Transcript Hurricanes

Hurricanes
• Sustained winds at least 74 mph
• Circulation (c-clkws in N Hem; clkws in S
Hem)
synonyms
• Hurricane : north Atlantic, eastern North
Pacific
• Typhoon: western North Pacific
• Baguio : Philippines
• Cyclone : India, Australia
Typhoon Angela
Progression of tropical storms
leads to hurricane:
1. tropical disturbances
disorganized groups of thunderstorms; some
spin: 90% fizzle out
2. tropical depressions : wind speeds 23 mph
3. tropical storm : wind speeds 39 mph (named)
4. hurricane : wind speed 74 mph
Begin as “easterly waves”
Most Atlantic hurricanes begin in Africa
Pacific High
Azores Bermuda High
Monsoonal Low
NE Trades
ITCZ
Turned SW by Coriolis deflection
equator
ocean
Africa
SE Trades
• At this point, storm is a “tropical
disturbance”
• If storm moves off African continent
towards Atlantic, becomes “easterly wave”
(carried west by equatorial easterlies)
• Of approx. 100 each year, 6 become hurricanes
Requirements:
• Warm ocean surface
– (>78°F)
– Warm to depth of 200 ft
– Vast
– Basin or current
• Surface convergence
• Coriolis deflection
– Do not form at equator (5 – 20 °N,S)
Hurricane source regions (red arrows) :
Anatomy of a hurricane
• Spiral bands of
thunderstorms organized
around low pressure
center
• Surface winds converge
towards central LOW
– Warm, moist air
• Air aloft diverges around
a central HIGH
anatomy of a hurricane
•
eyewall
– Tallest clouds
– Most rain
• 100 in / day
– Highest wind speed
•
Wind speeds strongest
here due to conservation of
angular momentum
•
eye
– air is sinking
– Clear skies
– Warmer temperatures
– 15 miles diameter; 1
hour
• Source of fuel: latent heat of condensation
Process of hurricane development:
• Hurricane starts as tropical storm,
clustered thunderstorms spinning around a
central low:
– Heavy rain
– High winds
– Release of latent heat (condensation)
• Increases temperature of cluster
• Central pressure drops
• The lower the Low, the higher the winds
speeds towards the center
• Bringing warm flow of warm, moist air
• More latent heat is released
• Pressure continues to drop
Edge of storm:
Outflow sinks and
warms
• Converging moist air and resulting release
of latent heat of condensation fuels the
process
• Rises, condenses in eyewall
• Coriolis deflection makes it spin
– Cclkws surface; clkws aloft (N. Hem)
Central low pressure
Average: 950 mb
Typhoon Tip (1979): 870 mb
Hurricane Gilbert (1988): 888 mb
Hurricane Katrina: 907 mb
IR shows height of clouds in eyewall
SH_ _ !
We’re in the middle
of a ………..
…hurricane???
At top of storm:
Ice crystals spiral out of storm
• Create a blanket of cirrostratus cloud
cover
– Obscures view from above of spiral bands of
clouds
– On satellite images, hurricanes look like they
have uniform thickness and density but they
are really strongly banded
Andrew
Katrina
Hurricane Destruction:
• High Winds (exceed 74 mph)
• High rainfall (can be as great as several
meters per day)
flooding
• Storm surge
• May contain clusters of short-lived
tornadoes
Storm surge
• Sea surface rises and high waves push
onshore
• 2 processes:
• Piling up of water as heavy winds drag surface
water forward to land, causing sea surface to rise
• Low pressure in storm center causes water to rise
– For every 1 mb drop in pressure, water level rises 1 cm
• Storm surge usually increases sea surface
by 3 – 6 feet, but can be extreme
Hurricane Camille: storm surge 25 feet
1900 Galveston Hurricane: storm surge 14.5 ft.
8000 drowned (book: Isaac’s Storm)
1970 Bay of Bengal / Bangladesh :
storm surge 40 ft.
300,000 – 500,000 fatalities
Zones of high storm surge and
wind speed:
• Right-hand side of storm (relative to
direction it is moving) in front (“forward
right flank”)
– Additive effect of wind speed and storm speed
Hurricane Andrew
Andrew’s destruction
August 1992
– $ 26.5 billion
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164 mph peak gust
26 deaths
17’ storm surge
1 tornado
922 mb central pressure
Began as tropical wave off west coast Africa,
August 14, became Tropical Storm Andrew on
Aug 17, became hurricane on Aug. 22, Category
4 Aug 23, blasted Florida Aug. 24
Floyd’s destructiveness
September 1999
• 155 mph sustained winds; gusts 185 mph
• 921 mb
• One of the largest peacetime evacuations
in US
• 52 deaths
• rain : 2-3 inches / hour
• $1 billion
Gilbert
Gilbert
Saffir-Simpson Scale
CATEGORY DAMAGE
1
2
3
4
5
minimal
moderate
extensive
extreme
catastrophic
WINDS
74 -95mph
96 -110
111-130
131 -155
> 155
STORM SURGE
4 -5 ft.
6 -8 ft.
9 -12 ft
13 -18 ft
> 18 ft
Naming hurricanes
• WWII named by Air Corps and Navy
meteorologists after wives and girlfriends
• Gradually added men’s names
• North Pacific basin after 2000, given Asian
names, not necessarily personal names
(flowers, birds, etc)
• Hurricane watch: landfall > 24 hours
• Hurricane warning: landfall < 24 hours
•
Arlene
Bret
Cindy
Dennis
Emily
Franklin
Gert
Harvey
Irene
TD #10
José
Katrina
Lee
Maria
Nate
Ophelia
Philippe
Rita
TD #19
Stan
Tammy
STD #22
Vince
Wilma
Alpha
Beta
Gamma
Delta