1. Upper-Level Frontogenesis
Cliff Mass
University of Washington

2. Early Days
In the first half of the 20th century there was no concept of upper-level fronts.
Most studies described a polar front that extended from the surface to the tropopause.
The tropopause was considered an extensive and impenetrable barrier between the troposphere and stratosphere.

3. 1920s-1950: Polar Front
Bjerknes and Palmen 1937

4. Conceptual Models of the Tropopause

5. During the 50’s and 60’s Some Nations Conducted Upper-Ground Testing of Nuclear Weapons

6. Radioactivity
It was thought the above ground tests were not a problem:
Radioactivity injected into the stratosphere would stay there.
Radioactivity injected into troposphere (in remote areas!!) would fall out rapidly or would be removed by precipitation.
But that did not prove to be the case. High concentrations of radioactivity showed up in the U.S. and other locations. High levels of strontium-90 were found in milk, for example.

9. Somehow radiation from Pacific tests were getting into the troposphere over the U.S. and then either dry deposited or were scavenged out by rain.

10. How did the radioactive material get into the midlatitude troposphere if the tropopause was like a plastic sheath??

11. Connection to Upper Level Fronts

12. Upper- level fronts and stratosphere-troposphere transport
To answer this question, a number of synoptic studies and field experiments took place in the 1950s and 1960s.
They described new meteorological animals:
the upper level front
Tropopause folding and gaps
Stratosphere-troposphere exchange

13. The First Study of Upper Level Fonts: Reed 1957

18. Vertical Cross Section

20. A Series of Aircraft-Based Field Experiments Described the Structure of Upper Level Fronts for A First Time

22. Radioactivity
Measurements of radioactivity showed that high levels of radioactivity in the stratosphere were entering the troposphere through upper level fronts.

23. Potential Vorticity As a Tracer of Air Parcel Origin
Potential vorticity is high in the stratosphere because of the large stability there. Ertel Potential Vorticity (PV):
The aircraft studies found stratospheric values of potential vorticity transported into the troposphere through upper level fronts.

25. Additional Aircraft Data Showed the Details of Upper-Level Fronts/Trop Fold

26. “Official” Definition

27. Upper Level Front Characteristics
Usually associated with midlatitude jet.
Can extend down to mb
Close association with upper-level troughs.
Often associate with substantial clear air turbulence.
Associated with a folding or “extrusion” of the tropopause.
Can also be associated with high ozone values, particularly in mountain stations.

28. Schmatic of upper level trop folding

29. Tropopause Folding

30. Stratospheric Air Injected into the Troposphere

31. Ozone and Upper Level Fronts
Ozone levels are generally higher in the stratosphere than the troposphere.
Ozone can be injected into the troposphere through upper level fronts
The enhanced upper-level fronts can frequently be observed at the surface, particularly at mountain and higher-elevation observation sites.

32. Ozone Measured By Aircraft

33. Many studies have document such stratospheric ozone in the troposphere

34. Remotely Sensed Ozone During an Upper Level Front

35. Simulated PV Structure at Same Time

36. Good correlation between Ozone and PV: not a surprise!

37. Trop Pressure A Good Tool http://www. atmos. washington

38. Clear-Air Turbulence (CAT) Associated with Upper-Level Fronts

40. Can be damaging and cause injuries and even death (unbelted)

41. CAT Associated With Upper Level Front and the Lower Stratosphere

42. Turbulence

43. Richardson Number (small indicates more turbulence)

44. Turbulence is Maximum Above and Below the Jet Due to Large Shear
Jet Core Level
Large Shear
Impact on PV insert

45. Upper Level Frontogenesis for the December 14-16, 1987 Storm

47. 400 hPa
1000 hPa

48. Why Upper Level Fronts?
What is the relative importance of tilting and confluence/deformation?
Upper level short waves/jet streaks are key players.
As we shall see, three dimensional motions are essential.

49. Upper Level Frontal Frontogenesis
Tilting frontogenesis associated with differential vertical motion is often dominant, particularly for shortwaves in NW flow
Horizontal confluent frontogenesis is also important, but usually secondary.

50. Frontogenesis

51. Tilting Frontogenesis

52. Tilting Frontogenesis

53. Keyser et al (1986) 2-D Primitive Equation Simulation
Clear differential
Vertical motion
Across upper
front

54. Confluence can become important at the upper front moves around the upper trough

55. The End