Transcript Document

Chapter 11: Hurricanes
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Tropical weather
Anatomy of a hurricane
Hurricane formation and
dissipation
Naming Hurricanes and
Tropical Storms
Some notable hurricanes
Hurricane watches,
warnings and forecasts
Modifying hurricanes
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Tropical Weather
Tropics:
23.5N-23.5S
 streamlines
 tropical wave
easterly wave,
2500 km wavelength,
10-20 knots speed
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Q: Why is there a convergence
east of the trough? A: see figure.
• 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,
and instead of isobars, streamlines are used
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Anatomy of a Hurricane
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hurricane (typhoon, cyclone): > 64 knots
eye; eyewall
spiral rain band
• 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
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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
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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
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The Right Environment
Convergence trigger
ITCZ, easterly waves, midlatitude fronts to tropics
 Weak trade wind inversion
 Weak upper wind
e.g., during a La Nina event (for more hurricanes over Atlantic)
 Some Coriolis force: 5-20deg latitude
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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.
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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
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Hurricane Stages of Development
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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.
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
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The Storm Dies Out
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cutting off the storm’s
energy supply by moving
over cooler ocean
Landfall: lose energy source
and increased friction to
reduce wind
A
B
C
D
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).
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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
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Hurricane Movement
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role of the ITCZ
northward movement due
to subtropical highs
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
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January
Fig. 7.27 (a)
July
Fig. 7.27(b)
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Q: Why is there a lack of
hurricanes adjacent to
South America? A:
because of cooler water,
vertical wind shear, and
unfavorable ITCZ
position.
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Hurricane paths
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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
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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, 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.
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Check
your
names
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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
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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/
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defined primarily based on wind speed
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Landfall from
1900 to 2009
over the Gulf
or Atlantic
coasts of U.S.
Categories 35 are
considered
major
hurricanes
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Katrina, 2005:
$75B damage;
>1500 deaths;
High winds, large waves, and
large storm surge caused
disastrous breeches in the levee
system
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Figure 4, p. 319
Visible satellite imagery on May 2, 2008. The storm
surge and flood waters killed more than 140,000 people.
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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
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Red: hurricane warning;
pink: hurrican watch;
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Modifying Hurricanes
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cloud seeding to reduce maximum wind
attempt to increase ordinary thunderstorms (rather than severe
thunderstorm in a hurricane;
difficult to verify the effect
monomolecular films
attempt to reduce surface evaporation;
difficult to verify the effect
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Hurricanes in a warmer world
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
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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
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