Transcript Chapter 11: Hurricanes

Chapter 11:
Hurricanes
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Tropical weather
Anatomy of a hurricane
Hurricane formation and dissipation
Some notable hurricanes
Hurricane watches, warnings and forecasts
Modifying hurricanes
Tropical Weather
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Tropics:
23.5N-23.5S
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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
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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
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
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Convergence trigger
ITCZ, easterly waves, midlatitude fronts to tropics
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Weak trade wind inversion
Weak upper wind
during El Nino over Atlantic
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Some Coriolis force: 5-20deg latitude
The Developing Storm
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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
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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
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tropical disturbance: unorganized mass of
thunderstorms, weak wind
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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
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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
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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
Fig. 11-10, p. 307
Naming Hurricanes and
Tropical Storms
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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
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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
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Flooding: due to heavy
rain and storm surge; cause
most human casualties
Table 11-2, p. 313
Table 11-3, p. 316
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
Hurricane Watches, Warnings
and Forecasts
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Hurricane watch:
24-48 hr before landing
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hurricane warning:
storm will strike an area
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Forecasts:
improvement in path;
not in strength
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Wrong forecasts
also cause Economic loss
Modifying Hurricanes
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cloud seeding to reduce maximum wind
monomolecular films