1. AOS 101
Cyclone Structure
April 22/24
April 29/May 1

2. Cyclone COLD WARM OCCLUDED STATIONARY Symbols:
Point in direction of front movement
COLD
WARM
OCCLUDED
STATIONARY

3. Warm Front
WARM
COOL

4. Associated Weather (WF)
Gradual Slope
Stratiform rain
long lasting light rain
occurs on cool side of front
Temperature increases prior to frontal passage
Wind becomes southerly after passage

5. Cold Front
WARM
COOL

6. Associated Weather (CF)
Much Steeper Slope
More intense (convective) rain
Thunderstorms for a shorter period
occurs on warm side of front
Temperature decreases after frontal passage
Wind becomes northerly after passage

7. COOL AIR L
LIGHTER RAIN
COLD AIR
WARM AIR
HEAVIER RAIN

8. Finding a Front Temperature (dewpoint) Gradient
Change in wind direction
Converging winds at the front
“Kink” or “trough” in isobars (lower pressure)
Banded precipitation

10. Horizontal Cyclone Structure

11. Cyclone Life Cycle Stationary Front Wave Cyclogenesis
Maximum Intensity
Occluding
Dissipation

12. Stationary Front COLD WARM North-to-south temperature gradient
Winds parallel to the front
COLD
WARM

13. Wave COLD WARM Winds gain a northerly/southerly component
Wave forms on the front
Usually caused by an upper-level trough (next week)
COLD
WARM

14. Cyclogenesis COLD L WARM Closed cyclonic circulation froms
Surface pressure minimum is observed
Cold fronts and warm fronts become distinct
COLD L
WARM

15. Maximum Intensity L COLD WARM Warm air overruns cold air at warm front
Cold air quickly moves around to the south
Cold front moves faster, “catches” warm front
Storm deepens to lowest pressure L
COLD
WARM

16. Comma Head

17. Occlusion
Cold front catches warm front leaving warm air aloft
Cold air encircles cyclone center
Another cyclone may form at triple point L L
COLD
WARM

18. Occluded Front
WARM
COLD
COOL

19. Dissipating Cold air completely encircles low
All warm air is pushed aloft
Cyclone can no longer feed off of temperature differences

20. Vertical Cyclone Structure

21. Upper-level terminology
TROUGH: area of lower heights
RIDGE: area of higher heights L H

22. L H Vorticity Advection VORTICITY: how much the flow curves
Trough = positive; Ridge = negative L
NEGATIVE VORTICITY H
POSITIVE VORTICITY

23. L H Downwind of trough axis = Positive V.A.
Downwind of ridge axis = Negative V.A. L
NEGATIVE VORTICITY
NEGATIVE VA
POSITIVE VA H

24. Mechanisms for Deepening Cyclones
Warm air advection (WAA) near the ground
Look at surface temperature and winds
Positive vorticity advection (PVA) at upper levels
Look at upper-level (e.g. 500 hPa) winds and trough/ridge axes.
Both promote upward vertical motion (ascent).

25. Vertical Structure: Beginning
STATIONARY FRONT
PVA
POSITIVE VORTICITY

26. Vertical Structure: Cyclogenesis
WAA L
Westward tilt with height
POSITIVE VORTICITY

27. Vertical Structure: Occlusion
WAA L
PVA
POSITIVE VORTICITY

28. EXAMPLE 0000 UTC 10 November 1998
300 mb flow which resulted in a massive cyclone development over the midwest.
PVA downwind of trough
PVA
TROUGH AXIS
300 hPa Height, Wind Speed

29. Surface Pressure, 500-1000 Thickness
EXAMPLE 0000 UTC 10 November 1998
Weak cyclone downwind of trough
Fronts are relatively weak L
TROUGH AXIS
Surface Pressure, Thickness

30. EXAMPLE 1200 UTC 10 November 1998 12 hours later…
Trough moves west, strong jet rotates around trough
PVA
TROUGH AXIS

31. EXAMPLE 1200 UTC 10 November 1998
Cyclone has deepened to 977 hPa, has moved closer to trough axis.
Intense WAA north of cyclone
WAA L
TROUGH AXIS

32. EXAMPLE 0000 UTC 11 November 1998 12 hours later… Trough is now cutoff
PVA
TROUGH AXIS

33. EXAMPLE 0000 UTC 11 November 1998
Surface cyclone practically under trough axis
i.e. The cyclone is “vertically stacked.”
No PVA or WAA = no more intensification L
TROUGH AXIS

34. Main Points
Cyclones will tend to form downwind of an upper-level trough in an area of PVA.
As the cyclone develops, WAA will occur on the warm front further deepening the cyclone
Whole system will tilt westward with height
While developing, cyclone will move closer to trough axis
While occluding, cyclone will move nearly under trough axis in an area void of PVA/WAA.