Transcript Slide 1

Hurricane Development
Nick D’Anna
ESS501
What is a Hurricane?
• A tropical cyclone
• Counter-clockwise circulation
around a Low-Pressure System.
• Sustained winds exceeding 64 knots
(74 mph)
• Usually about 500km in diameter
Structure of Hurricane Jeanne
September 26, 2004
http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003024/index.html
Eyewall
Eye
Spiral Rain Bands
Yellow illustrates 1 “/hr of rainfall
http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003024/jeanne_09_26.0185.tif
Formation:
• Hurricanes do not just appear.
• There is a sequence of steps to their
formation.
• Certain conditions must be met for the
sequence to progress.
Thunderstorms
Tropical wave
Tropical depression
Tropical storm
HURRICANE
First Thunderstorms:
Tropical regions (23½°N - 23½°S) have the
right ingredients for frequent
thunderstorm activity.
There is little seasonal temperature
variation.
The daily heating and high humidity
encourage the formation of cumulus /
thunderstorm clouds
In addition to
Thunderstorms, hurricanes
require warm water (28°C).
Water with the appropriate
temperature will only be found in
tropical regions
A series of sea surface
temperature (SST) images
will depict the location of
the warm water.
JANUARY
-10.0
0.8
11.6
22.4
33.2 ( oC )
FEBRUARY
MARCH
APRIL
MAY
JUNE
JULY
AUGUST
SEPTEMBER
OCTOBER
NOVEMBER
DECEMBER
JANUARY
Air will rise over
the warm water.
Air moves in to
replace the air
that rose up. The
converging air
will further rise.
As it rises, it
cools and water
vapor will
condense. This
will release
latent heat, and
further heat the
column of air.
The ITCZ shows up as a band of
clouds near the equator
The ITCZ may be the birth of
some hurricanes, but it is
uncommon.
Hurricanes more frequently
start off as Easterly Waves.
An Easterly Wave
also called a Tropical Wave
Often the seed
of a hurricane
A group of
thunderstorms
that has
organized
The lines on the
illustration are
streamlines
Tropical Wave
• originate near the African coast as a shear
instability in the flow (spin off from the
African easterly jet – seasonal jet)
• have wavelengths of about 2500 km
• position is found in a trough of streamline
pattern
• convergence/upward motion on east side
• divergence/downward motion on west
side
• travel from east to west at 10-20 knots
This is a scheme of a series of tropical waves
coming off the coast of Africa toward the
Caribbean Sea. As the waves travel across
the Atlantic, they may organize and grow
into tropical depressions.
How could a wave become a
hurricane?
1. It has to stay over the warm water
(28°C)
2. It must be far enough from the
equator in order to start to spiral.
#1. Warm water
• Water vapor turning into liquid water is
the real “fuel” of a hurricane.
• The initial cause for rising air is the
intense solar heating, but as this air rises,
it cools adiabatically, and water vapor
condenses into cloud droplets.
• The condensation process releases large
amounts of latent heat (540 cal/gram).
• This in turn heats the column of air,
further causing it to rise, creating a
pressure gradient force.
Tropical Depression
• Winds near the center are constantly
between 20 and 34 knots (23 - 39 mph).
• Designated at the first appearance of a
lowered pressure and organized
circulation in the center of the
thunderstorm complex.
• At least one closed isobar.
Tropical Depression
Tropical Storm
• Maximum sustained winds are between 35-64
knots (39-73 mph).
• Assigned a name.
• The storm becomes more organized and begins
to become more circular in shape -- resembling a
hurricane.
• Tropical storms can cause a lot of problems even
without becoming a hurricane. However, most
of the problems a tropical storm causes stem
from heavy rainfall.
Tropical Storm
Hurricane: eye & spiral bands
develop as a result of rotation
Satellite images of the various stages
in a hurricanes development
#2. Must be some distance
from the equator
• At the equator the Coriolis force is zero.
• In order for the storm to start to spiral it
must be large and about 10 degrees from
the equator.
300 mb
300 mb
Two columns of air (blue blocks). The wax
paper represents the pressure gradient.
300 mb
300 mb
Heat one column with intense solar radiation
(shown here with a flashlight).
300 mb
100 mb
???
300 mb
The air molecules expand when heated. They become
less dense and rise (remember warmer air is thicker
air). This creates a pressure gradient.
Outward upper air
flow is clockwise
Air flows down the pressure gradient. The
direction of flow is deflected to the right due
to the Coriolis force.
Stacked
on air
300 mb
300 mb
Stacked
on air
300 mb
The air essentially leaves the center column and
stacks on the outer columns. Creating differences
in surface pressure
300 mb
300 mb
The air at the surface will
rush toward the L. When the
air collides it will be forced
to rise, creating a positive
feedback loop to drive the
system.
300 mb
Counter-clockwise
surface flow
In addition to (#1)warm water and
being (#2)far enough from the equator
the storm needs:
3. Weak winds aloft, high wind
shear will spread out the energy
4. Upper air outflow must be
greater than the surface
convergence.
#3. Weak winds aloft
• The rising column of air needs stability.
• Wind shear will disrupt that and the rising
column’s energy will disperse.
How wind sheer lessons the chance of
a hurricane developing:
No or little
shear, good
vertical
development
Good for
hurricanes
High amount of
shear, less
vertical
development
Not good for
hurricanes
Hurricane in Therapy
#4. Upper-level outflow of air
must be greater than the
surface inflow
This is not so obvious.
A commonly used analogy is that of a
vacuum. As long as the air is exiting the
vortex from the top more air will be
“sucked” up into it from the bottom.
As air leaves the upper High, air is
“sucked” in toward the surface low. This is
what creates the intense winds.
H
L
In well developed hurricanes,
there is a downward flow of
air inside the eye.
This is caused by the eyewall
being heated more than the air
inside the eye, creating a
temperature/pressure
gradient.
Eye
Not heated
by release
of latent
energy.
Eyewall
super
heated
by latent
energy
Creates
pressure
gradient
and
downward
flow.
Eyewall
super
heated
by latent
energy
The air flowing down the eye is heated by
compression and water droplets evaporate. This
allows clear skies to be seen from the eye of the
storm.
H
L
Composite image of Charley, Jeanne,
Ivan, & Frances
2004 Hurricane Tracks
The 2004 Hurricane Season
SUMMARY
• Form only at lower latitudes because
they require deep layer of warm
water.
• Hurricane season is summer and fall
since that is when ocean is warmest
• Rotation of storm needs some
Coriolis force so they cannot form at
the Equator
SUMMARY (con’t)
•May form at point of surface
convergence - possibilities include
easterly waves, ITCZ, organized mass
of thunderstorms, old front from middle
latitudes
•Certain conditions aloft hinder
development - sinking air
suppresses convection and strong
winds aloft shear apart storms
SUMMARY (con’t)
• Still debate about exact mechanism
that drives storm - but certain is that
warm ocean surface is source of
energy
SUMMARY
Hurricane development
• Stages that are somewhat arbitrarily
defined
• Tropical disturbance/tropical wave is
just a mass of thunderstorms with
evidence of weak circulation
• Tropical depression when winds are
23-39mph and central pressure a few
millibars lower than surrounding
pressure
References
• http://www.usatoday.com/weather/hurricane/hurrican
e-resources.htm
• http://www.usatoday.com/weather/graphics/hurricane
/hurricane_explainer/flash.htm
• http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/s
tages/home.rxml
• http://apollo.lsc.vsc.edu/classes/met130/notes/chapter1
6/east_waves.html
• http://brookscole.com
• Ahrens, Donald C. Meteorology Today, 7th ed.
Thomson Brookscole. 2003.
• Ahrens, Donald C. Essentials of Meteorology, 3rd ed.
Thomson Brookscole. 2001.
References (con’t)
• http://www.atmos.washington.edu/2003Q3/101/notes/
Hurricanes.html
• http://www.palmbeachpost.com/storm/content/weathe
r/special/storm/
• http://www.sullivanet.com/misc/hurricanes/hurrfear.g
if
• http://cagle.slate.msn.com/news/Hurricanes/1.asp
• http://svs.gsfc.nasa.gov/index.html
• Inquiring Minds: Weather Phenomena. Hurricanes.
TV Ontario © 1998. http://www.unitedstreaming.com
• http://www.physicalgeography.net/fundamentals/7u.html