Overview of Hurricanes

Report
Carl Schreck
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 Winds < 35 kts
 Numbered, but not
named
 Semi-organized group of
thunderstorms
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 Winds 35–63 kts
 ≈ 11 per year in the
Atlantic
 Named
 More circular/spiral, but
no eye
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 Winds ≥ 64 kts
 ≈ 6 per year in the Atlantic
 Eye develops
 Different names around the
world:




Hurricane in the Atlantic and
East Pacific
Typhoon in West Pacific
Cyclone in Indian Ocean and
South Pacific
“Tropical Cyclone” is the
generic/scientific term
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 Winds ≥ 96 kts
 ≈ 2-3 per year in the
Atlantic
 Category 3–5 on the
Saffir–Simpson Scale
 Strong and dangerous
storms
 Well-defined, circular eye
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 Deep warm ocean layer


Warm oceans are the energy
source for hurricanes
But hurricanes can mix cold
water upward if the warm
water is too shallow
 Conditionally unstable
atmosphere

Warm air rises, but only if it’s
warmer than its surroundings
 Moist mid-troposphere

Dry air 2-3 km from the
surface can cause cold
downdrafts
ww2010.atmos.uiuc.edu
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 Pre-existing convection

Needs organized
thunderstorms to get things
going
 Cyclonic low-level vorticity


Counter-clockwise spinning
winds help organize the
thunderstorms
Associated with low pressure
 Weak vertical wind shear

Tilting winds can knock the
storm over
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 Wind shear is defined as the wind vector
difference between the 850 and 200 mb
level (arbitrary)
Good – latent
heat can
concentrate in
one area
Bad – convection
torn apart
High westerly shear
Low easterly shear
In general, low values (< 20 kt) of vertical wind
shear are desired.
Courtesy of C.C. Hennon, UNC Asheville
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Berry et al. (2007, Mon. Wea. Rev.)
 Thunderstorm systems with
wave-like properties
 Move westward with a period
of 3–6 days
 Common over West Africa
 Provides storms with cyclonic
vorticity and convection
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 Warm Ocean Temperatures
 10°N to 20°N
 Supply of Tropical Waves
from Africa
 20°W to 60°W or 80°W
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Zipser et al. (2009, BAMS)
climate.nasa.gov
 Dry air from Sahara can
weaken storms
 Dust blocks the sun and
cools the ocean
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Knapp et al. (2010, BAMS)
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 Bars depict number
of named systems
(open/yellow),
hurricanes
(hatched/green),
and category 3 or
greater (solid/red),
1886-2004
nhc.noaa.gov/climo
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 Historical record of
tropical cyclone locations
and intensities
 Constructed after each
season by the warning
agencies
 Best estimate using all
available data
nhc.noaa.gov
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 Weather stations
 Buoys
 Ships
 Radar
 Aircraft
Reconnaissance
 Visible/Infrared
Satellites
 Microwave satellites
 Satellite estimates
account for vast
majority of the
record
Courtesy of Jack Beven (NOAA/NHC)
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 Weather stations
 Buoys
 Ships
 Radar
 Aircraft
Reconnaissance
 Visible/Infrared
Satellites
 Microwave satellites
 Satellite estimates
account for vast
majority of the
record
Courtesy of Jack Beven (NOAA/NHC)
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Which image on the right is the
closest match for this image of
Hurricane Katrina?
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