Chapter 13
Extratropical Cyclones
Chapter overview:
• Cyclone characteristics
• Cyclone evolution
• Cyclone tracks
• Lee cyclogenesis
• Stacking and tilting: Vertical structure of extratropical cyclones
• Cyclogenesis
o Boundary layer processes
o Diabatic heating
o Upper level processes - Troughs and ridges
o Upper level processes - The jet streak
• Divergence, convergence, cyclogenesis, and cyclolysis
• Overview of a mature extratropical cyclone
Cyclone Characteristics
Extratropical: Refers to
something outside of the tropics
(i.e. in the middle or high
latitudes)
Extratropical cyclone: A cyclone
that occurs in the middle or high
latitudes
Cyclogenesis: Development or intensification of a cyclone.
What causes surface pressure to decrease and cyclogenesis to occur?
Cyclolysis: Weakening or dissipation of a cyclone.
Extratropical cyclone
characteristics:
- pressure
- wind
- size
- temperature distribution
- fronts
- clouds and precipitation
The airflow in a cyclone
occurs in three dimensions.
Ahead of the cold front, in
the warm sector, warm air
moves towards and over
the warm front. This is
referred to as a warm air
conveyor belt.
Behind the cold front, cool,
dry air descends.
Cyclone Evolution
Panel a
One condition that favors cyclogenesis is the presence of a baroclinic zone.
Initially a stationary front is located in the mid-latitudes.
Panel b
An area of lower pressure forms (cyclogensis) along a kink on the polar
front. This is referred to as a frontal wave.
What causes this area of lower pressure to form?
How does the stationary front change once the frontal wave develops?
What causes clouds and precipitation to form at this stage in the cyclone
life cycle?
Panel c
The incipient cyclone typically moves towards the east or northeast and
grows into an open wave in 12 to 24 hours.
How has the pressure of the cyclone changed from panel b to panel c?
How can you tell this from the weather maps shown on the previous page?
How do the winds change in response to the change in cyclone pressure?
What is the spatial distribution of precipitation along the warm and cold
fronts?
Warm sector: The region of warm air between the advancing cold and
warm fronts.
What types of weather occur in the warm sector?
Panel d
As the cyclone continues to move northeastward the pressure continues to
fall, the winds continue to increase, and the wave develops into a mature
cyclone.
How does the size of the warm sector change from panels c to d?
What causes this change in size of the warm sector?
What type of weather occurs northwest of the cyclone center?
Panels e and f
What type of front forms as the cold front catches up to the warm front?
The cyclone is referred to as being occluded at this time.
What temperature air surrounds the
cyclone at this point in time?
How does this differ from earlier periods in
the cyclone life cycle?
The cool, dry air that wraps around the
cyclone at this time is referred to as a dry
tongue.
At this point in time the cyclone begins to weaken (cyclolysis).
Triple point: The point where the occluded front, warm front, and cold front
meet.
Sometimes a new low may form at the triple point. This low is referred to as
a secondary low.
The typical life cycle of an
extratropical cyclone is shown
to the left.
Over what period of time does
the cyclone life cycle described
above / to the left occur?
During its lifetime the cyclone
transports cold air equatorward
and warm air poleward.
Thus cyclones act to reduce the
temperature gradient between
the tropics and the polar
regions.
Cyclone Tracks
Cyclogenesis is favored:
- in areas of strong baroclinicity / along fronts
- east of upper level troughs
- east of mountain ranges (m)
- where cold air moves over warm water (w and kw)
Regions of cyclolysis are shown by G on the map above.
The map below shows preferred locations for cyclogenesis and associated
cyclone tracks over the United States.
Lee Cyclogenesis
What happens to westerly
flow that passes over a
north-south oriented
mountain range?
Lee side low: A low
pressure center that
develops on the downwind
(lee) side of a mountain
range
Lee cyclogenesis:
Development of a cyclone
on the lee side of a
mountain range
Stacking and Tilting: Vertical Structure of Extratropical Cyclones
How does a thermal low (shown in Ch 10) change with height?
Extratropical cyclones are dynamic lows that deepen with height.
In an intensifying
extratropical cyclone the
area of low pressure (or
height) will tilt westward
with increasing height.
What causes this
westward tilt with height?
Where is cold air located relative to the surface cyclone and why is it found
in this location?
What is the pressure at the surface (aloft) in this cold column of air?
Cyclogenesis
Cyclogenesis is associated with:
- decreasing surface pressure
- increased rotation (vorticity)
- upward motion
Net divergence in a column of the atmosphere will cause the surface
pressure to decrease and cyclogenesis to occur.
What processes result in divergence
or convergence in a column of the
atmosphere?
- boundary layer effects
- diabatic heating
- upper level processes
• Troughs / ridges
• Jet streaks
Boundary layer effects
Friction in the boundary layer causes wind to spiral in towards a low
pressure center causing convergence and thus weakens the low.
What type of vertical motion does this induce?
What types of weather are associated with this vertical motion?
Diabatic heating
What processes can
cause diabatic heating to
occur?
How will diabatic heating
alter the pressure at the
surface (aloft)?
Upper level processes - Troughs and ridges
Why are upper level heights
low over the polar regions and
high in the tropics?
The waviness of upper level
height contours is a result of
uneven heating of the
atmosphere and the rotation of
the planet.
Wavelength: The distance
between adjacent troughs (or
ridges)
Longwave: A wave with
a wavelength of several
thousand kilometers.
Longwaves are also
known as planetary
waves or Rossby waves.
Shortwave: A small
ripple or disturbance
imbedded within a
longwave
The westward tilt with height of developing extratropical cyclones results in
upper level troughs being located to the west of the surface cyclone.
How does the
gradient wind speed
change as air moves
from an upper level
trough to an upper
level ridge?
Does this result in
upper level
divergence or
convergence?
When an extratropical cyclone becomes occluded the upper level trough is
located above the surface cyclone and the system is referred to as being
vertically stacked.
Once a cyclone becomes vertically stacked how will the location where
upper level divergence occurs relative to the surface cyclone position
change?
Upper level processes - The jet streak
A jet streak is an area of stronger
winds embedded in the jet
stream.
How does the wind speed change
as an air parcel enters a jet
streak?
What impact does this have on
the balance between pressure
gradient and Coriolis forces acting
on the air parcel?
How does the atmosphere respond to this imbalance between the pressure
gradient and Coriolis forces?
In the entrance of the jet
streak an ageostrophic
flow is directed from the
right to the left side of the
jet streak.
In the exit of the jet streak
an ageostrophic flow is
directed from the left to the
right side of the jet streak.
In a straight jet streak divergence occurs in the right entrance and left exit
regions and convergence occurs in the left entrance and right exit regions.
The figure below is a vertical slice through the exit region of the jet streak
shown above.
Where does cyclogenesis
(cyclolysis) occur relative to
jet streak?
Where does rising (sinking)
motion occur relative to the
jet streak?
For a curved jet streak
divergence and convergence
only occur on the left side of the
jet.
Divergence, convergence, cyclogenesis, and cyclolysis
Panel a
A stationary front is located at the surface, with a strong temperature
gradient.
What does this temperature gradient imply about how the wind speed will
change with height?
The jet stream will be located above the stationary front.
Panel b
A shortwave trough moves into the region disturbing the flow.
Where does divergence (convergence) occur relative to this trough?
The surface pressure will decrease ahead of the upper level trough.
What causes sinking (rising) motion in the column at location 1 (2)?
How does the temperature and relative humidity change as air is forced to
rise? What type of weather is associated with this rising motion?
Latent heat released as clouds form will warm the atmosphere and result in
lower surface pressure in the column at location 2.
As low pressure develops at the surface at location 2 what happens to the
surface winds and how does the stationary front change in response to
these winds?
Cold air advection occurs behind the surface low and warm air advection
occurs ahead of the surface low.
How does the intensity of the upper level trough (ridge) change in response
to this pattern of temperature advection?
What impact does this have on the upper level divergence and
convergence?
Panel c
As cold air wraps all the way around the low pressure center at the surface
the cyclone occludes.
Once the cyclone has occluded the upper level trough is located above the
surface cyclone and the system becomes vertically stacked.
Where is upper level divergence occurring relative to the location of the
surface low pressure center at this time?
Where is surface convergence occurring relative to the location of the
surface low pressure center at this time?
How will the intensity of the surface low pressure system change in
response to this upper level divergence and surface convergence?
Once the cyclone becomes occluded cyclolysis will occur.
The figure at the left
summarizes the
processes causing
convergence and
divergence throughout
the life cycle of an
extratropical cyclone.
Cyclogenesis will occur
if there is net divergence
in the column.
Cyclolysis will occur if
there is net convergence
in the column.
Overview of a Mature Extratropical Cyclone