Download 3. Hurricanes/Typhoons/severe form of Cyclones

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13. Severe Weather
3. Hurricanes/Typhoons/severe form of Cyclones
A&B: Ch. 12
Formation
o between 5- 20° (N/S) over oceans
ƒ below 5° Coriolis effect to weak for rotation
ƒ polewards 20°: ocean surface to cool
(needs TSS > 27 °C)
Storms Categorized by wind speed
Worldwide
Tropical
<61 km h-1
Lots! •
depression
Tropical
61-115 km h-1 80-100 •
storm
Hurricane
> 115 km h-1
~40
ß
increasing
wind speed
decreasing
number
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Characterisitcs of a Hurricane
o Rotary circulation
o 100 -1500 km across - average 600 km
o eye wall:
greatest winds
heaviest rains
o eye
o precipitation ceases, winds subsides
o air descends gradually - heats up by
compression
o warmest part of the storm
Hurricane cross section:
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Hurricane temperatures
Rotation Physics:
Principle of Conservation of Angular Momentum
V × R ≈ constant
V
V
R
R
V
velocity of an object
around center of rotation
radius (distance from the
center axis)
~ 1/R
“The closer to the center,
the faster the wind”
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o As air parcel moves towards the center of the storm
the rotational velocity must increase (ice-skater effect)
o Moist air flowing faster & faster into the center of the
storm forced to rise - CONVERGENCE
o Rotating around the eye wall
Distribution of Hurricane Wind Speeds
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Hurricanes spawn tornadoes
Influences on Hurricanes Development and Fate
o In center: centrifugal force very strong ⇒ divergence
⇒ air sinks ⇒ clear ⇒ “the eye”
o Near eye-wall: air rises ⇒ cools and releases energy latent heat of vaporization
o Source of energy - latent heat of vaporization. Needs:
o Ocean water > 27 °C (~81°F)
⇒ Ocean temperatures influence
o hurricane season (length, activity)
o Locations of hurricanes
o water depth influences heating (⇒ shallow Caribbean)
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Energy for hurricane dissipates:
(i)
when it moves over land
ƒ Lose source of moisture (latent heat!)
ƒ Rougher surface
⇒ reduces wind speed
⇒ causes air to move more directly into
center of the storm - reduces the pressure
difference
(ii) when it moves further N or S on oceans
ƒ Cooler T
(iii) reach a location where the large scale flow aloft is
unfavorable (counteracting)
Path of hurricanes
o Strongly influenced by
o Hadley cell - trade winds and westerlies
o Warm ocean currents
Atlantic hurricanes
o ~10 named storms a year
o ~ 6 hurricanes a year
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Origin in easterly waves (trade-wind zone):
o Rainfall western Sahel and Gulf of Guinea
o Strength of upper level winds in Atlantic
o Strength of upwelling (i.e. pool warm water)
Hurricane Paths:
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Fate of a Hurricane:
Hurricane Camille
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Damage
o High wind speeds
o Heavy rain/ flooding
o Storm surge: Low pressure system ⇒ water level
higher (usually greatest cause of damage!)
Detecting and tracking
o Flights
o Satellites
o Numerical models
o On ground instrument packages
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