Download Hurricanes

Tropical Cyclones and Hurricanes
Read Chapter 11
Charley
Ivan
Katrina's landfall
The Hurricane
 A low pressure system of
tropical origin that produces
sustained surface winds
>=74mph
 Organized convection
around center
 Tropical cyclone is generic
name given low-pressure
systems forming over warm,
tropical seas (typhoon in
western pacific, super
typhoon if winds >150 mph)
The Hurricane: Birth Regions
 Not over land
 Not high latitudes, but not at the equator
either
(Graphic by Robert Simmon, NASA GSFC)
Active Season last year,
Inactive this year
2006
8
5
2
A Recipe for Hurricane Formation
 1) Sea surface temperature (SST) >80°F
(26.5°C) & deep layer of warm water
 2) Conditional/absolute instability through a
deep layer of troposphere- Important for
thunderstorms!
 3) Moist air in mid-troposphere
 4) Weak vertical wind shear
 5) Genesis region at least 5° away from
equator
 6) Source of low-level cyclonic spin
Ingredient #1: Warm water
 Warm water ensures
high evaporation rates,
thus high low level
dew points…promotes
instability &
convection!
 Tropical cyclones can
rapidly strengthen if
they move over patch
of warm water
Atlantic Hurricane Season Peaks ~
Sept.10 – around when ocean is
warmest
Why is a relatively deep layer of warm
water needed?
 Strong hurricane wind can lead to
upwelling of colder water from below as
high winds at surface mix the ocean water
– Cooler surface waters lead to less
evaporation, more stabilized atmosphere
Ingredient # 2: Conditional Instability- How
convection leads to pressure falls
Animation on CD – let’s take a look
Ingredient #3: Mid-Tropospheric
Moisture
 Dry mid-level air promotes hurricane
decay by leading to precipitation
evaporating => downdrafts => cooler,
more stable air near ground => less
convection
#4) Vertical wind shear separates
low level circulation from tall
thunderstorms
Less wind shear =>
better environment for
hurricane formation
Ingredient #5: Cyclonic vorticity (spin)
 Earth vorticity is increased as one head toward poles,
it’s zero at equator (related to Coriolis force)
 Earth vorticity needed for cluster of t-storms to obtain
cyclonic spin
 Tropical cyclone formation at least 5° from equator
Ingredient #6: Tropical Waves
 Emerge every 3-4 days off
coast of Africa from JuneOctober, about 60/year
 When waves spawn
hurricanes in east-central
Atlantic, storm is termed
“Cape Verde”
storm…Cape Verde
season is from August to
October
Intertropical Convergence Zone
(ITCZ): The source for tropical
waves
Stages of Development I
 Tropical disturbance: disorganized blob of
thunderstorms, often forms within the ITCZ
– If atmosphere and ocean conditions favor
development, we enter a positive feedback loop
 Latent heat release in thunderstorms through
condensation
 Increased pressure aloft
 Upper-level divergence, latent heating &
compressional warming lead to lower surface
pressure
 Increased inflow deflected to right and cyclonic
circulation enhanced, more evaporation into air
=>more fuel for t-storms…
 Animation of this process on CD
…->Tropical Depression
 organized thunderstorms, circular wind flow
around center, wind speed less than 39 mph
– Given number-letter tag (i.e. Tropical
Depression 5-A)
Stages of Development II
 Tropical Storm: sustained wind speeds
from 39-73mph with higher gusts
– Storm gets a name
 Six-year cycle
(http://www.nhc.noaa.gov/aboutnames.shtml)
 Names reused or retired if storm is historic
 If all names used in a season, storms named
with Greek alphabet
– If conditions remain favorable, positive
feedback loop continues, leading to…
Stages of Development:
Hurricane
 Wind speed (sustained)
at least 74 mph
 An eye forms
 Spiral bands
 Conservation
of angular
momentum
 Strength dictated by
SSTs
Strength of tropical cyclone related to
number & intensity of thunderstorms
around eye
Winds fastest near surrounding calm eye
of hurricane…known as eye wall
Relative
Velocity
Strongest winds in
a tropical cyclone
are in the right
front quadrant
How does the eye form?
 As air spirals into center of
storm, outward acting
centrifugal force
increases…eventually,
centrifugal force equals
PGF and air stops
spiraling inward….leads to
increased convergence
and rising air motion
 Diameter ~ 25 mi
 Smaller eye-stronger
hurricane
Loop of Hurricane Katrina showing
cyclonic flow
 http://www.atmo.arizona.edu/products/radar/
kat_lix_rad.gif
Importance of anticyclonic flow aloft
above a hurricane…increased upperlevel divergence
Hurricane Felix
Vertical Wind Profile in a
Hurricane
 Speedy ring of air at
surface in eye wall
expands outward as it
rises due to upperlevel divergence
 Wind speed decreases
with height conserving
angular momentum
Hurricane: Saffir-Simpson
Scale (in need of revision?)
This graphic was created by Lynn A. Dombrowski,
Ed. D.
Storm Surge
 Water, not wind, is by far the biggest killer
(includes both storm surge and fresh water flooding)
 Right front quadrant is most dangerous
(N.Hemisphere)
– Strongest winds (rotation + translation velocities)
– Winds blow on-shore which piles up water
Apartment Complex before Camille
Apartment Complex after Camille
Demise
 Loss of fuel
– Moves into colder water
– Moves over land
 Wind shear
– Tilts the thunderstorms
– Lowest pressure not concentrated, pressure
gradient relaxes and winds weaken
 Increased friction (landfall)
Symbols on Weather Maps
 Tropical Storm Symbol
 Hurricane Symbol
Hurricane Watches/Warnings
 Watch - Issued when there is a possibility
that a part of the coastline could experience
hurricane conditions within 36 hours
 Warning – Issued when hurricane
conditions are expected for a part of a
coastline within 24 hours
Understanding National Hurricane
Center (NHC) Forecasts
 http://www.nhc.noaa.gov/archive/2005/KAT
RINA_graphics.shtml