1. Mid-Latitude Cyclones: Vertical Structure
Lecture 13
CASE STUDY DUE NEXT WEEK!

2. Review
Extratropical cyclones generally first develop along an intersection of two airmasses (like a stationary front)
As the cyclone develops, warm and cold fronts form, and the cold front slowly approaches the warm front
Once an occluded from forms, the cyclone is normally at its most intense, and will begin to weaken afterward
This is because it is no longer near a region of a horizontal temperature gradient (which is why it developed in the first place)

3. Review Continued Finding fronts on weather maps is very useful
It is often useful to first find the area of lowest pressure, since fronts typically originate from it
In the case of most fronts (except occluded fronts), there should be a large temperature change across them
All fronts should also have a fairly sharp wind shift from one side to another
Other factors, like precipitation, cloud cover, and moisture gradients can indicate a front

4. Review Continued
Last week, we discussed the surface structure of mid-latitude cyclones which are crucial in maintaining a temperature equilibrium on our planet.
We know that the winds move counter-clockwise and converge around a surface low low pressure center (this is because of the frictional force at the surface)
This Convergence/Divergence suggests that there must be movement of air in the vertical (continuity of mass)
Also, the flow in the upper troposphere is generally in geostrophic balance, so there is no friction forcing convergence/divergence.

5. Upper Levels
Ridge Trough Ridge

6. Vorticity
Vorticity is simply a measure of how much the air rotates on a horizontal surface
Positive vorticity is a counterclockwise (i.e. cyclonic) rotation
Negative vorticity is a clockwise (i.e. anticyclonic) rotation
Therefore, troughs contain positive vorticity, and ridges contain negative vorticity
Trough
Ridge

7. Let’s Revisit …
Vorticity < 0
Vorticity < 0
Vorticity > 0

8. Negative vorticity advection Positive vorticity advection

9. Vorticity Advection and Vertical Motion
* Positive vorticity advection (PVA) results in divergence at the level of advection
* Negative vorticity advection (NVA) results in convergence at the level of advection

10. Vorticity Advection and Vertical Motion
Remember that convergence at upper levels is associated with downward vertical motion (subsidence), and divergence at upper levels is associated with upward vertical motion (ascent).
Then, we can make the important argument that . . .

11. Upper Tropospheric Flow and Convergence/Divergence
Downstream of an upper tropospheric ridge, there is convergence, resulting in subsidence (downward motion).
Likewise, downstream of an upper tropospheric trough, there is divergence, resulting in ascent (upward motion).

12. Upper Tropospheric Flow and Convergence/Divergence
Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressure, and of course, downstream of an upper tropospheric trough axis is a favored location for a surface low pressure center.

13. Upper Tropospheric Flow and Convergence/Divergence
Surface cyclones also move in the direction of the upper tropospheric flow!
The surface low pressure center in the diagram above will track to the northeast along the upper level flow

14. Vertical Structure of Cyclones
What else do these diagrams tell us?
Because the surface cyclone is downstream from the upper tropospheric (~500 mb) trough axis, mid-latitude cyclones generally tilt westward with height!

15. Vertical Structure of Cyclones
To the right is another depiction illustrating the same point:
500 mb positive vorticity advection results in divergence and ascent, inducing a surface cyclone.

16. Cyclone Growth And Decay
Based on what we’ve learned, the position of the surface cyclone in relation to the upper level structure is key to development
A cyclone will grow if it is below an area of PVA, and weaken if below an area of NVA
Commonly, a cyclone will intensify until it becomes situated in an unfavorable location in relation to the upper levels

17. An Example: Time 1 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

18. An Example: Time 1 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

19. Time 2 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

20. Time 2 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

21. Time 3 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

22. Time 3 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

23. Summary of Event
At time 1, the upper levels and lower levels are perfectly set up for the surface cyclone to intensify
At time 2, the upper trough is almost above the surface cyclone, so the intensification slows
By time 3, the upper trough is exactly over the surface cyclone, so the intensification has halted

24. Cyclone Decay
Recall that due to friction, air blows across isobars near the surface
This means that the air is always converging at the center of low pressure areas
Therefore, unless there is at least enough divergence at upper levels to counteract the convergence at low levels, the surface cyclone will weaken because more mass will be added to the air column
This will force the surface pressure to rise

25. Cyclone Intensification/Weakening
How do we know if the surface cyclone will intensify or weaken?
If upper tropospheric divergence > surface convergence, the cyclone will intensify (the low pressure will become lower)
If surface convergence > upper tropospheric divergence, the cyclone will weaken, or “fill.”
Think of an intensifying cyclone as exporting mass, and a weakening cyclone as importing mass.