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ATS/ESS 452: Synoptic Meteorology
- Cyclone Structure
Observed Structure of Extratropical Systems
DJF
Fig. 6.1 (Meridional Cross Sections of
Temperature and Zonal Winds)
•
Meridional temperature gradient
characteristics
-
Compare winter vs. summer in
NH
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Temperature gradient
•
•
JJA
•
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Larger in NH winter, less
in NH summer
Not as much seasonal
change in SH
•
SH has more water,
heats up faster/cools
off slower…
moderates
temperature
Can see migration of ITCZ
(Thermal Equator)
Sfc temps are warmer in
NH summer than SH
summer
Observed Structure of Extratropical Systems
DJF
Fig. 6.1 (Meridional Cross Sections of
Temperature and Zonal Winds)
•
Zonal wind characteristics
-
-
JJA
Jet stream much stronger in
NH winter than in NH
summer
Smaller seasonal difference
in SH winter vs. summer
Jet core is located just below
the tropopause
Jet core is found at the
latitude where the thermal
gradient (as averaged
through the troposphere) is
greatest:
•
•
•
30-35 degrees N during
winter
40-45 degrees N during
summer
Deduce that the jet
stream has a lot to do
with frontal
boundaries!
H
L
H
L
L
Fig. 6.2 (Longitudinal Distribution of Average Zonal 200 mb Winds for NH Winter)
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Characteristics
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Large differences in zonal 200-mb wind speed with longtiude; strongest winds found near
30° N latitude
Two NH jet maxima in mid latitudes
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East Asian Jet and Eastern North America Jet
Synoptic disturbances frequently develop near these maxima
Semi-permanent low surface pressure is observed near the left exit region of each jet
maximum (Aleutian and Icelandic Lows)
Semi-permanent high pressure is observed near the left entrance region of each jet
maximum (Siberian and NW Canada Highs)
Develop 4-quadrant jet theory for straight flow… correlate with semi-permanent surface
pressure features.
Two NH jet minima in mid-latitudes
Fig. 6.2 (Longitudinal Distribution of Average Zonal 200 mb Winds
for NH Winter)
•
And remember  jet streams are exhibitive of strong thermal
gradients… notice where both average jet maxes are located
• Along eastern coasts of continents were strong warm ocean
currents are riding along continental boundaries
• Strong thermal gradients in those locations!
Fig. 6.3 (Mean 500-mb Height
Contours for January in the NH)
•
Characteristics
•
Large asymmetries in this
pattern are observed from one
longitude to another
 Mean troughs are located
just east of the North
American and Asian
Continents. These mean
troughs correspond to jet
maxima in Fig. 6.2
 Mean ridges are found off
the west coasts of North
America and Asia. These
ridges correspond to wind
minimums in Fig. 6.2.
•
Relating the trough and ridge
positions to surface weather
patterns, we can see that
 Low sfc pressure is located
east (downstream) of 500mb troughs
 High sfc pressure is located
east (downstream) of 500mb ridges
How might this 500-mb wave pattern be linked to the distribution
and influence of ocean and continent positions in the midlatitudes?
•
•
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During winter, the atmosphere cools as it flows from west
to east across cold continents, leading to lower 500-mb
heights on the east coasts
However, the atmosphere warms as it flows west to east
across warmer oceans, leading to higher 500-mb heights
over eastern ocean basins
Winds are stronger near troughs because the height gradient
between tropics and poles is very strong
Fig. 6.4 (Vertical Cross Sections from Charleston SC to Omaha NE across a Jet Stream Wind
Maximum)
a)
Wind and Temperatures (top panel)
•
Characteristics:
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Strong jet core around 300-mb (Polar Jet)
Tropopause Break  Jet core will cause a break or fold in the tropopause
Warm at Charleston  tropopause is high
o
(warm temperature = large thickness)
o
(low temperature = low thickness)
Cool at Omaha  tropopuase is low
Jet sits between warm/cold areas; also notice how the frontal surface slopes to the
west and is connected to the jet
Jet stream is over maximum thermal gradient
Fig. 6.4 (Vertical Cross Sections from Charleston SC to Omaha NE across a Jet Stream Wind Maximum)
b)
Wind and Potential Temperature (bottom panel)
•
Characteristics:

Potential Temp increases with height
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¶q
¶z
This gradient is less in warm air; greater in cool air
is very large in the stratosphere; less in troposphere
Unsaturated parcel will follow the isentropes up/down; so you know vertical motion of
parcel if you know what the winds are doing

Rising air east of front; Subsidence (sinking air) behind front
Stability and Isentropic Spacing
Static Stability and Potential Temp
Very Stable Conditions
• When there is a tight vertical packing of
isentropes (θ quickly increasing with height),
then there exists a very stable atmosphere
• This tight vertical gradient of θ indicates an
inversion
• In Fig. 6.4b, the greatest stability is indicated
at:
– The tropopause and upward into the stratosphere
– On the cold side of the polar front
Conditional Stability
• When there is a weak vertical gradient of
isentropes (θ slowly increases with height), then
there exists a conditionally stable atmosphere
– A dry parcel in this environment is stable
– A moist parcel in this environment is unstable
• Conditional stability is observed:
– In the warm sector ahead of the polar front,
particularly
• Between the surface and 850-mb
• In the upper troposphere between 500-mb and the
tropopause
– In a shallow layer below 850-mb behind the polar
front
• On the synoptic scale, the atmosphere is
always stably stratified (because convection
would immediately stabilize any unstable
regions)
• So, (dθ/dz) > 0
Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)
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Baroclinic vs. Barotropic Atmospheres
o Barotropic Atmosphere
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No intersection of isoheights and isotherms are allowed
This means no temperature advection (isotherms parallel
isoheights)
No thermal wind; no vertical wind shear
Tropics are very near barotropic
o Baroclinic Atmosphere
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Allows vertical wind shear
Has thermal wind
Allows temperature advection
Main region of baroclinic instability is in mid-latitudes
•
Large horizontal temperature gradients!
Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)
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Frontal zones exhibiting strong thermal gradients and vertical
wind shear (i.e. thermal wind) are called “baroclinic zones”
•
This thermal wind can reach excessive values, and “baroclinic
instability” may result. This is normally associated with a jet
streak (i.e. wind maximum)
•
In a baroclinically-unstable environment, a small disturbance with a
developing baroclinic zone (i.e. thermal gradient) will amplify and
develop by drawing energy from the jet stream flow.

Thus, you have a down-scale cascade of energy from the mean flow to the
eddies
 Cyclone = eddy; mean flow = jet stream
 The jet stream controls surface cyclones/anti-cyclones
•
Most mid-latitude synoptic-scale systems are the result of baroclinic
instability (see figure of hyper-baroclinic zone).
•
Figure 6.5 shows 500-mb heights (heavy solid lines), 1000-mb
heights (thin lines) and 1000-500-mb thickness (dashed lines)

Frontal locations are also indicated
Heavy solid
lines = 500-mb
contours
Thin lines =
1000-mb
contours
Dashed lines =
thickness
•
•
What is happening in this figure (6.5)?

Temperature advection is represented by thickness advection

Open 500-mb shortwave trough amplifies, then begins to close off (aloft)

Surface frontal wave cyclone amplifies and becomes more vertically stacked

Surface cyclone becomes occluded
The fact that the developing surface low and upper-level trough are offset from one another guarantees strong thermal advection
Heavy solid
lines = 500-mb
contours
Thin lines =
1000-mb
contours
Dashed lines =
thickness
More detail…
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•
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Panel a)
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The developing mid-latitude cyclone is downstream of the upper-level trough
Sfc isobars don’t quite form advection boxes with the thickness contours
Panel b)
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Advection boxes are now formed (stronger temp advection)
Surface low deepens
Surface cyclone moves closer to upper-level trough
Panel c)
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Occulusion occurs  significant temperature advection
Upper level low forms and cyclone is becoming vertically stacked (i.e. sfc low directly
underneath upper-level low)
Lose westward tilt with height  eventually lose strong thermal advection
• Boundary b/w cold, dry
air and warm, moist air
• Thermal gradient is
already an area of natural
lower pressures, causing
air to blow towards the
boundary
• A counter-clockwise
circulation may develop,
which will act to take
warm air up from the
south and cold air down
from the north
– This is called cyclogenesis
• Mass convergence in the center of
developing circulation, causing the air
to want to rise
• IF the upper-levels are favorable, then
we should have a region of
divergence aloft, above the
developing low pressure center.
– This will help pull the air that is
converging at the surface upward and
continue to develop the low pressure
area
– If the upper-levels are unfavorable,
then the cyclone will not grow and
the mass convergence at the surface
will just pile up and fill in the low,
causing it to decay
• If the upper levels are favorable, the
mid-latitude cyclone will continue to
develop, via increasing temperature
advection
– This will act to strengthen the
temperature gradient, and thus
strengthening the upper-levels (i.e.,
divergence), and deepening the low
• As the cyclone reaches
maturity, the central
pressure will be at its
lowest and occlusion will
begin
– Cold front catches up to
warm front
• Once the system is fully
occluded (warm air is
above cold air),
temperature advection
weakens, and mass
convergence acts to fill in
the low and the system
decays
•
The west-ward tilt with height observed in developing
baroclinic systems means that:
 The surface trough is overlain by an upper-level ridge
(between 200 and 300 mb)
 The surface ridge is overlain by an upper-level trough
(between 200 and 300 mb)
•
See Handout
Fig. 6.6 (West-to-East Cross
Section through a Developing
Baroclinic Wave)
•
This figure shows the
westward vertical tilt of
pressure troughs and
ridges with height

This westward tilt of
troughs and ridges is
necessary in order for
the mean jet stream
flow to give up
potential and kinetic
energy to the
developing baroclinic
system
**Westward tilt allows jet
stream to give up energy
to eddies at the sfc**
Healthy Westward Tilt of Pressure Systems with
Height
H
L
Z
H
W
L
E
Healthy Westward Tilt of Pressure Systems with
Height
•
Note that the thermal ridges and troughs depicted in Fig. 6.6
tilt eastward with height and are out of phase with the
pressure ridges and troughs. This means that:
 Baroclinic ridges (highs) are cold at the surface and warm
aloft
 Baroclinic troughs (lows) are warm at the surface and cold
aloft
Vertically Stacked Systems
• As cyclones mature and become occluded, they become vertically stacked with their upperlevel system
H
L
H
L
Z
W
E
Vertically Stacked Systems
•
As a result, the pressure trough and temperature trough lose
their vertical tilt. They also become aligned. (This is common as
cyclones move poleward toward 60N and 60S latitudes)
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The same happens with the pressure ridge and temperature ridge
as an aging polar high moves equatorward and merges with the
subtropical high near 30N and 30S
i.e. systems began to lose their baroclinic nature
•
As a consequence, the thermal advection with these systems
becomes quite weak (or non-existent). Thus, energy conversion
from the jet stream to these surface systems becomes negligible
 No thermal contrasts, then the jet stream retreats
Vertically Stacked Systems
•
These vertically-stacked systems become more barotropic in
nature (equivalent barotropic) with the wind direction being
uniform with height (no wind shear)
Westward tilt of pressure systems
with height
Vertically stacked
Unhealthy Eastward Tilt
• If the pressure trough displays an eastward tilt with height, there is an upscale energy cascade whereby the eddies must give their energy back to the
mean flow
H
L
Z
H
W
L
E
• Thus, individual cyclones and anticyclones will
weaken or dissipate as they lose energy to the
larger jet-stream westerlies
• The surface cyclone or anticyclone must give
up its kinetic and potential energy to the jet
stream
• The surface system rapidly dissipates
• This is characterized on weather maps by the
upper trough (ridge) tending to “outrun” the
surface low (high)
• This is common with landfalling cyclones on
the West Coast
• A simplistic diagram of baroclinic troughs and ridges
is displayed in your handout
– The surface trough (i.e. convergence) is overlain by the
upper-tropospheric ridge (i.e. divergence)
– Thus, mass continuity requires that upward vertical
motion occur in the vicinity of baroclinic surface lows
– The surface ridge (i.e. divergence) is overlain by the
upper-tropospheric trough (i.e. convergence)
– Thus, mass continuity requires that downward vertical
motion occur in the vicinity of baroclinic surface highs