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Tropical Meteorology
Jeff Gawrych
Met. 10
Introduction
• Tropical weather is very different than our
mid-latitude weather.
– No fronts, little day-to-day variation
• Tropical weather systems form due to warm
air rising (convection).
• Thermodynamics says
– Heat added to system will increase the
temperature and increase ability to perform work
• Warm oceans are fuel
• Latent heat release when warm air rises and condenses
• Tropical storms and hurricanes act like heat engines
Background
• Tropical cyclones/Hurricanes are perhaps the most
severe of the weather phenomena. They can cause
both massive property damage and loss of life.
– in 1972 a TC killed an estimated 300,000-500,000 people in
Bangladesh.
– in 1992 Hurricane Andrew hit the southern US.
– in 1999 created massive flooding & mud slides in Honduras and
Guatemala
– in 2004, 4 hurricanes hot the Florida coast. Charley, Frances, Ivan,
Jeanne.
– http://www.cnn.com/SPECIALS/2004/hurricanes/interactive/hurrican
e.paths/index.html
ITCZ
• Inter-Tropical Convergence Zone
– defined as the result of the equatorial
Hadley cell where air converges near the
equator, rises and diverges near the
tropopause
• Creates large thunderstorms called hot
towers.
• Usually between 5 and 10 degrees
latitude.
Definition
•
•
•
•
These phenomena are commonly known as
Tropical Cyclones
_______________
(Indian Ocean & South Pacific),
Hurricanes*
_______________
(Atlantic and Eastern Pacific)
Typhoons
_______________
(North Pacific).
* Named for Carib god of evil: Huracan
• Tropical storms differ considerably from mid-latitude
cyclones: For example, tropical storms have
–
– Warm core
– No fronts
Strongest wind
near the surface
Stages of development
•Tropical Disturbance:
mass of thunderstorms with slight wind circulation
Some organization
•Tropical depression:
some closed isobars and winds 20-34 knots (more organization)
•Tropical storm:
packed isobars, winds 35-64 knots
•Hurricane:
when winds above 64 knots (74 mph)
Atlantic Hurricanes…
• Begin with easterly waves
– Form as the Sahara Desert intensely
warms up in the summertime
– Wind blows to the west at ~ 15 deg. N
– Triggers thunderstorm formation
Hurricane Classification
Recipe for Tropical Cyclone Formation
1. Warm ocean temperatures (T> 80F):
– Typical in subtropical Atlantic/Pacific in
summer/early fall
– N. H. hurricane season: June-November
2. Rotation:
– Thunderstorms must organize to produce
converging air
Coriolis force=0
– Cannot form at equator
3. Weak winds: Strong wind can disrupt organization
Hurricane movement
• Steered by prevailing winds
– Easterly winds of tropical latitudes
– Westerly winds of middle latitudes
• Middle latitude high/low pressure systems
can cause quick changes in hurricane
position.
• Often challenging to predict!!
Hurricane George
Evolution
• Tropical
cyclones are
driven by the
warm ocean
temperature
• Thus, they will
weaken and
eventually die
when they move
over
–
–
Evolution
• Tropical
cyclones are
driven by the
warm ocean
temperature
• Thus, they will
weaken and
eventually die
when they move
over
– land or
– colder waters.
Formation
12%
12%
30%
7%
12%
15%
12%
Hurricane Structure
• Eye of hurricane
– Extremely low pressure,
– weak winds and generally clear skies
• Eye wall
– Intense thunderstorms
– Strongest winds, heaviest precip
• Spiral rain bands
– Areas corresponding to upward motion and heavy rain
– Alternating with rain free areas
Why is eyewall the most
intense part of the Hurricane?
• Law of Conservation of angular
momentum
– Wind speed (v) X distance from axis of
rotation (r) must remain constant
• Close to eye --> r is small --> v must be large
• Far from eye --> r is large --> v is small
Hurricane Destruction
• Winds can have speeds over 100mph.
– Extensive damage to buildings and vegetation.
• Storm surge:
abnormal rise in ocean level responsible for most
damage via flooding
• Hurricane may spawn tornadoes.