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

2. Tropical Weather Tropics: Tropics: 23.5N-23.5S 23.5N-23.5S streamlines streamlines tropical wave tropical wave easterly wave, easterly wave, 2500 km wavelength, 2500 km wavelength, 10-20 knots speed 10-20 knots speed The tropics are close to the equator,The tropics are close to the equator, where the Coriolis force is too small where the Coriolis force is too small to balance the pressure gradient to balance the pressure gradient force. Thus winds are not geostrophic, force. Thus winds are not geostrophic, and instead of isobars, streamlines are used and instead of isobars, streamlines are used Q: Why is there a convergence east of the trough? A: see figure. 2

3. Anatomy of a Hurricane hurricane (typhoon, cyclone): > 64 knots hurricane (typhoon, cyclone): > 64 knots eye; eyewall eye; eyewall spiral rain band spiral rain band Hurricanes are quite similar to, yet also quite different from mid-latitude storms.Hurricanes are quite similar to, yet also quite different from mid-latitude storms. Q: 1 knot equals a) 1 km/hr b) 1 mile/hr c) 1 nautical mile/hr 3

4. Q: What would be your experience if you cross a hurricane? A: heavy rain, followed by no rain; very heavy rain and very strong wind near eyewall, followed by clear sky in the eye due to high pressure aloft; very heavy rain near the eyewall again with the reverse of wind direction 4

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; Peak in early September 5

6. The Right Environment Convergence trigger Convergence trigger ITCZ, easterly waves, midlatitude fronts to tropics ITCZ, easterly waves, midlatitude fronts to tropics Weak trade wind inversion Weak trade wind inversion Weak upper wind Weak upper wind e.g., during a La Nina event (for more hurricanes over Atlantic) e.g., during a La Nina event (for more hurricanes over Atlantic) Some Coriolis force: 5-20deg latitude Some Coriolis force: 5-20deg latitude Q: Why are relatively strong temperature inversion and strong wind shear required for supercell? A: very weak inversion would lead to ordinary thunderstorms only and weak wind shear would self-destroy the storm by downdraft. Q: Why are weak inversion and weak upper wind are required for hurricanes? A: warm ocean surface is the energy source that is not much affected by downdraft; strong inversion prevents the formation of thunderstorms; strong wind disrupts the organized pattern of convection. 6

7. The Developing Storm Heat engine: heat taken at high T, converted into work, then ejected at low T. 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 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 Maximum wind depends on surface and tropopause temperature difference and the potential of sea surface evaporation 7

8. Hurricane Stages of Development tropical disturbance: unorganized mass of thunderstorms, weak wind tropical disturbance: unorganized mass of thunderstorms, weak wind tropical depression: 20-34 knots, closed isobars tropical depression: 20-34 knots, closed isobars tropical storm: 35-64 knots, with a name tropical storm: 35-64 knots, with a name hurricane: > 64 knots, with a name hurricane: > 64 knots, with a name This progression of stages is followed in reverse order as a storm weakens.This progression of stages is followed in reverse order as a storm weakens. Q: Where does the strongest wind occur in a hurricane? a) in the hurricane eye, b) around the eyewall, c) in the spiral rain band 8

9. The Storm Dies Out cutting off the storm’s energy supply by moving over cooler ocean cutting off the storm’s energy supply by moving over cooler ocean Landfall: lose energy source and increased friction to reduce wind Landfall: lose energy source and increased friction to reduce wind A D B C Q: In the figure, which storm is dying out? a) storm A, b) storm B, c) storm C, d) storm D Q: How to draw the vertical distribution of isobars associated with a hurricane? A: minimum surface pressure at hurricane center; high pressure near tropopause at center (see the drawing in class). 9

10. Q: Hurricanes can form over oceans with surface temperature a) > 17 C, b) > 22 C, c) > 27 C Q: Do hurricanes usually form at the equator? a) yes, b) no Q: Hurricane wind is stronger than a) 34 knots, b) 44 knots, c) 54 knots, d) 64 knots Q: Favorable conditions for hurricane development include a) strong inversion and strong upper wind b) strong inversion and weak upper wind c) weak inversion and strong upper wind d) weak inversion and weak upper wind 10

11. Hurricane Movement role of the ITCZ role of the ITCZ northward movement due to subtropical highs northward movement due to subtropical highs influence of the westerlies influence of the westerlies Q: Westward-moving North Atlantic hurricanes often take a turn towards the north as they approach North America, because of a) the Bermuda High, b) easterly wave, c) westerly wind 11

12. January Fig. 7.27 (a) July Fig. 7.27(b) 12

13. Q: Why is there a lack of hurricanes adjacent to South America? A: because of cooler water, vertical wind shear, and unfavorable ITCZ position. 13

14. 14 Hurricane paths

15. Hurricane vs midlattitude storms Q: What do hurricanes and midlatitude storms have in common? A: both are low pressure systems with counterclockwise motions (in Northern Hemisphere). Q: What do they differ in energy source? A: hurricanes derive energy from warm ocean and latent heat of condensation; while midlatitude storms derive energy from horizontal temperature difference 15

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17. Naming Hurricanes and Tropical Storms past practices: female names 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 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, Z not used over north Atlantic Letters Q, U not used over eastern north Pacific Whenever a hurricane has had a major impact, (as a category 3 or higher), its name is retired for at least 10 years. 17

18. 18 Check your names

19. Devastating Winds and the Storm Surge Q: The strongest hurricane wind is located at the place where the hurricane rotational wind and the hurricane movement wind are: a) in the same direction, b) in opposite direction, c) perpendicular to each other Q: Does the Ekman transport increase or decrease the coastal sea level in the figure? a) increase, b) decrease, c) no effect Q: What would cause high ocean at the center? a) surface low pressure, b) high pressure 19

20. Devastating Winds and the Storm Surge High waves would raise sea level Ocean tides (due to gravity of sun and moon) also raise sea level Storm surge (i.e., sea level rise by several meters) could be caused by the combination of Ekman transport, low pressure, high waves, and tides Flooding: due to heavy rainfall and storm surge; cause most human casualties http://www.nhc.noaa.gov/surge/ 20

21. defined primarily based on wind speed 21

22. 22 Landfall from 1900 to 2009 over the Gulf or Atlantic coasts of U.S. Categories 3- 5 are considered major hurricanes

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24. Figure 4, p. 319 Katrina, 2005: $75B damage; >1500 deaths; High winds, large waves, and large storm surge caused disastrous breeches in the levee system 24

25. 25 Visible satellite imagery on May 2, 2008. The storm surge and flood waters killed more than 140,000 people.

26. Hurricane Watches, Warnings and Forecasts Hurricane watch: Hurricane watch: 24-48 hr before landing 24-48 hr before landing hurricane warning: hurricane warning: storm will strike an area storm will strike an area Forecasts: Forecasts: improvement in path; improvement in path; not in strength not in strength Wrong forecasts Wrong forecasts also cause economic loss also cause economic loss 26 Red: hurricane warning; pink: hurrican watch;

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28. Modifying Hurricanes cloud seeding to reduce maximum wind cloud seeding to reduce maximum wind attempt to increase ordinary thunderstorms (rather than severe thunderstorm in a hurricane; attempt to increase ordinary thunderstorms (rather than severe thunderstorm in a hurricane; difficult to verify the effect difficult to verify the effect monomolecular films monomolecular films attempt to reduce surface evaporation; attempt to reduce surface evaporation; difficult to verify the effect difficult to verify the effect 28

29. Hurricanes in a warmer world 29 Hurricanes are very active in 2005; Tropical Atlantic sea surface temperature (SST) is warmer than climatology. Q: Will hurricane activities increase in a warmer world? a) yes, b) no, c) not so sure

30. 30 Q: list three triggers for the initial development of hurricane development. Q: Where are showers found: a) western side of tropical easterly wave, b) eastern side of the wave Q: What is the “fuel” that drives the hurricane? a) ocean surface evaporation, b) easterly wave, c) ITCZ Q: What factors tend to weaken hurricanes? A: wind shear, passage over cooler water, passage over land. Q: For a hurricane moving northward, which side will the strongest wind located? a) southern side, b) western side, c) northern side, d) eastern side