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Tropical Cyclones
Lecture 11
November 18, 2009
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Review from last week
• A severe thunderstorm is defined as a
thunderstorm that produces
- Hail of 1 inch diameter (in central US) or larger and/or
wind gusts 58 mph or greater or a tornado
• Occurs most frequently during the spring
and summer when there are the following
atmospheric conditions:
– Conditionally unstable atmosphere
– Moisture
– Upward vertical motion (“Lifting”)
– Wind shear
Review from last week
• A tornado is defined as “a violently rotating
column of air descending from a
thunderstorm and IN CONTACT with the
ground.” –NWS
•Rising air within the thunderstorm
updraft tilts the rotating air from
horizontal to vertical.
•An area of rotation, 2-6 miles wide,
now extends through much of the
storm.
•Most strong and violent tornadoes
form within this area of strong rotation
Tropical Cyclones
• A tropical cyclone is a low pressure
system that develops over tropical or
subtropical waters (between 30º N and 30º
S)
• Develop in areas without a horizontal
temperature gradient
• Does not have fronts
• Has a warm core center
• Has an organized circulation
Hurricane Katrina- 2005
Tropical Cyclone Names
• Depending upon location, tropical
cyclones have different names around the
world
• In the
– Atlantic/Eastern Pacific Oceans: hurricane
– Western Pacific: typhoon
– Indian Ocean: tropical cyclone
Hurricane
Typhoon
Cyclone
• Do not typically form within 5° latitude of the
equator due to the lack of sufficient Coriolis Force
Tropical Cyclone Names
• Since 1953, Atlantic tropical storms have
been named from lists made by the
National Hurricane Center (NHC)
• In 1979, men’s names were introduced
and they alternate with the women’s
names
• Six lists are used in rotation
Tropical Cyclone Names
• The only time that there is a change in the
list is if a storm is so deadly or costly that
future use of its name would be
inappropriate
– Essentially the storm name is “retired”
– For example, Andrew 1992, Katrina 2005,
Gustav and Ike 2008
• In the event that more than 21 tropical
cyclones occur in a season, additional
storms will take names from the Greek
alphabet
– Alpha, Beta, Gamma, Delta, etc
How Do Tropical Cyclones Develop?
• Several favorable environmental conditions
must be in place before a tropical cyclone
can form:
– Warm ocean waters (at least 80°F)
– Potentially unstable atmosphere favorable to
convection
– Moist air near the middle of the troposphere
– Some initial disturbance such as a thunderstorm
complex, which may slowly develop
– Low values of vertical wind shear between the
surface and upper troposphere
• If these conditions persist for several days, a
tropical cyclone may form
How Do Tropical Cyclones Strengthen?
• Tropical cyclones strengthen and maintain
themselves through latent heat release
• As water vapor rises, it cools and condenses
• Large quantities of latent heat release warm
the air, causing it to be less dense than its
surroundings
• The ideal gas law tells us that the pressure
should then decrease
• The storm must be over warm ocean waters
to supply enough moisture
Stages of Hurricane Development
• However, when these disturbances first
appear, they are called tropical depressions
– Not named yet
• Once the disturbance has developed with
surface wind speeds stronger than 39 mph,
the storm is classified as a tropical storm
• After further strengthening and surface wind
speeds greater than 74 mph, the system is
upgraded to a hurricane
• Atlantic Hurricane season is June 1 –
November 30
Air converges in the center in a cyclonic pattern and diverges at the top.
In the center of the storm, called the “eye”, the storm is mostly cloud-free
http://www.srh.noaa.gov/jetstream//tropics/tc_structure.htm
Tropical Cyclone Structure
• The main parts of a tropical cyclone are
the eye, the eyewall, and the rainbands
• The eye
– Strong rotation of the cyclone causes a
vacuum of sinking air at the center which
suppresses cloud formation
– This creates a pocket of generally clear, calm
conditions in the center
– Typically 20-40 miles across
– Will usually develop when the winds exceed
74 mph but cause of eye formation still not
fully understood
Tropical Cyclone Structure
• The eyewall
– Consists of a ring of thunderstorms that
produce heavy rains
– Usually produces the strongest winds of the
storm
– Changes in the structure of the eyewall and
eye can cause changes in wind speed
– Double eyewalls can form if the eye changes
size
Tropical Cyclone Structure
• The rainbands
– Curved bands of clouds and thunderstorms that
trail away from the eyewall in a sprial fashion
– Capable of producing heavy bursts of rain and
wind, as well as tornadoes
– Sometimes gaps in the spiral bands where no
rain or wind is found
– if one were to travel between the outer edge of a
hurricane to its center, one would progress from
light rain and wind, to dry and weak breeze, then
back to increasingly heavier rainfall and stronger
wind, over and over again
RAINBAND
EYEWALL
EYE
HURRICANE KATRINA
What is storm surge?
• As the surface winds converge toward the
center of the storm, they “pile up” ocean
water
• Also, the decreased surface pressure raises
the sea level slightly
• Strong waves also can increase the water
height
What causes the most damage?
• A hurricane can cause damage by many
different aspects
– Very strong winds can damage structures
– Heavy rainfall can cause flooding
– Storm surge can inundate low-lying areas
– Occasionally, lightning strikes and/or
tornadoes can cause damage
• Storm surge generally causes the most
damage and deaths, particularly in strong
hurricanes
Damage from Hurricane Ivan
2005 Atlantic Hurricane Season
• Was the most active Atlantic hurricane
season in recorded history
• Forecasts fell far short of the actual activity
• Most tropical storms, hurricanes, and
Category 5 hurricanes recorded in one
season
• 27 named tropical storms, 17 became
hurricanes
• 7 major hurricanes
• 4 Category 5 hurricanes – Emily, Katrina,
Rita, Wilma
• Widespread economical effects
Hurricane Katrina - 2005
• Katrina was the 6th strongest hurricane on
record for the Atlantic Ocean
• After entering the Gulf, rapidly intensified
from Cat. 3 to Cat. 5 in 9 hours on August 28
• Lowest pressure as Category 5 storm was
902 mb with max sustained winds of 175 mph
• Weakened slightly before landfall to a
Category 3
• Hurricane force winds extended 120 miles
from the center
Katrina’s Path
Made landfall August
29, 2005 as Cat. 3
Time Lapse of Hurricane Katrina
• http://www.youtube.com/watch?v=_SLXY
RJnYm0&feature=related
Katrina eye
viewed from
NOAA
Hurricane
Hunters
August 28,
2005
Katrina’s Impact
• Largest natural disaster in the history of
the U.S.
• Attributed to over 1830 deaths and 700
missing people, mostly in Louisiana and
Mississippi
• Damages well in excess of $100 billion
• Federal disaster declarations covered
90,000 sq miles (size of United Kingdom)
• 8-16 inches of rain
• Breaching of levees caused 80% of New
Orleans to be flooded