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NATS 101
Lecture 30
Hurricanes
Supplemental References for
Today’s Lecture
Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd
Ed. 505 pp. Prentice Hall. (ISBN 0-13-027394-5)
Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp.
McGraw-Hill. (ISBN 0-697-21711-6)
Types of Tropical Cyclones
Cyclone Type
Tropical Depression
Tropical Storm
Hurricane/Typhoon
Winds
25-39 mph
40-74 mph
 75 mph
Most Depressions do not develop into Storms
Majority of Storms reach Hurricane status
Some Hurricane Extremes
Lowest Central Pressure
Pressure
Pacific: Typhoon Tip 1979
870 mb
Atlantic: Hurricane Gilbert 1988
888 mb
Costliest Hurricanes
Cost-Loss
Hurricane Andrew 1992
$25 billion
Hurricane Katrina 2005
 $200 billion?
Bangladesh Cyclone 1970
300,000 dead
Andrew 1992 Time Sequence
2005 Atlantic Hurricanes NASA
Note cooler water in wake of
Dennis, Emily and Katria
Link to Older NASA Satellite Animations
U.S. Hurricane Deaths and Costs
Williams, The Weather Book
Hurricane Lecture Overview
• What are the primary differences between
hurricanes and extratropical cyclones?
• When and where do hurricanes form?
• How do hurricanes intensify?
• What is the structure of a hurricane?
• What kind damage do hurricanes inflict?
• When and where do hurricanes dissipate?
Differences Between Tropical
and Extratropical Storms
Williams, The Weather Book
No Fronts
Warm at Storm Center Aloft
Strongest Winds near Surface
Forms over Tropical Oceans
Diameter of 200-500 miles
Energy Source: Energy Fluxes
from Warm Ocean
Strong Fronts
Cold at Storm Center Aloft
Strongest Winds Aloft
Forms outside Tropics
Diameter of 500-1000 miles
Energy Source: Horizontal
Temperature Contrast
Where Hurricanes Form?
Hurricanes
Hurricanes
Tropical
Cyclones
Hot Bed!
Typhoons
Williams, The Weather Book
• Hurricanes go by different names in different regions of the world.
• Form over warm tropical waters, equatorward of 20 latitude…
Not on equator (poleward of 5 ) b/c non-zero Coriolis is needed.
• Occur most frequently over Western North Pacific Ocean.
Atlantic Hurricane Frequency
Danielson et al. Fig. 13.2
Occur in Warm Season
Maximum Likelihood
when Sea Surface
Temperatures are
Warmest-September
Average of ~6 Per Year
Large Yearly Variability
Fewer in El Nino Years
More in La Nina Years
Atlantic Hurricane Tracks
Danielson et al. Fig. 13.12
• Atlantic hurricanes tend
to form in the Middle
Tropical Atlantic Ocean
and Caribbean Sea
• They usually propagate
westward before turning
northward
• They dissipate rapidly
over land
Hurricane Steering
Large-scale flow controls
where hurricanes go.
Williams, The Weather Book
Hurricane Necessary Ingredients
Williams, The Weather Book
Warm Water with T  82oF
Deep Warmth > 200 ft
Converging Surface Winds
Seedling Low Required
Conditionally Unstable Air
Supports Deep Convection
Widespread, Deep Humid Air
Supplies More Latent Heat
Weak Vertical Wind Shear
Shear Shreds Storm Apart
Diverging Winds Aloft
Where do Seedling Vortices Come?
Lots of Places and Ways
Remnant mid-lat circulation
Remnant MCC circulation
Danielson et al. Fig. 13.14
Vortices along ITCZ
Easterly Waves
3D Flow within Hurricanes
Winds aloft spiral outward clockwise
Eyewall winds spiral
upward c’clockwise
Surface winds spiral
inward c’clockwise
Winds inside eye
spiral downward
clockwise
Aguado and Burt, Fig. 12-3
Thermal Structure of Hurricane
Aguado and Burt, Fig. 12-4
Radar of Andrew’s Landfall
Most intense rainfall is
along the eyewall.
Fastest surface winds
are along the eyewall.
Region inside of eye is
dry with light winds
Rita TRMM "Hot Towers"
Rita TRMM Rain Rate
Danielson et al. Fig. 13.25
Floyd hourly rain loop from RSMAS
Eye of Hurricane Luis 1995
Luis Visible Eye Animation
Asymmetry of Hurricane Winds
20 kts
Region of
Maximum
Storm Surge
100 kts
80 kts
80 kts
60 kts
Aguado and Burt, Fig. 12-10
Hurricane Intensity Scale
(> 980 mb)
(965-980 mb)
(945-964 mb)
(920-944 mb)
(< 920 mb)
Williams, The Weather Book
Primary Hurricane Hazards
• Wind Damage
Large-Scale Hurricane Circulation Itself
Embedded Tornadoes
• Flooding
Heavy Rains Far Inland, 5”-10” Common
Storm Surge along Shoreline
Hurricanes Spawn Tornadoes
• Tornadoes embedded
within an overland
hurricane tend to be
weak (category F1-F2)
• But they are embedded
within an environment
with 65+ kt winds.
• Causes hurricane wind
damage to be localized.
Aguado and Burt, Fig. 12-11
Inland Flooding-Agnes 1972
• Even weak hurricanes can be catastrophic,
hundreds of miles inland.
• Agnes 1972, category 1 storm for a few hours.
• Agnes merged with a slow-moving ET cyclone.
• Up to 15” of rain in 24 h fell over Pennsylvania.
• Previous flood records exceeded by 6 ft.
• Damage > $10B in inflation adjusted dollars.
• Costliest U.S. storm prior to Andrew and Katrina.
Storm Surge I
Williams, The Weather Book
• Low atmosphere pressure raises a mound of water inside eye.
• Water rises about 1 cm for every 1 mb decrease in pressure.
• Inward spiraling winds push more water toward hurricane eye.
• Deep hurricanes only raise about 1 meter of water over deep ocean.
Water can sink downward and flow away from the surface.
Storm Surge II
Williams, The Weather Book
• In shallow water near land, water can not flow away under surface.
• But winds continue to push water inward towards storm’s center.
• Winds along hurricane’s right flank also push water against shore.
• Water piles up along shoreline and rushes inland. The big effect!
• Effect is worse where ocean floor slopes gently - Gulf of Mexico!
Link to COMET Surge Animation
Storm Surge III
Williams, The Weather Book
• If hurricane hits at high tide, the two effects superimpose.
• A 2 ft tide plus a 10 ft surge rises water 12 ft above mean sea level.
• Penetration of storm-whipped waves inland worsens damage.
• Waves cause far more destruction than the high water alone.
Winds and Storm Surge
Danielson
et al.RSMAS
Fig. 13.20
Floyd wave height forecast
from
Surge Damage
• Richelieu Apartments before and after landfall of Camille 1969.
• Camille was a Category 5 hurricane.
• Sustained winds > 180 mph!
• Storm surge was 24 feet along the coast!
• Many tired citizens took refuge in apartments.
Sadly, several died.
http://en.wikipedia.org/
Hurricane Decay
Andrew Central Pressure
Danielson et al. Fig. 13.26
Hurricanes weaken when they make landfall (or go over cool water).
Intense surface energy fluxes are cut off and friction increases.
Summary: Hurricanes
• What are differences between hurricanes and
extratropical cyclones?
Many significant ones! See earlier slide.
• Where and when do hurricanes form?
5-20 latitude over oceans during warm season
• How do hurricanes intensify?
Energy source is surface energy fluxes from the
underlying warm ocean
Summary: Hurricanes
• What is the structure of a hurricane?
Eyewall - strongest winds, heaviest rain
Eye - dry with light winds
• What kind damage do hurricanes inflict?
Can be catastrophic due to high winds,
torrential rains, and coastal storm surges
• When and where do hurricanes dissipate?
At landfall or when they go over cold water
Assignment for Next Lecture
• Topic – Air Pollution
• Reading - Ahrens, pg 317-324, 327-340
• Problems - 12.1, 12.5, 12.14, 12.15, 12.19, 12.23