Download Hurricanes and Tropical Storms

Hurricanes and Tropical
Storms
Naming Convention
Hurricanes: extreme tropical storms over Atlantic and Eastern
Pacific Oceans
Typhoons: extreme tropical storms over western Pacific Ocean
Cyclones: extreme tropical storms over Indian Ocean and
Australia
Ocean Temperatures and
Hurricanes
Hurricanes depend on large pools of warm water
Annual Hurricane Frequency
There are no hurricanes in the southern Atlantic Ocean!
The strongest hurricanes occur in the western Pacific Ocean
(often referred to as “super-typhoons.”)
Hurricane Characteristics
Definition: Hurricanes have sustained winds of 120
km/hr (74 mph) or greater.
Size: Average diameters are approx. 600 km (350
mi). This is 1/3 the size of a mid-latitude cyclone
(synoptic storm system).
Duration: Days to a week or more.
Strength: Central pressure averages 950 mb but
may be as low as 870 mb (lower the pressure –
stronger the storm).
Power: The power released by a single hurricane
can exceed the annual electricity consumption by
the US and Canada combined.
Hurricane Seasons
Hurricanes obtain their energy from latent heat release in cloud
formation processes.
Hurricanes occur where a deep layer of warm water exists
during the time of the highest SSTs (sea surface temps).
Northern Hemisphere: August through September are most
active months.
Southern Hemisphere: January through March are the most
active months
Latent Heat: Release Me!
Hurricanes draw their power from
warm, extremely humid air found only
over warm oceans.
The key energy source is the latent
heat that's released when water vapor
condenses into cloud droplets and
rain. Tropical storms and hurricanes
grow best in a deep layer of humid air
that supplies plenty of moisture.
When a substance changes phase,
energy must be supplied in order to
overcome the molecular attractions
between the constituent particles. This
energy must be supplied externally,
normally as heat, and does not bring
about a change in temperature.
When water is in the vapor
state, as a gas, the water
molecules are not bonded to
each other. They float around
as single molecules.
When water is in the liquid state,
some of the molecules bond to each
other with hydrogen bonds. The
bonds break and re-form continually.
Latent Heat
We call the energy needed to change
phases latent heat (the word "latent"
means "invisible"). The latent heat is the
energy released or absorbed during a
change of state.
To get the molecule of water vapor to
become liquid again, we have to take the
energy away, that is, we have to cool it
down so that it condenses (condensation is
the change from the vapor state to the liquid
state). When water condenses, it releases
latent heat.
Latent Heat Values
The amount of latent heat involved depends to some extent on the
temperature at which the process is occurring. The figures below
are those normally found in meteorology texts and are for
temperatures found in the atmosphere, such as 0 Celsius (32 F).
Latent heat of condensation (Lc): Refers to the heat gained by
the air when water vapor changes into a liquid. Lc=2500 Joules per
gram (J/g) of water or 600 calories per gram (cal/g) of water.
Latent heat of fusion (Lf): Refers to the heat lost or gained by the
air when liquid water changes to ice or vice versa. Lf=333 Joules
per gram (J/g) of water or 80 calories per gram (cal/g) of water.
Latent heat of sublimation (Ls): Refers to the heat lost or gained
by the air when ice changes to vapor or vice versa. Ls=2833
Joules per gram (J/g) of water or 680 calories per gram (cal/g) of
water.
Latent heat of vaporization (Lv): Refers to the heat lost by the air
when liquid water changes into vapor. This is also commonly
known as the latent heat of evaporation. Lv= -2500 Joules per
gram (J/g) of water or -600 calories per gram (cal/g) of water.
Latent Heat
As water vapor evaporates from the warm ocean
surface, it is forced upward in the convective clouds that
surround the eyewall and rainband regions of a storm.
As the water vapor cools and condenses from a gas
back to a liquid state, it releases latent heat. The release
of latent heat warms the surrounding air, making it lighter
and thus promoting more vigorous cloud development.
The release of latent heat warms the surrounding air,
making it lighter and thus promoting more vigorous cloud
development.
Animation:
http://svs.gsfc.nasa.gov/vis/a000000/a001600/a001605/c
loud.mov
Energy of Latent Heat
Air parcel dew point temperature
(oC)
Approximate amount of water vapor
(g) in an air parcel (kg) at saturation
(by the way, in text books referred to
as the saturation mixing ratio)
Approximate amount of potential
heating due to latent heat release
(calories) if all the water vapor
condenses
0
4
2360
10
8
4720
20
16
9440
30
32
18880
40
64
37760
Condensation releases latent heat. This causes the temperature
of a cloud to be warmer than it otherwise would have been if it did
not release latent heat. Anytime a cloud is warmer than the
surrounding environmental air, it will continue to rise and
develop.
The more moisture a cloud contains, the more potential it has to
release latent heat.
Temperatures in the middle of the rising air in cumulonimbi there may
be as much as 15-20C (28-38F) warmer than air outside the storm. For
Hurricane Rita, observations showed that, at 700 mb (approximately
10000 feet), the interior temperature of Rita was 31C and about 20C
warmer than the outside of the storm at the same elevation.
Hurricane Structure
A central eye is surrounded by large cumulonimbus thunderstorms occupying
the adjacent eyewall.
Weak uplift and low precipitation areas are separated by individual cloud
bands.
Temperature Structure
Hurricanes characterized
by a strong thermally direct
circulation with rising of
warm air near center of
storm and sinking of cooler
air outside.
The “warm core” of the
hurricane serves as a
reservoir of potential
energy which is continually
being converted to kinetic
energy by thermally direct
circulation
Pressure Structure
of Hurricanes
The horizontal
pressure gradient with
altitude decreases
slowly
At about 400 mb, the pressure
inside the storm is approx. that of
outside of the storm.
The upper portions of the storm
are blanketed by a cirrus cloud
cap due to overall low
temperatures.
From surface to
400 mb: cyclonic
circulation.
From 400 mb to
tropopause: anticylonic
circulation.
Hurricanes and Tropical
Storms
Naming Convention
Hurricanes: extreme tropical storms over Atlantic and Eastern
Pacific Oceans
Typhoons: extreme tropical storms over western Pacific Ocean
Cyclones: extreme tropical storms over Indian Ocean and
Australia
Ocean Temperatures and
Hurricanes
Hurricanes depend on large pools of warm water
Annual Hurricane Frequency
There are no hurricanes in the southern Atlantic Ocean!
The strongest hurricanes occur in the western Pacific Ocean
(often referred to as “super-typhoons.”
Hurricane Characteristics
Definition: Hurricanes have sustained winds of 120
km/hr (74 mph) or greater.
Size: Average diameters are approx. 600 km (350
mi). This is 1/3 the size of a mid-latitude cyclone
(synoptic storm system).
Duration: Days to a week or more.
Strength: Central pressure averages 950 mb but
may be as low as 870 mb (lower the pressure –
stronger the storm).
Power: The power released by a single hurricane
can exceed the annual electricity consumption by
the US and Canada combined.
Hurricane Seasons
Hurricanes obtain their energy from latent heat release in cloud
formation processes.
Hurricanes occur where a deep layer of warm water exists
during the time of the highest SSTs (sea surface temps).
Northern Hemisphere: August through September are most
active months.
Southern Hemisphere: January through March are the most
active months
Latent Heat: Release Me!
Hurricanes draw their power from
warm, extremely humid air found only
over warm oceans.
The key energy source is the latent
heat that's released when water vapor
condenses into cloud droplets and
rain. Tropical storms and hurricanes
grow best in a deep layer of humid air
that supplies plenty of moisture.
When a solid substance changes
phase, energy must be supplied in
order to overcome the molecular
attractions between the constituent
particles. This energy must be
supplied externally, normally as heat,
and does not bring about a change in
temperature.
When water is in the vapor
state, as a gas, the water
molecules are not bonded to
each other. They float around
as single molecules.
When water is in the liquid state,
some of the molecules bond to each
other with hydrogen bonds. The
bonds break and re-form continually.
Latent Heat
We call the energy needed to change
phases latent heat (the word "latent"
means "invisible"). The latent heat is the
energy released or absorbed during a
change of state.
To get the molecule of water vapor to
become liquid again, we have to take the
energy away, that is, we have to cool it
down so that it condenses (condensation is
the change from the vapor state to the liquid
state). When water condenses, it releases
latent heat.
Latent Heat Values
The amount of latent heat involved depends to some extent on the
temperature at which the process is occurring. The figures below
are those normally found in meteorology texts and are for
temperatures found in the atmosphere, such as 0 Celsius (32 F).
Latent heat of condensation (Lc): Refers to the heat gained by
the air when water vapor changes into a liquid. Lc=2500 Joules per
gram (J/g) of water or 600 calories per gram (cal/g) of water.
Latent heat of fusion (Lf): Refers to the heat lost or gained by the
air when liquid water changes to ice or vice versa. Lf=333 Joules
per gram (J/g) of water or 80 calories per gram (cal/g) of water.
Latent heat of sublimation (Ls): Refers to the heat lost or gained
by the air when ice changes to vapor or vice versa. Ls=2833
Joules per gram (J/g) of water or 680 calories per gram (cal/g) of
water.
Latent heat of vaporization (Lv): Refers to the heat lost by the air
when liquid water changes into vapor. This is also commonly
known as the latent heat of evaporation. Lv= -2500 Joules per
gram (J/g) of water or -600 calories per gram (cal/g) of water.
Latent Heat
As water vapor evaporates from the warm ocean
surface, it is forced upward in the convective clouds that
surround the eyewall and rainband regions of a storm.
As the water vapor cools and condenses from a gas
back to a liquid state, it releases latent heat. The release
of latent heat warms the surrounding air, making it lighter
and thus promoting more vigorous cloud development.
The release of latent heat warms the surrounding air,
making it lighter and thus promoting more vigorous cloud
development.
Animation:
http://svs.gsfc.nasa.gov/vis/a000000/a001600/a001605/c
loud.mov
Energy of Latent Heat
Air parcel dew point temperature
(oC)
Approximate amount of water vapor
(g) in an air parcel (kg) at saturation
(by the way, in text books referred to
as the saturation mixing ratio)
Approximate amount of potential
heating due to latent heat release
(calories) if all the water vapor
condenses
0
4
2360
10
8
4720
20
16
9440
30
32
18880
40
64
37760
Condensation releases latent heat. This causes the temperature
of a cloud to be warmer than it otherwise would have been if it did
not release latent heat. Anytime a cloud is warmer than the
surrounding environmental air, it will continue to rise and
develop.
The more moisture a cloud contains, the more potential it has to
release latent heat.
Temperatures in the middle of the rising air in cumulonimbi there may
be as much as 15-20C (28-38F) warmer than air outside the storm. For
Hurricane Rita, observations showed that, at 700 mb (approximately
10000 feet), the interior temperature of Rita was 31C and about 20C
warmer than the outside of the storm at the same elevation.
Hurricane Structure
A central eye is surrounded by large cumulonimbus thunderstorms occupying
the adjacent eyewall.
Weak uplift and low precipitation areas are separated by individual cloud
bands.
Temperature Structure
Hurricanes characterized
by a strong thermally direct
circulation with rising of
warm air near center of
storm and sinking of cooler
air outside.
The “warm core” of the
hurricane serves as a
reservoir of potential
energy which is continually
being converted to kinetic
energy by thermally direct
circulation
Pressure Structure
of Hurricanes
The horizontal
pressure gradient with
altitude decreases
slowly
At about 400 mb, the pressure
inside the storm is approx. that of
outside of the storm.
The upper portions of the storm
are blanketed by a cirrus cloud
cap due to overall low
temperatures.
From surface to
400 mb: cyclonic
circulation.
From 400 mb to
tropopause: anticylonic
circulation.
Hurricane Eye and
Eyewall
A shrinking eye
indicates storm
intensification
The hurricane eye is an area of
descending air, relatively clear sky and
light winds: 25 km (15 mi) diameter on
average.
The eyewall is
moves at a speed
of 20 km/hour and
the calm weather
associated with the
eye will last less
than 1 hour
The eyewall is comprised of the strongest winds, the largest
clouds and the heaviest precipitation with rainfall rates as high
as 2500 mm/day (100 inches/day).
Hurricane Formation
Tropical Disturbance: Clusters of small
thunderstorms.
Tropical Depression: When at least 1 closed
isobar is present (organized center of low
pressure).
Tropical Storm: Further intensification to wind
speeds of 60 km/hr (37 mph).
Hurricane: Hurricane status is gained when the
winds reach a sustained 120 km/hr (74 mph).
Tropical Disturbances and Easterly Waves
Some tropical disturbances form from mid-latitude troughs migrating
towards lower latitudes, some form from ITCZ convection, but most
develop from “easterly waves.”
Easterly Waves, or undulations in the trade wind patterns, spawn
hurricanes in the Atlantic.
Only 10% of tropical disturbance become more organized, rotating
storms.
Conditions necessary for Hurricane Formation
Hurricanes only form over deep (several 10s of meters)
water layers with temperatures in excess of 27 degrees
C.
Poleward to about 20 degrees, water temperatures are
usually below this threshold.
Coriolis effect is an important contributor, hurricanes do
not form from equator to 5 degrees.
Need unstable atmosphere: available in western parts of
oceans but not in eastern parts of ocean.
Strong vertical shear must be absent. (both magnitude
and direction).
Vertical shear: changing of wind speed and/or direction
with height of the atmosphere.
Hurricane Movement
Tropical depressions and
disturbances are largely
regulated by trade wind
flow – move westward.
Tropical storms &
hurricanes: upper-level
winds and ocean
temperatures gain
importance.
Fully developed
hurricanes will move
poleward (stronger upper
winds will steer the
hurricanes northward).
Hurricane Dissipation
After making landfall, a hurricane may die
completely within a couple of days (no longer
has moisture to feed into storm).
Even as storm weakens, it can still have large
effects on land (especially flooding).
Hurricanes will also dissipate over cooler waters
or if they encounter strong vertical shear.
Animation:
http://svs.gsfc.nasa.gov/vis/a000000/a001600/a
001605/coldwater.mov
Hurricane Damage
Heavy rainfall
Strong winds
Tornadoes (generally F0-F2)
Storm surge – rise in water level induced
by the hurricane.
Tornado formation in hurricanes
Most hurricanes
contain clusters of
tornadoes.
Most tornadoes occur
in right front quadrant
of the storm.
It appears that slowing
of wind by friction at
landfall contributes to
the formation of these
tornadoes.
Storm Surges
Process 1: Hurricane winds drag surface
waters forward and pile water near coasts.
Process 2: Lower atmospheric pressure
raises sea level (for every 1mb pressure
decrease, sea level raises 1 cm)
Storm surges raise sea level by a 1-2 m
for most hurricanes, as much as 7 m (a
real problem when coastal locations are at
or below sea-level)
Hurricane Wind
Structure
Winds and surge
are typically
strongest at right
front quadrant of
storm where wind
speeds combine
with speed of
storm’s movement
to create the
highest area of
potential impact.
Trends for Atlantic
Mid 1990s-now: A significant increase in
the numbers of hurricanes and intense
hurricanes making landfall in US.
1970s-mid 1990s: Lower than normal
incidence of Atlantic Hurricanes.
Debate: Is the recent increase in
hurricanes part of a natural cycle, global
climate change or a combination?
Forecasting
National Hurricane Center responsible for
Atlantic and eastern/central Pacific.
Data gathered through satellite, surface
observations and aircraft using
dropsondes (dropping of instruments
through hurricane)
Computer models assist in predictions.
Hurricane Watch and Warning
Hurricane Watch: if an approaching
hurricane is expected to make landfall
within 24 hours.
Hurricane Warning: if the time frame is
less, then it’s a warning.
Naming of Hurricanes
When a tropical disturbance reaches a
tropical depression, the storm will be given
a name.
The name comes from an “A-W” list given
by World Meteorological Organization
(WMO).
The names of hurricanes with devastating
effects are retired.
Hurricane Intensity Scale
Saffir-Simpson Scale
Five Categories: the larger numbers indicate lower central
pressures, greater winds and stronger storm surges