Chapter 11: Hurricanes Tropical weather Anatomy of a hurricane Hurricane formation and dissipation Some notable hurricanes Hurricane watches, warnings and forecasts Modifying hurricanes

Tropical Weather Tropics: 23.5N-23.5S streamlines tropical wave easterly wave, 2500km wavelength, 10-20 knots speed The tropics are close to the equator, where the Coriolis force is too small to balance the pressure gradient force. Thus winds are not geostrophic.

Anatomy of a Hurricane hurricane (typhoon, cyclone): > 64 knots eye; eyewall spiral rainband Hurricanes are quite similar to, yet also quite different from mid-latitude storms.

Sinking air and clear sky in the eye due to high pressure aloft Figure 11.3: A model that shows a vertical view of air motions and clouds in a typical hurricane in the Northern Hemisphere. The diagram is exaggerated in the vertical. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 11-3, p. 302

Hurricane Formation and Dissipation Hurricane 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 Environment Convergence trigger ITCZ, easterly waves, midlatitude fronts to tropics Weak trade wind inversion Weak upper wind during El Nino over Atlantic Some Coriolis force: 5-20deg latitude

The Developing Storm Heat engine: heat taken at high T, converted into work, then ejected at low T. Hurricane: heat taken from warm ocean, converted into kinetic energy (wind), lost at its top through radiational cooling Maximum wind depends on surface and tropopause temperature difference and the potential of sea surface evaporation

The Storm Dies Out cutting off the storm’s energy supply by moving over cooler ocean Landfall: lose energy sourse and increased friction to reduce wind

Hurricane Stages of Development tropical disturbance: unorganized mass of thunderstorms, weak wind tropical depression: 20-34 knots, closed isobars tropical storm: 35-64 knots, with a name hurricane: > 64 knots, with a name This progression of stages is followed in reverse order as a storm weakens.

Hurricane vs midlattitude storms Hurricane: 1) derive energy from warm ocean and latent heat of condensation; 2) warm core low; 3) high center aloft; 4) sinking air in the eye; 5) strongest wind near surface; 6) stronger wind; 7) smaller size Midlatitude storms: 1) derive energy from horizontal temperature difference; 2) cold core low; 3) intensifies with height; 4) rising air at center; 5) strongest wind aloft in the jet stream; 6) wind not as strong; 7) larger size Both are low pressure systems with counterclockwise motions

Hurricane Movement role of the ITCZ northward movement due to subtropical highs influence of the westerlies Because of the Bermuda High, westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America.

Lack of hurricanes adjacent to South America: cooler water vertical wind shear, unfavorable ITCZ position Figure 11.10: Regions where tropical storms form (red shading), the names given to storms, and the typical paths they take (red arrows). Fig. 11-10, p. 307

Naming Hurricanes and Tropical Storms past practices: female names current protocol: female and male names; in alphabetic order so that the first hurricane starts 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 the Storm Surge location of strongest winds Ekman transport 10-15 m high waves High ocean due to low pressure storm surge: several meter rise of sea level Flooding: due to heavy rain and storm surge; cause most human casualties

Table 11-2, p. 313

Table 11-3, p. 316

High winds, large waves, and large storm surge caused Katrina, 2005: $75B damage; >1200 deaths; High winds, large waves, and large storm surge caused disastrous breeches in the levee system Figure 4: The paths of eight hurricanes that impacted Florida during 2004 and 2005. Notice that in 2004 hurricanes Frances and Jeanne made landfall at just about the same spot along Florida’s southeast coast. The date under the hurricane’s name indicates the date the hurricane made landfall. Figure 4, p. 319

Hurricane Watches, Warnings and Forecasts 24-48 hr before landing hurricane warning: storm will strike an area Forecasts: improvement in path; not in strength Wrong forecasts also cause Economic loss

Modifying Hurricanes cloud seeding to reduce maximum wind monomolecular films