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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
http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003024/jeanne_09_26.0185.tif
Yellow illustrates 1 “/hr of rainfall
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.
Two columns of air (blue blocks). The wax paper represents the
pressure gradient.
300 mb
300 mb
300 mb
300 mb
Heat one column with intense solar radiation (shown here with a
flashlight).
300 mb
300 mb
100 mb ??
?
The air molecules expand when heated. They become less dense and rise (remember warmer air is thicker air). This creates a pressure gradient.
Air flows down the pressure gradient. The direction of flow is deflected to the right due to the Coriolis force.
Outward upper air flow is clockwise
The air essentially leaves the center column and stacks on the outer columns. Creating differences in surface pressure
Stacked on air
Stacked on air
300 mb
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
300 mb
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
High amount of shear, less vertical development
Good for hurricanes
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.
L
H
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.
Eyewallsuper
heated
by latent energy
EyeNot
heated by
release of latent energy.
Creates pressure gradient
and downward flow.
Eyewallsuper
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.
L
H
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
SUMMARYHurricane 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/
hurricane-resources.htm• http://www.usatoday.com/weather/graphics/
hurricane/hurricane_explainer/flash.htm• http://ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/hurr/stages/home.rxml• http://apollo.lsc.vsc.edu/classes/met130/notes/
chapter16/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/
weather/special/storm/• http://www.sullivanet.com/misc/hurricanes/
hurrfear.gif• 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
