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 900-800 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. Clear-Air Turbulence (CAT) Associated with Upper-Level Fronts

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

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

41. Richardson Number (small less stable)

42. Turbulence

43. And why is highest PV near the jet core?

44. Turbulence is Maximum Above and Below the Jet Due to Large Shear 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?

49. Upper Level Frontal Frontogenesis • Tilting frontogenesis associated with differential vertical motion is often dominant! • Horizontal confluent frontogenesis is also important, but usually secondary.

50. Frontogenesis