The Extratropical Cyclone and Frontal Analysis

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The Extratropical Cyclone and Frontal Analysis
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Upper Level Flow Outside the Tropics

• The relatively small amounts of solar radiation
  absorbed in the higher latitudes creates a
  typical pattern of colder temperatures near the
  poles and warmer temperatures at lower latitudes.
• This thermal pattern creates forces that produce
  prevailing westerly winds aloft.

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Upper Level Flow Outside the Tropics (Cont.)
N
Cold air low 500 hPa (mb) heights
E
5200 m
Upper Level
5400 m
Coriolis Effect
Gravita-tional Acceler-ation (acts like pressure
gradient force)
5600 m
Westerly Winds
5800 m
warm air greater 500 hPa (mb) heights
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Upper Level Flow Outside the Tropics (Cont.)

• Upper level westerly winds flow around the pole
  in a wavelike pattern called the circumpolar
  vortex.
• Each wave consists of a ridge of greater heights
  and a trough of lower heights.

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Circumpolar Vortex
NP
Equator
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Circumpolar Vortex
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Ridges and Troughs
N
E
Ridge
Cold air aloft
Warm air aloft
Trough
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Longwaves

• Longwaves have wavelengths of thousands of
  kilometers. They generally move slowly from west
  to east, but they may become stationary or
  retrogress slowly from east to west. They
  represent the large scale, global flow.

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Longwaves (Cont.)

• Longwaves influence the locations of large
  regions of warm versus cold temperatures, wet
  versus dry conditions, the position of the jet
  streams and storm tracks.

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Shortwaves

• Shortwaves tend to have wavelengths less than
  3000 km. They move rapidly from west to east
  around the longwaves. Shortwaves represent
  smaller pools of cold and warm air aloft. The
  pools of cold air aloft may lead to instability,
  outbreaks of rain or snow showers, or in extreme
  cases thunderstorms.

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Shortwaves (Cont.)
shortwave ridge
shortwave trough
warm air aloft
cold air aloft
STABLE
UNSTABLE
warm air in lower levels
warm air in lower levels
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Ridges and Troughs (Cont.)

• Warm air tends to rise ahead of the trough axis,
  while cold air tends to sink behind the trough
  axis.
• The rising warmer air may generate clouds and
  precipitation ahead of the trough axis, while the
  sinking colder air may generate clearing skies
  behind the trough axis.

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Ridges and Troughs (Cont.)
N
E
Rising air Clouds Precip.
Ridge
Cold air aloft
Sinking air Clearing skies
Warm air aloft
Trough
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Surface Pressure Changes

• Surface pressure decreases when there is net
  divergence in a column of air above a location.
• Surface pressure increases when there is net
  convergence in a column of air above a location.

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Surface Pressure Changes (Cont.)
Net Convergence
Net Divergence
Surface Pressure decreases
Surface Pressure increases
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Confluence and Difluence

• Confluence means that the wind directions are
  blowing toward each other at an angle.
• Difluence means that the wind directions are
  blowing away from each other at an angle.

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Confluence and Difluence (Cont.)
N
E
Confluence
Difluence
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Convergence and Divergence

• Convergence means that there is more mass (i.e.
  air) entering a region than is leaving that
  region.
• Divergence means that there is more mass (i.e.
  air) leaving a region than is entering that
  region.

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Convergence and Divergence (Cont.)
N
E
20 m s-1
10 m s-1
10 m s-1
20 m s-1
Convergence
Divergence
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Jet Streams

• Jet streams are bands of winds where the air is
  moving faster than the surrounding air.
• Jet streaks are local wind maxima located within
  jet streams.

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Jet Streams (Cont.)

• Two major types of jet streams are normally
  observed in the upper troposphere.
• polar front jet stream
• subtropical jet stream

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The Polar Front

• The polar front is the name given to the boundary
  between warmer, tropical air and colder, polar
  air.

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The Polar Front (Cont.)
N
E
Colder, drier polar air
Polar
Front
Warmer, moister tropical air
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Polar Front Jet Stream

• The density differences between the colder, polar
  air and the warmer tropical air create a large
  pressure gradient force that accelerates the air
  near the front.
• In the upper troposphere where frictional
  acceleration is small the pressure gradient force
  generates the high winds speeds of the polar
  front jet stream.

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Polar Front Jet Stream (Cont.)
tropopause
X
Polar Front Jet Stream
Colder polar air
Warmer tropical air
Polar Front
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Subtropical Jet Stream

• The subtropical jet stream is typically found on
  the poleward side of the subtropical high
  pressure systems.
• Large areas of convection (i.e. thunderstorms)
  over the tropical oceans sometimes produce large
  masses of rising air.

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Subtropical Jet Stream (Cont.)

• As the rising air diverges in the upper
  troposphere it generates the higher wind speed
  observed in the subtropical jet stream.

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Subtropical Jet Stream
Strong upper level westerly winds Subtropical
Jet
When the air reaches the upper troposphere it is
forced to move horizontally. The large mass is
funneled into a small vertical distance, which
creates high wind speeds and the Subtropical Jet
Stream.
Convection generates large masses of rising air
over a large area.
Tropical Ocean
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Vorticity

• Vorticity is defined as tendency for air to
  rotate cyclonically (i.e. counterclockwise in the
  northern hemisphere.)
• Vorticity may be generated by changes in the
  direction of the flow called curvature vorticity
  and by changes in the wind speed called shear
  vorticity.

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Vorticity (Cont.)

• Relative vorticity represents the tendency for
  air to rotate cyclonically relative to the
  surface of the Earth.
• Relative vorticity shear vorticity
• curvature vorticity

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Vorticity (Cont.)

• Absolute vorticity represents the tendency for
  air to rotate cyclonically with respect to an
  absolute or inertial frame of reference.
• Absolute vorticity relative vorticity
• vorticity
  associated with
• the rotation of
  the Earth

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Curvature Vorticity
N
Counterclockwise turning produces positive
vorticity in the upper level trough.
E
Clockwise turning produces negative vorticity in
the upper level ridge.
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Shear Vorticity
N
Deacrease in wind speed toward north generates
positive vorticity.
E
20 m s-1
40 m s-1
60 m s-1
40 m s-1
20 m s-1
Increase in wind speed toward the north generates
negative vorticity.
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Combined Curvature and Shear Vorticity
N
Shear vorticity gt 0 Curvature vorticity lt 0
E
Upper Level Trough
Shear vorticity gt 0 Curvature vorticity gt 0
Jet Stream (highest wind speed)
Shear vorticity lt 0 Curvature vorticity lt 0
Upper Level Ridge
Shear vorticity lt 0 Curvature vorticity gt 0
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Vorticity (Cont.)

• The combination of shear and curvature generally
  produces the largest magnitudes of positive
  vorticity to the north of the wind speed maximum
  in the trough.
• The vorticity minimum (i.e. most negative
  vortiticy) is typically found to the south of the
  wind speed maximum in the ridge.

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Vorticity and Surface Pressure Systems

• Since large-scale surface low pressure systems
  rotate counterclockwise in the northern
  hemisphere, they tend to be associated with
  regions of positive vorticity.
• High pressure systems rotate clockwise and are
  associated with regions of negative vorticity.

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Advection

• Advection is defined as the horizontal movement
  of air.
• A variable (e.g. temperature, mixing ratio,
  vorticity) is advected when the wind blows across
  a gradient of that variable.

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Thermal Advection

• Warm advection occurs when the wind blows across
  the gradient of temperature (often represented by
  isotherms) from the higher toward the lower
  temperature.

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Warm Advection
N
Cold air
E
-5C
0C
5C
10C
Warm air
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Thermal Advection (Cont.)

• Cold advection occurs when the wind blows across
  the gradient of temperature from the lower toward
  the higher temperature.

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Cold Advection
N
Cold air
E
-5C
0C
5C
10C
Warm air
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No Advection

• No advection of a variable occurs if the wind
  blows parallel to the gradient (i.e parallel to
  the isolines) of that varable.
• No advection occurs because the winds always
  bring in air with the same magnitude of the
  variable in question.

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No Thermal Advection
N
Cold air
E
-5C
0C
5C
10C
Warm air
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Positive Vorticity Advection

• Positive vorticity advection (PVA) occurs when
  the winds blow across the vorticity gradient from
  the higher toward the low vorticity.
• PVA will tend to increase the vorticity at a
  point if all other factors are negligible.

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Positive Vorticity Advection
N
Negative Vorticity
E
5x10-5 s-1
10x10-5 s-1
15x10-5 s-1
20x10-5 s-1
PositiveVorticity
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Negative Vorticity Advection

• Negative vorticity advection (NVA) will occur
  when the winds blow across the vorticity gradient
  from the lower toward the higher vorticity.
• NVA will tend to decrease the vorticity at a
  point when all other factors are neglgible.

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Negative Vorticity Advection
N
Negative vorticity
E
4x10-5 s-1
8x10-5 s-1
12x10-5 s-1
16x10-5 s-1
Positive vorticity
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Extratropical Cyclone

• The large low pressures systems that are analyzed
  on surface weather maps outside the tropics are
  called extratropical or mid-latitude cyclones.

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Fronts

• Extratropical cyclones are typically associated
  with fronts.
• A front represents the boundary between two
  different air masses.
• An air mass is a large body of air that has
  similar thermal and moisture characteristics in
  the horizontal directions.

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Air Masses

• Air masses form over source regions, which are
  large, relatively flat areas over the Earths
  surface with relatively uniform characteristics.
• Air masses typically have a surface high pressure
  system at their center.

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Source Regions
Air sinks and spreads out from the same
geographic region.
H
The air takes on the characteristics of the
surface.
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Source Regions (Cont.)
N
E
H
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Typical Air Mass Types

• Arctic very cold and very dry
• Continental polar cold and dry
• Maritime polar cool and moist
• Maritime tropical warm and moist
• Continental tropical hot and dry

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Fronts (Cont.)

• Fronts are typically classified according to the
  direction of their movement.
• A warm front indicates that the warm air mass is
  pushing the colder air mass away.

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Warm Front
N
E
Colder air mass
Warmer air mass
Warm front moves north as warmer air pushes out
colder air.
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Fronts (Cont.)

• A cold front indicates that the colder air mass
  is pushing out the warmer air mass.

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Cold Front
N
Cold front moves south as colder air pushes out
warmer air.
E
Colder air mass
Warmer air mass
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Stationary Front

• A stationary front is drawn when the boundary
  between the two air mass is not moving.

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Stationary Front
N
E
Colder air mass
Warmer air mass
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Occluded Front

• During the development of the extratropical
  cyclone, the cold front typically moves faster
  than the warm front. When the cold front catches
  up to the warm front, it is drawn as an occluded
  front.

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Occluded Front
N
E
Advancing cold air mass
Retreating cold air mass