Chapter 11: Hurricanes

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

2. 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 theCoriolis force is too small to balance the pressuregradient force. Thus winds are not geostrophic.

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

4. Sinking air and clear sky in the eye due to high pressure aloft Fig. 11-3, p. 302

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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

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

12. Lack of hurricanes adjacent to South America: cooler water vertical wind shear, unfavorable ITCZ position Fig. 11-10, p. 307

13. 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.

14. 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

15. Table 11-2, p. 313

16. Table 11-3, p. 316

17. Katrina, 2005: $75B damage; >1200 deaths; High winds, large waves, and large storm surge caused disastrous breeches in the levee system Figure 4, p. 319

18. Hurricane Watches, Warnings and Forecasts • Hurricane watch: 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

19. Modifying Hurricanes • cloud seeding to reduce maximum wind • monomolecular films