Download What is a `hurricane`?

What is a ‘hurricane’?
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Where do they form? Why?
Why do they move east to west rather than west
to east?
Why do they have names?!? And how do they
pick the names?
What is the ‘eye”?
The ‘eye wall”?
How do hurricanes compare to other storms?
Typhoons? mid-latitude cyclones?
http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html
Sept 06 2004
“IVAN IS FORECAST TO SLOWLY DEEPEN OVER
THE NEXT 36 HOURS REACHING 130 KT IN 48
HOURS “
DEEPEN? What does this mean? How does it relate to
hurricane intensity?
“ESTIMATED MINIMUM CENTRAL PRESSURE 951
MB MAX SUSTAINED WINDS 110 KT WITH GUSTS
TO 130 KT. “
What is an “MB”? A “KT”? What is the significance of these
What does ‘warm water’ have to
do with hurricanes?
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NWS TPC/NATIONAL HURRICANE
CENTER MIAMI FL 5 AM EDT MON SEP 06
2004
“NOW THAT FRANCES HAS MOVED OVER THE WARM
WATERS OF THE NORTHEAST GULF OF
MEXICO...THUNDERSTORMS ACTIVITY HAS STEADILY
INCREASED NEAR THE TIGHT CIRCULATION CENTER
DURING THE PAST 6 HOURS. THE LAST RECON REPORT
AT AROUND 05Z INDICATED A RAGGED EYE WAS TRYING
TO FORM…
NOW THAT THE CENTER OF FRANCES IS BACK OVER WARM
WATER...SOME RE-STRENGTHENING IS POSSIBLE.
What are the ‘levels’ or ‘layers’
of the of the atmosphere?
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“UPPER-LEVEL OUTFLOW IS STILL QUITE
IMPRESSIVE”
“A STRONG DEEP-LAYER ANTICYCLONE
IS FORECAST TO REMAIN TO THE
NORTH OF THE HURRICANE STEERING
IVAN INTO THE EASTERN CARIBBEAN
SEA BY DAY TWO AND TOWARD THE
SOUTHWESTERN PORTION OF
HISPANIOLA ON DAY FOUR.”
“STRONG UPPER LEVEL SHEAR…”
How are Hurricanes tracks
established?
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THE NHC MODEL GUIDANCE IN GOOD
AGREEMENT ON FRANCES MOVING WESTNORTHWESTWARD AROUND THE
SOUTHWESTERN PORTION OF THE
SUBTROPICAL RIDGE AXIS THAT EXTENDS
EAST-WEST ALONG 32N LATITUDE. THE
PRIMARY DIFFERENCE IN THE MODELS IS HOW
SHARP OF A TURN FRANCES WILL MAKE UPON
REACHING 32N. THE GFS APPEARS TO MAKE
TOO SHARP OF A NORTHEASTWARD TURN
GIVEN THE CURRENT LARGE DIAMETER OF
FRANCES. THE OFFICIAL FORECAST IS CLOSE
TO THE PREVIOUS FORECAST TRACK AND IS
CLOSE THE GUNA MODEL CONSENSUS.
Huh?
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“THERE IS VERY GOOD
AGREEMENT THROUGH DAY FIVE
WITH NOGAPS TO THE RIGHT OF
THE AIDS ENVELOPE AND THE
UKMET TO THE LEFT OF THE AIDS
ENVELOPE. THE OFFICIAL
FORECAST IS SLIGHTLY RIGHT OF
THE PREVIOUS FORECAST TRACK
AND IN VERY GOOD AGREEMENT
WITH GUNS AND GUNA”.
So what is a ‘MODEL” and how
does it work?
How does RADAR and Satellite
imagery assist in evaluating
hurricanes?
06 Sept. 04 Visible Satellite Image
A SMALL EYE IS EVIDENT ON THE VISIBLE IMAGERY. …. AND
SOMEWHAT RAGGED-LOOKING BANDING FEATURES
Francis
06 Sept 04 IR Satellite Image
Hermine?
IVAN
The Dvorak Technique Explained
The Dvorak technique is a method using enhanced
Infrared and/or visible satellite imagery to quantitatively
estimate the intensity of a tropical system. Cloud
patterns in satellite imagery normally show an indication
of cyclogenesis before the storm reaches tropical storm
intensity. Indications of continued development and/or
weakening can also be found in the cloud features. Using
these features, the pattern formed by the clouds of a
tropical cyclone, expected systematic development, and a
series of rules, an intensity analysis and forecast can be
made. This information is then standardized into an
intensity code
How do statistics and probability
relate to hurricane forecasts?
Note the difference in size between the forecast cones…
note the degradation of certainty over time, and the higher
certainty of the track for hurricane ‘Ivan’
First question….
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Where do hurricanes form? Why?
Atmospheric heating….
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Imbalances in solar NRG are the driving
force behind all weather.
Direct sunlight imparts more NRG to the
surface
High angle sunlight has very much lower
NRG content
The atmosphere is heated from below…
Figure 3.3
What significance do the
tropics of Cancer and
Capricorn have?
Figure 3.2
Figure 3.5
Note the influence of clouds… which interrupts the latitudinal patterns
Figure 3.7
DECEMBER 21
Figure 3.8
Figure 2.6
Figure 2.13
Figure 2.3
So how about those names?
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For several hundred years, hurricanes in the West
Indies were often named after the particular saint’s day
on which the hurricane occurred.
Using women’s names became the practice during
World War II
In 1979 male and female names were included in lists
for the Atlantic and Gulf of Mexico
Names associated with storms that have caused
significant death and/or damage are usually retired
from the list.
name lists have been agreed upon at international
meetings of the World Meteorological Organization
http://mywebpages.comcast.net/herbwx/hurrsixyr.html
Historical hurricane tracks and names
http://weather.unisys.com/hurricane/atlantic/index.html
Tropical Weather & Waves
Figure 16.1
Tropical winds
typically blow
from the east,
and when they
encounter a
slow moving
trough of low
pressure, called
a tropical
wave, the
winds initially
converge and
lift to produce
showers and
thunderstorms.
Structure of a Hurricane
Tropical cyclones are
the international
name of hurricanes,
which typically form
from an organized
mass of storms formed
along a tropical wave.
In this image of
Hurricane Elena, the
central area of broken
clouds is the eye,
surrounded by an eye
wall cloud and spiral
rain bands, with a
total diameter nearing
500 kilometers.
Figure 16.2
Hurricane Wind Profile
Figure 16.3
The low pressure core of the hurricane is surrounded by several
thunderstorms, each with updraft and downdraft cycles.
The wind and moisture cycle is repeated as:
surface moist air converges in a counterclockwise pattern at the eye,
rises to create high pressure aloft, condenses, precipitates, dries,
diverges outward in a clockwise pattern, sinks, and warms.
3-D Radar Image of Hurricane
Figure 16.4
Several key features of a hurricane are shown in this radar
composite image, including overshooting clouds, the area of
strongest echoes (heaviest rain), and the eyewall.
Formation by Organized Convection
Figure 16.5A
Figure 16.5B
One theory explains that hurricane formation requires cold air above an
organized mass of thunderstorms, where the release of latent heat warms
the upper troposphere, creates high pressure aloft, which pushes air
outward and causes a low to deepen at the surface.
Air moving toward this low intensifies the cycle.
Formation by Heat Engine
Another theory of hurricane development proposes that
a heat engine cycle, fueled by warm moist input air and
the release of heat when it converts to cool dry air.
Differences in the input and output temperatures
determine the amount of work on the ocean and winds
that is performed.
Hurricane Stages of Development
The initial
components of a
hurricane may
form as a tropical
disturbance, grow
into a tropical
depression when
winds exceed 20
knots, become a
tropical storm
when winds
exceed 35 knots,
and finally then
qualify as a
hurricane when
winds exceed 64
knots.
Figure 16.6
Erratic Paths of Hurricanes
Figure 16.8
Historical charts of hurricane location may reveal
erratic, and hard to predict, patterns of movement.
As this figure shows, hurricanes may occasionally
double back.
Further, when removed from the ocean and without a
moisture source to supply energy, they may still
continue an inland journey.
In the North Atlantic, on average 3 storms per year
move inland and bring damaging winds and rain.
North Atlantic Hurricanes
Composite infrared
imagery of
Hurricane Georges
reveals the pattern
of a seasonal threat
for Central and
North America
coastlines.
Tropical cyclones at
the same latitude
survive longer in
the Atlantic than
Pacific Ocean
because of warmer
Atlantic Ocean
waters.
Figure 16.9
Hurricane Damage & Warning
Figure 16.11
Figure 16.10
Hurricanes have their highest wind speeds on the side where
storm pushing winds amplify cyclonic, or counterclockwise,
rotational winds. In coastal areas, flooding is aggravated by the
hurricane low pressure triggering higher tides and Ekman
transport piling up water.
Hurricane Watch & Warning
Figure 16.5A
The National Hurricane Center in Florida issues a hurricane watch 24 to
48 hours before a threatening storm arrives, and if it appears that the
storm will strike within 24 hours, a hurricane warning is issued.
While some consider the warning area too large, causing unneeded
evacuation, such evacuations have saved many lives.
Hurricane Hugo, with peak winds near 174 knots, caused tremendous
damage.
Hurricane Saffir-Simpson Winds
Figure 16.13A
Figure 16.14
In 1989 Hugo caused nearly $7 billion in damages in the U.S.,
killing 49 in the Caribbean and United States.
Current classification of hurricanes is based on their wind speed,
however, and not on human or property damage.
Hurricanes range from category 1 to 5, with winds of 64 to more
than 135 knots.
Hurricane Names and Cost
Category 5 Hurricane
Andrew (1992) was
the costliest US storm,
but it ranks as less
intense than 1935 and
1969 hurricanes.
Hurricane names are
chosen from an
alphabetical list of
male and female
names for the Atlantic
and Pacific, some of
which are retired if
the storm was
especially damaging.
Figure 16.14
Hurricane Andrew Devastation in Homestead, Florida
August 24, 1992
Figure 16.15
Likelihood for Landfall
Between 1900 and
1999, only two
category 5
hurricanes have
made landfall
along the Gulf or
Atlantic.
Numerous
category 1, and
less damaging
storms, that do
make landfall may
not cause much
damage, but bring
needed rainfall.
Figure 16.16