U.S. coastlines in any given year

Short term prediction

• Location, movement, intensity closely monitored– Ship reports– Satellites– Radar– Buoys– Aircraft QuickTime™ and a

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Warnings and Watches

• Watch: issued for large areas that might be affected– Usually issued a few days before landfall

• Warning: usually issued within 24 hours of storm striking area (+probability of striking)– Usually issued for large area (over 300 miles) to

compensate for wide swath, variations in landfall



Example of watches and warnings posted for Hurricane Erin, 1995

Warnings and Watches

• Significantly reduced loss of life from hurricanes

• Not amount of damage

Evacuation

• Can be a problem as more people move into coastal areas

• Evacuation time estimates for areas– 72 hours New Orleans– 50-60 hours Ft. Myers, FL– 30-39 hours Miami– Problem: usually don’t have detailed

knowledge of landfall during these periods

Mitigating Damage

• Similar to earthquake planning• Building codes

– Withstand winds– Keep roofs on– Protecting windows

• Land use– Low-lying areas for parks, golf courses, not

houses

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• Hurricanes Part 2

Hurricane Principles Hurricane Isabel, 2003. Outline Definitions Formation and Conditions Needed...

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