Hurricanes
Tropical Weather
• Tropics:
the belt between the Tropic of Cancer (23.5N) and the Tropic
of Capricorn (23.5S)
• The weather is very different from that of the mid-latitudes
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♦ Temperature: small seasonal changes (the noon sun is always high)
♦ Precipitation: either wet all seasons (Af) or distinct dry and rainy
seasons (Aw, Am) depending on the location of the ITCZ
♦ Winds: mostly easterlies
The surface temperature of the ocean is relatively high all year around
resulting is high evaporation rate and water content in the atmosphere.
In the NH the ocean temperature is maximum August-September. This
is the time when large amount of latent heat is released in the
atmosphere.
Satellite Infrared Image
Tropical Waves
• Streamlines:
depict actual wind flow
at the surface (account for friction as
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well). Exhibit regions of surface
convergence and divergence
Tropical wave: a weak trough of low
pressure disturbing the streamlines
Tropical waves have wavelengths
~2500 km, move slowly to the west.
In the zone of divergence:
♦ West side of the trough
♦ Downward air motion
♦ Fair weather
In the zone of convergence:
♦ East side of the trough
♦ Upward air motion
♦ Precipitation and storms
Occasionally the tropical wave will
intensify and become a hurricane
Hurricanes
• A(64hurricane
is a tropical cyclone with sustained winds over 74 mi/hr
knots), formed over the tropical region of the oceans
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♦ Forms around centers of very low surface pressure
♦ Surface winds in the NH are counterclockwise and in the SH are
clockwise.
♦ The winds are spiraling and intensifying towards the center
♦ Typical diameter ~ 500 km
♦ The center of the storm (the hurricane eye~40 km) is cloud free
and with very weak winds.
♦ The eye is surrounded by a ring (the eye wall) of intense winds and
storms.
♦ The clouds are aligned into spiral rain bands
Aka typhoons (NE Asia), cyclones (India) and tropical cyclones
(Australia). “Tropical cyclone” is the common international term for a
storm which developed in the tropics.
Summary of the 2004 Hurricane Season
Summary of the 2005 Hurricane Season
Summary of the 2006 Hurricane Season
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Hurricane Movement
The direction of movement of
hurricanes is determined by
the prevailing winds.
In the tropics they move to
the west, northwest.
The direction depends on the
pressure and the wind system
in the rest of the atmosphere.
Over the oceans they follow
closely the flow around the
subtropical highs.
The strength depends very
much on the temperature of
the ocean water.
H
H
H
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The Structure of a Hurricane
The surface winds are cyclonic (counterclockwise) and
toward the low pressure center.
In the center the converging air is swirling and
moving upwards.
The rising air condenses, forms clouds, and releases
latent heat that heats the core of the storm.
The warm central core is a column of warmer air that
is taller than the surrounding air and corresponds to a
center of high pressure aloft.
The resulting winds aloft a therefore anticyclonic and
outward of the high pressure center.
The Structure of a Hurricane
Hurricane Katrina’s eye wall
The Structure of a Hurricane
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Forming Conditions:
Warm and moist air that is conditionally unstable.
♦ The peak of the hurricane season is late summer and
early fall.
♦ A cold air mass aloft can create unstable conditions
Surface air convergence.
♦ Converging winds along the ITCZ
♦ Preexisting atmospheric disturbance
♦ Tropical wave
Upper air divergence - a divergence in the jet stream.
Hurricanes typically form in the tropics 10-24 degrees
latitude.
♦ not at the equator (why?)
♦ away from the subtropical highs (why?)
Where is the energy coming from?
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• During evaporation latent heat is taken from the ocean
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The energy is ultimately derived from the warm ocean
water.
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surface into the atmosphere. The rising warm moist air
further transports the energy upward.
During condensation at higher levels, the heat is released
in the form of sensible heat and the air aloft is heated
This creates higher pressure aloft and drives diverging
anticyclonic winds aloft.
The diverging winds aloft intensify the low pressure center
at the surface. This drives stronger convergence of the
surface winds, stronger winds and bigger waves (stronger
air-ocean friction) -> positive feedback.
When the hurricane moves over land the supply of moisture
in the “engine” is cut off and the storm dissipates.
Middle Latitude Cyclones/Tropical
Cyclones/Tornadoes
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• Difference:
In common: the central pressure is LOW and the surface
winds are cyclonic
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♦ the size: MLC > TC >T
♦ the pressure drop: MLC < TC < T
What does that tell us?
♦ The pressure gradient force is biggest in tornadoes, is
smaller in hurricanes and is smallest in middle latitude
cyclones
♦ PGF is balanced by the Coriolis force, which is
proportional to the wind speed
♦ The winds are strongest in tornadoes, less strong in
hurricanes and weakest in middle latitude cyclones.
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Storm Surge and Flooding.
The storm surge results from several different effects:
♦ Heavy rainfall
♦ Elevated sea level due to the low pressure center.
♦ Large ocean waves generated by the winds (high surf)
♦ Strong winds push the ocean water towards the beach
(Eckman transport)
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Storm surge due to
Ekman transport:
the net transport
of water is to the
right of the surface
wind
Notice the velocity
difference of the
rain bands on both
sides of the eye.
Saffir-Simpson Scale
1900-1999
Florida (Keys) 1935
Camille 1969
Andrew 1992
Naming Hurricanes