1. ATS/ESS 452: Synoptic Meteorology
Cyclone Structure

2. Observed Structure of Extratropical Systems
Fig. 6.1 (Meridional Cross Sections of Temperature and Zonal Winds)
Meridional temperature gradient characteristics
Compare winter vs. summer in NH
Temperature gradient
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
DJF
JJA
SH has a lot of water, which warms up slower and cools off slower (high specific heat) when compared to land. NH has a lot more land… it heats up very quickly.

3. Observed Structure of Extratropical Systems
Fig. 6.1 (Meridional Cross Sections of Temperature and Zonal Winds)
Zonal wind characteristics
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!
DJF
JJA
**Jet stream found right along the largest temperature gradients in mid-latitudes
** We can deduce that the jet stream has a lot to do with frontal boundaries!

4. L H L H L
Fig. 6.2 (Longitudinal Distribution of Average Zonal 200 mb Winds for NH Winter)
Characteristics
Large differences in zonal 200-mb wind speed with longtiude; strongest winds found near 30° N latitude
Two NH jet maxima in mid latitudes
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
**Stongest winds found near 30 deg N
Variations in windspeed along latitude
**Jet streams are exhibitive of strong thermal gradients… notice where both of the average jet maxes are located. Strong warm ocean currents riding along continential boundaries create strong thermal gradients in those locations!

5. 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!
**Stongest winds found near 30 deg N
Variations in windspeed along latitude
**Jet streams are exhibitive of strong thermal gradients… notice where both of the average jet maxes are located. Strong warm ocean currents riding along continential boundaries create strong thermal gradients in those locations!

6. 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 500-mb troughs
High sfc pressure is located east (downstream) of 500-mb ridges

7. How might this 500-mb wave pattern be linked to the distribution and influence of ocean and continent positions in the mid-latitudes?
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

8. Wind and Temperatures (top panel) Characteristics:
Fig. 6.4 (Vertical Cross Sections from Charleston SC to Omaha NE across a Jet Stream Wind Maximum)
Wind and Temperatures (top panel)
Characteristics:
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
(warm temperature = large thickness)
Cool at Omaha  tropopuase is low
(low temperature = low thickness)
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

9. b) Wind and Potential Temperature (bottom panel) Characteristics:
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
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
Remember that dry adibates = isentropes = lines of potential temperature

10. Stability and Isentropic Spacing
Static Stability and Potential Temp

11. 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

12. 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

13. On the synoptic scale, the atmosphere is always stably stratified (because convection would immediately stabilize any unstable regions)
So, (dθ/dz) > 0

14. Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)
Baroclinic vs. Barotropic Atmospheres
Barotropic Atmosphere
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
Baroclinic Atmosphere
Allows vertical wind shear
Has thermal wind
Allows temperature advection
Main region of baroclinic instability is in mid-latitudes
Large horizontal temperature gradients!
Remember we used the barotropic assumption previously to greatly simplify the vorticity equation to explain the dynamics of Rossby waves. However, our part of the world is really baroclinic

15. Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)
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)
Remember that dry adibates = isentropes = lines of potential temperature

16. 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 mb thickness (dashed lines)
Frontal locations are also indicated
Cyclone = eddy
Mean flow = jet stream
*The jet stream controls surface cyclones & anti-cyclones (for the most part)… mountain areas can interfere with the surface and decouple it from the upper-levels

17. What is happening in this figure (6.5)?
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 off-set from one another guarantees strong thermal advection

18. More detail… Panel a) Panel b) Panel c)
Heavy solid lines = 500-mb contours
Thin lines = 1000-mb contours
Dashed lines = thickness
More detail…
Panel a)
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)
Advection boxes are now formed (stronger temp advection)
Surface low deepens
Surface cyclone moves closer to upper-level trough
Panel c)
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

19. 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

20. 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

21. 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

22. 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

23. 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**

24. Healthy Westward Tilt of Pressure Systems with HeightZ H L W E

25. 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
This allows for significant temperature advection to take place in the presence of cyclones

26. Vertically Stacked Systems
As cyclones mature and become occluded, they become vertically stacked with their upper-level system H L Z
Note that sub-tropical highs are vertically stacked… but its not a baroclinic high (barotropic) H L W E

27. 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)
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
So basically, with a vertically stacked system, the surface cyclone pulls back into the colder air (it’s under a low height center!)
**When the system becomes totally vertical, that means it has moved away from the energy source (thermal gradient) and it will soon begin to die out

28. 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)

29. Westward tilt of pressure systems with height
Vertically stacked

30. Unhealthy Eastward Tilt
If the pressure trough displays an eastward tilt with height, there is an up-scale energy cascade whereby the eddies must give their energy back to the mean flow H L Z
**So now, the sfc features are giving its energy back to the jet
** This is really the kiss of death for any weather system**  this is important stuff to look for when doing weather analysis
**Huge deep lows will very likely die very quickly…. See the large pacific lows that come in on the west coast H L W E

31. 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

32. 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