Transcript Ch12: Tropical Storms and Hurricanes

Where Do the Hurricanes Come From?
Review of last lecture
• Forces acting on a cloud/rain droplet. Terminal velocity.
How does it change with cloud drop radius?
• Growth mechanisms for rain and snow
Summary of Precipitation processes:
Condensation
Collisioncoalescence
Bergeron
Process
Riming/
Aggregation
Rain
Snow
(can change to rain, sleet,
or any other type of
precipitation depending on
underlying atmosphere
Observation of tropical cyclones:
Aircrafts (The hurricane hunters)
Observation of tropical cyclones:
Satellite (1960s-Now)
Genesis and tracks of tropical cyclones
2.5
16
4.4
8.9
5.4
3.4
4.3
– Different names: hurricanes, typhoons, cyclones
– Genesis region generally in the tropics, especially between 5N-15N and
5S-15S.
Necessary environmental conditions for
tropical cyclone formation
1.
2.
3.
4.
5.
6.
SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest
frequency in late summer to early autumn when water is warmest.)
Warm ocean mixed layer is thick enough to supply energy (this is why they
weaken quickly upon landfall)
Unstable atmosphere with a moist lower/middle troposphere (central and
western ocean basins)
Low vertical windshear (Otherwise upward transfer of latent heat disrupted)
Coriolis force (do not form between 5N-5S where Coriolis force is too weak)
Pre-existing low-level rotating circulations (tropical waves and other
disturbances)
Tropical waves often cause tropical cyclone
formation (e.g. African easterly waves,
Madden-Julian Oscillation)
Four stages of tropical cyclone formation
Vortex w/ max
wind >74 mph
Vortex w/ max
wind 39-73 mph
Vortex w/ max
wind <38 mph
Organized convection
>200 km, >24 hours
Hurricane Intensity Scale
Saffir-Simpson scale classifies hurricanes into five categories
based on:
- central pressures (ended in 1990s)
- maximum sustained wind speeds (used now)
Categories 3, 4, 5 are collectively called major hurricanes.
They account for 21% of hurricane landfall in U.S., but
cause 83% of the damage.
Hurricane names
• Named alphabetically
• 6 lists in rotation
• When there is a
hurricane that is too
devastating, the name
will retire and be
replaced by a new
name
2007
Andrea
Barry
Chantal
Dean
Erin
Felix
Gabrielle
Humberto
Ingrid
Jerry
Karen
Lorenzo
Melissa
Noel
Olga
Pablo
Rebekah
Sebastien
Tanya
Van
Wendy
2008
Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gustav
Hanna
Ike
Josephine
Kyle
Laura
Marco
Nana
Omar
Paloma
Rene
Sally
Teddy
Vicky
Wilfred
2009
Ana
Bill
Claudette
Danny
Erika
Fred
Grace
Henri
Ida
Joaquin
Kate
Larry
Mindy
Nicholas
Odette
Peter
Rose
Sam
Teresa
Victor
Wanda
2010
Alex
Bonnie
Colin
Danielle
Earl
Fiona
Gaston
Hermine
Igor
Julia
Karl
Lisa
Matthew
Nicole
Otto
Paula
Richard
Shary
Tomas
Virginie
Walter
2011
Arlene
Bret
Cindy
Don
Emily
Franklin
Gert
Harvey
Irene
Jose
Katia
Lee
Maria
Nate
Ophelia
Philippe
Rina
Sean
Tammy
Vince
Whitney
2012
Alberto
Beryl
Chris
Debby
Ernesto
Florence
Gordon
Helene
Isaac
Joyce
Kirk
Leslie
Michael
Nadine
Oscar
Patty
Rafael
Sandy
Tony
Valerie
William
Movie time!
How do the meteorologists study and
predict hurricanes?
Horizontal Structure of tropical cyclones
• Total diameter: about 600km
• Lifetime: up to a week or more
• Make up: many thunderstorms
arranged in pinwheel formation
• Three components:
1. Central eye - clear skies, light
winds (25 km diameter)
2. Eye wall - strongest winds
(can be 200 mph), max precip. (up
to 2500 mm/day)
3. Spiral rainbands
• Cylonic inflow, anticyclonic
outflow. Outflow creates cirrus
shield that often obscures
pinwheel structure
Vertical Structure: Wind flow
Vertical Structure: Thermal
• Warm core fueled by thermal
energy - core
warmer than
outside at upper
levels
• Minimum surface
pressure occurs in
the region of central
eye
• Maximum winds
and rainfall occur in
the region of eye
wall
Vertical Structure: Feedbacks
Sea surface temperature
1. Convergence-Convection (CISK): positive
2. Surface flux-Convection (WISHE): positive
3. Air-sea interaction: negative
Tropical cyclone Destruction and Fatalities
Destruction caused by:
• Hurricane-scale winds
• Rainfall
• Storm surge (winds blowing coast-ward + lower atmosphere pressure)
• Fine-scale Tornadoes
Destruction most intense on right side of cyclone (wind + storm speed)
Hurricane-spawned tornadoes
• More than half of the landfall
hurricanes produce at least one
tornado
• Hurricane Buelah (1967) produced
141 tornadoes
• Most likely occur in the right-front
quadrant
• Generally less intense than those
that occur in the great plains
Tropical
cyclone
forecast
model
By Tim Marchok of
NOAA/GFDL
Tropical cyclone forecast skill: Track
Tropical cyclone forecast skill: Intensity
Summary
• Tropical cyclone genesis: 6 necessary conditions,
4 stages
• Tropical cyclone tracks
• Tropical cyclone structure: 3 major components,
rotation direction of inflow and outflow, location of
maximum wind and rainfall, 3 feedbacks
• Tropical cyclone destruction: 4 reasons? Which
side has the most intense destruction?
• Tropical cyclone forecast: track and intensity
Currently which skill is better?