1. AOS 100: Weather and Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell

2. Miscellaneous
New Homework

3. Review of September 8th: Forces and Vorticity
Geostrophic balance is a result of a balance between the PGF and the Coriolis Force, and the resultant wind is called the geostrophic wind
This is not a good approximation at the surface, because of friction
Friction always acts opposite the wind, and causes the wind to slow
Since the Coriolis Force is proportional to the strength of the wind, the Coriolis Force also slows

4. Review Continued However, the PGF remains the same strength
This results in a new force balance, which causes the wind to blow slightly across isobars toward lower pressure
Upper levels are pretty close to geostrophic balance, due to the reduced friction
When looking at a map of constant pressure, we can use heights the same we use pressure when looking at a surface map
Vorticity advection determines where convergence or divergence will occur at upper levels

5. Review Continued

6. Review Continued
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

7. From: http://www. noaanews. noaa. gov/stories2009/20091007_septusstats

8. From: http://www. noaanews. noaa. gov/stories2009/20091007_septusstats

9. Let’s Revisit … Vorticity < 0 Vorticity < 0 Vorticity > 0
Negative Vorticity Advection Positive Vorticity Advection

10. Diagnosing Vorticity Advection
To determine vorticity advection, first find the locations of maximum (positive) vorticity and minimum (negative) vorticity
Then, determine what direction the wind flow is
Areas of negative vorticity advection (NVA) will be just downstream of vorticity minima, and areas of positive vorticity advection (PVA) will be just downstream of vorticity maxima

12. The Connection
Based on what we learned earlier, areas of convergence at upper levels favor surface anticyclones, and areas of divergence at upper levels favor surface cyclones

13. The Connection
Given what we learned earlier, areas of convergence at upper levels favor surface anticyclones, and areas of divergence at upper levels favor surface cyclones

14. A 3-D Look

15. The Connection
As the locations of upper level troughs and ridges change, we might expect the positions of surface cyclones and anticyclones to change

16. The Big Picture

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

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

19. An Example: Time 1 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 2 Above: Upper Level Height and Wind Speed
Right: Surface Pressure

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

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

24. 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

25. 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

26. Revisited

27. About Contour Analysis …

28. An Example From Tuesday Morning:

29. Observations about Observations
Conventionally, only temperature, dewpoint, wind speed and direction, cloud cover, pressure, current weather, and visibility (if less than 10 miles) are shown
However, much of the planet goes unobserved
Large differences in temperature, dewpoint, etc. can exist from location to location
This is why we must do contour analysis in order to “fill in” the missing data

30. How Do We Do Contour Analysis?
You can think of it like a glorified version of “connect the dots”
However, for contour analysis, we have to “fill in” some of the missing data
The goal of contour analysis is to allow for easier interpretation of the current weather
Some things to remember:
- Lines never cross
- Always use a pencil so you can erase lines
Now for an example …