Chapter 11: Chapter 11: HurricanesHurricanes

Tropical weatherTropical weather Anatomy of a hurricaneAnatomy of a hurricane Hurricane formation and dissipationHurricane formation and dissipation Some notable hurricanesSome notable hurricanes Hurricane watches, warnings and Hurricane watches, warnings and

forecastsforecasts Modifying hurricanesModifying hurricanes

Tropical WeatherTropical Weather

Tropics: Tropics: 23.5N-23.5S23.5N-23.5S

streamlinesstreamlines tropical wavetropical wave easterly wave,easterly wave,

2500km wavelength,2500km wavelength,

10-20 knots speed10-20 knots speed

• The tropics are close to the equator, where theThe tropics are close to the equator, where theCoriolis force is too small to balance the pressureCoriolis force is too small to balance the pressuregradient force. Thus winds are not geostrophic.gradient force. Thus winds are not geostrophic.

Anatomy of a HurricaneAnatomy of a Hurricane

hurricane (typhoon, cyclone): hurricane (typhoon, cyclone): > 64 knots> 64 knots

eye; eyewalleye; eyewall spiral rainbandspiral rainband

• Hurricanes are Hurricanes are quite similar quite similar to, yet also to, yet also quite different quite different fromfrommid-latitude mid-latitude storms.storms.

Fig. 11-3, p. 302

Sinking air and clear sky in the eye due to high pressure aloft

Hurricane Hurricane Formation and Formation and

DissipationDissipationHurricane forms over tropical waters where winds are light, humidity is high in a deep layer, and surface temperature is warm, typically 26.5C (80F) or greater,over a vast area

SST > 28C

Over Atlantic

The Right EnvironmentThe Right Environment

Convergence triggerConvergence trigger

ITCZ, easterly waves, midlatitude fronts to ITCZ, easterly waves, midlatitude fronts to

tropicstropics Weak trade wind inversionWeak trade wind inversion Weak upper windWeak upper wind

during El Nino over Atlanticduring El Nino over Atlantic Some Coriolis force: Some Coriolis force: 5-20deg latitude5-20deg latitude

The Developing StormThe Developing Storm

Heat engine: heat taken at high T, converted into work, then ejected at Heat engine: heat taken at high T, converted into work, then ejected at low T.low T.

Hurricane: heat taken from warm ocean, converted into kinetic energy Hurricane: heat taken from warm ocean, converted into kinetic energy (wind), lost at its top through radiational cooling (wind), lost at its top through radiational cooling

Maximum wind depends on surface and tropopause temperature Maximum wind depends on surface and tropopause temperature difference and the potential of sea surface evaporationdifference and the potential of sea surface evaporation

The Storm Dies OutThe Storm Dies Out

cutting off the cutting off the storm’s energy storm’s energy supply by moving supply by moving over cooler oceanover cooler ocean

Landfall: lose Landfall: lose energy sourse and energy sourse and increased friction increased friction to reduce windto reduce wind

Hurricane Stages of Hurricane Stages of DevelopmentDevelopment

tropical disturbance: tropical disturbance: unorganized mass of unorganized mass of thunderstorms, weak windthunderstorms, weak wind

tropical depression: tropical depression: 20-34 knots, closed 20-34 knots, closed isobarsisobars

tropical storm: tropical storm: 35-64 knots, with a name35-64 knots, with a name

hurricane: hurricane: > 64 knots, with a name> 64 knots, with a name• This progression of stages is This progression of stages is followed in reverse order as a followed in reverse order as a storm weakens.storm weakens.

Hurricane vs midlattitude Hurricane vs midlattitude stormsstorms

Hurricane: 1) derive energy from warm ocean Hurricane: 1) derive energy from warm ocean and latent heat of condensation; 2) warm core and latent heat of condensation; 2) warm core low; 3) high center aloft; 4) sinking air in the eye; low; 3) high center aloft; 4) sinking air in the eye; 5) strongest wind near surface; 6) stronger wind; 5) strongest wind near surface; 6) stronger wind; 7) smaller size7) smaller size

Midlatitude storms: 1) derive energy from Midlatitude storms: 1) derive energy from horizontal temperature difference; 2) cold core horizontal temperature difference; 2) cold core low; 3) intensifies with height; 4) rising air at low; 3) intensifies with height; 4) rising air at center; 5) strongest wind aloft in the jet stream; center; 5) strongest wind aloft in the jet stream; 6) wind not as strong; 7) larger size6) wind not as strong; 7) larger size

Both are low pressure systems with Both are low pressure systems with counterclockwise motionscounterclockwise motions

Hurricane MovementHurricane Movement

role of the ITCZrole of the ITCZ northward northward

movement due to movement due to subtropical highssubtropical highs

influence of the influence of the westerlieswesterlies

• Because of the Bermuda High, westward-Because of the Bermuda High, westward-movingmovingNorth Atlantic hurricanes often take a North Atlantic hurricanes often take a turn towards the north as they turn towards the north as they approach North America.approach North America.

Fig. 11-10, p. 307

Lack of hurricanes adjacent to South America: cooler water vertical wind shear, unfavorable ITCZ position

Naming Hurricanes and Naming Hurricanes and Tropical StormsTropical Storms

past practices: past practices: female namesfemale names

current protocol: current protocol: female and male names; female and male names; in alphabetic order so that the first hurricane in alphabetic order so that the first hurricane starts with the letter A; then in Greek alphabetstarts with the letter A; then in Greek alphabet

•Letters Q, U, X, Y not used over north Atlantic

•Letters Q, U not used over north Pacific

•Whenever a hurricane has had a major impact, any country affected by the storm can request that the name of the hurricane be retired by the World Meteorological Organization.

Devastating Winds and Devastating Winds and the Storm Surgethe Storm Surge

location of strongest windslocation of strongest winds Ekman transport Ekman transport 10-15 m high waves10-15 m high waves High ocean High ocean due to low due to low

pressurepressure storm surge: storm surge: several several

meter rise of sea levelmeter rise of sea level Flooding: Flooding: due to heavy due to heavy

rain and storm surge; causerain and storm surge; cause

most human casualtiesmost human casualties

Table 11-2, p. 313

Table 11-3, p. 316

Figure 4, p. 319

Katrina, 2005: $75B damage; >1200 deaths;

High winds, large waves, andlarge storm surge caused disastrous breeches in the leveesystem

Hurricane Watches, Hurricane Watches, Warnings and ForecastsWarnings and Forecasts

Hurricane watch:Hurricane watch:

24-48 hr before landing24-48 hr before landing hurricane warning:hurricane warning:

storm will strike an areastorm will strike an area Forecasts:Forecasts: improvement in path;improvement in path;

not in strengthnot in strength

Wrong forecastsWrong forecasts

also cause Economic lossalso cause Economic loss

Modifying HurricanesModifying Hurricanes

cloud seeding to reduce maximum cloud seeding to reduce maximum windwind

monomolecular filmsmonomolecular films