Transcript Lecture 14

Lecture 14
• Tropical disturbances, tropical
depressions, tropical cyclones
• Other tropical systems, remember the
ITCZ
Tropical cyclones
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What are they?
How are they structured?
How and where do they form?
What is the life cycle?
Tropical cyclone tracks
How do they cause destruction?
Observing and forecasting them
Hurricane Mitch approaching Honduras on
Oct. 26, 1998
5-6 Atlantic,
9 in E Pacific,
16 W Pacific
Hurricane Mitch in W Caribbean Oct 26
IR images
Structure
• The eye, clear of clouds (8 km or more)
• eye wall: narrow circular rotating region of
intense thunderstorms
• Spiral rain bands
• Depend on warm water, warm air. Up
moist, down dry.
Cross-section of a hurricane (vertical
dimension exaggerated)
Large weather engines
• Driven by latent heat of water
• Air of tropics dry because of subsidence in the
subtropics and trade winds blowing that air
equatorw. Evaporation may take place and
great amounts of energy are collected that may
be released when the water vapor condenses
into liquid water again.
• Air is driven into the center because there the
pressure is lowest. Tight pressure gradient, wind
speed is increasing toward the eye; even more
energy in latent form
How strong a heat engine?
• “The energy released by condensation in a
single day in an average hurricane is at
least 200 times the entire world’s electrical
energy production capacity.”
• Can calculate the minimum possible
central sea-level pressure for a tropical
cyclone based on SSTs
Minimum possible central pressure for a
perfect tropical cyclone, avg Sept SST
Regions where most hurricanes form
What inhibits tropical cyclone development if
SSTs are above 26.5C?
• Location too close to the equator because
of the decreasing Coriolis effect and
therefore less spin
• Vertical shear. It tends to tear apart the
vertical structure in the storm
Hurricane Celia wind measurements
Winds are strongest on the NE side of the
storm because of its movement
Wind damage from hurricane
What is the life cycle?
• Easterly waves, westward propagating
disturbances (WPDs)
• Tropical depression, tropical cyclone
• hurricane/typhoon/cyclone of category
– 1 33-42 m/s
– 2 43-49 m/s
– 3 50-58 m/s
– 4 59-69 m/s
– 5 above 70 m/s
Decay
Vis satellite picture on 8/14/2001, easterly
wave and a tropical depression are evident
Tropical depression
later became tropical
storm Chantal
Easterly wave (scale 2000-3000 km)
Georges at different stages in its life cycle
(IR satellite images) 18-28 Sept 98
Hurricane Floyd’s track and SST Sept 99
Hurricane Allen ‘80
Category 5 storm that loses its powers when
traveling over land.
After landfall
Tracks
• Determined by the vertically averaged
background flow
• Under influence of subtropical highs
• It is also influenced by the perturbation the
storm creates on the background vorticity
gradient
• Additional ingradients still to be
determined
The names, dates and stages of tropical
storms in the Atlantic in 95
Two very damaging hurricane tracks
Weather Phenomena of the tropics
• Easterly waves
• Tropical disturbances, tropical depressions, tropical
storms
• Tropical upper tropospheric troughs (TUTTs)
• Intertropical convergence zones (ITCZs)
• Kelvin, Mixed-Rossby gravity (MRG), inertio-gravity
waves
• Madden Julian Oscillation (MJO)
African Easterly Waves form due to the instability of the African
Easterly jet. Period of 3-4 days, wavelength of 2000-2500 km
Easterly Waves
How do tropical cyclones form?
Note first baroclinic mode structure, warm ocean, low wind shear, moisture
How do tropical cyclones form ? (necessary but
not sufficient conditions)
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Warm ocean (at least 50m deep)
Potentially unstable atmosphere to moist convection
Relatively moist layers near mid-troposphere (5 km)
At least 500 km from equator (~5 deg latitude)
Pre-existing near surface disturbance (vorticity)
Low vertical wind shear (<10m/s from low to upper)
– Sometimes the pre-existing disturbance is an MCC
– Within envelope of the MJO
– From breakdown of ITCZ
Sfc pressure vs wind speed for tropical systems.
Hurricanes, p<980mb, V>118km/hr
Table 7u-1: Saffir-Simpson scale of hurricane intensity.
Category
Pressure
(mb)
Winds
(mph)
Winds
(kmph)
Surge
(meters)
Tropical
Depression
TD
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< 39
< 62
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Tropical
Storm
TS
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39-73
63-118
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Hurricane
1
> 980
74-95
119-153
1.2-1.5
Hurricane
2
965-980
96-110
154-177
1.6-2.4
Hurricane
3
945-965
111-130
178-209
2.5-3.6
Hurricane
4
920-945
131-155
210-250
3.7-5.4
Hurricane
5
< 920
>155
>250
>5.4
Type
A “typical” tropical cyclone measures about 600 to 800 km
horizontally and extends 15 km vertically
Energy
• Latent energy, gained through evaporation,
drives tropical cyclones. Within the storm, this
energy is efficiently converted to mechanical
energy reinforcing the storm's pressure gradient
and winds. The process depends on unsaturated
air slowly sinking from the upper atmosphere at
the periphery of the storm and gaining moisture
(latent heat) as it moves over the ocean surface
and towards the eye wall.
Hurricane Hortense (1996) note the spiral
structure
How much energy does a hurricane release?
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- Total energy released through cloud/rain formation: An
average hurricane produces 1.5 cm/day (0.6 inches/day) of rain
inside a circle of radius 665 km (360 n.mi) (Gray 1981). (More rain
falls in the inner portion of hurricane around the eyewall, less in the
outer rainbands.) Converting this to a volume of rain gives 2.1 x
10^16 cm3/day. A cubic cm of rain weighs 1 gm. Using the latent
heat of condensation, this amount of rain produced gives
• 5.2 x 10^19 Joules/day or
6.0 x 10^14 Watts.
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This is equivalent to 200 times the world-wide electrical generating
capacity - an incredible amount of energy produced!
Energy
• Notice the 1:400 ratio,
• KE of the hurricane : LH used
• Very inefficient heat engine
Tropical cyclone tracks and geography
• Tropical cyclones are steered by the background flow
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throughout the depth of the troposphere (eestward with
slight poleward component)
Depends on strength of subtropical ridge
May take a more northward component and then recurve
eastward if ridge is weak
However, a tropical cyclone will modify the background
flow and as in so many other cases, there is not clear cut
way of teasing the two apart.
Beta effect
A difficult forecast problem
Multiple storm tracks
Typical areas where hurricanes begin their
path and typical early paths
Locations where hurricanes began their
development (1950-2000)
Color coded tropical storm tracks