Jet Streak Dynamics

1. Jet Streak Dynamics Dr. Scott M. Rochette SUNY Brockport 16 October 2003

2. Basis of Presentation • Jet Stream/Streak Basics • Four-Quadrant Model • Role of the Ageostrophic Wind • Effect of Curvature • Satellite Imagery • Coupled Jet Streaks (both kinds!) • Summary

3. Jet Streams • Quasi-horizontal, intense, narrow stream of air, associated with strong vertical wind shear • Intense: > 25 m s-1 (> 15 m s-1 for lower trop.) • Narrow: width ~0.5-1 order of magnitude less than length • Strong VWS: at least 5-10 m s-1 km-1 (at least 0.5-1 order of magnitude greater than synoptic-scale shear) • Typically found at/near tropopause • Two types in mid-latitudes • Polar Front Jet (PFJ) • Sub-Tropical Jet (STJ)

4. Polar Front Jet • Associated with polar front, separating polar cell and Ferrel cell • Best defined at 250-300 hPa • During cold season: • Stronger (can reach/exceed 100 m s-1) • Farthest south (can approach 30° N) • During warm season: • Weaker (~50 m s-1 or less) • Confined mainly to northern latitudes (~50° N) • Strong horizontal/vertical temperature gradients (thermal wind argument)

5. Thermal Wind Argument • Difference in geostrophic wind between two levels (i.e., vertical wind shear) • Analogous to geostrophic wind, except parallel to thickness contours with cold air (low Z) to left • Proportional to thickness (mean temperature) gradient • Westerly wind should strengthen (weaken) with height below (above) tropopause • This leads to a relative wind maximum near tropopause

6. cold warm (http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polar_jet_form.html)

7. (http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polar_jet_form2.html)(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polar_jet_form2.html)

8. (http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polarjet_plan.html)(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polarjet_plan.html)

9. Sub-Tropical Jet • Found mainly between 20° and 35° N (south of PFJ) during cold season • Best defined at 200-250 hPa • Separates Ferrel cell and Hadley cell • Speeds approach 70 m s-1 • Relatively steady wrt intensity (cf. PFJ) • Weak temperature gradients (cf. PFJ) • Primarily the result of conservation of angular momentum (spinning skater)

10. Conservation of Angular Momentum • Angular momentum = mass x velocity x radial distance (distance between object and rotation axis) • Ice skater pulls arms in close to body  spins faster • Air near tropopause flows north in upper branch of Hadley cell • r decreases  v increases (to hold M constant) • Coriolis force deflects flow to right (southerly flow becomes westerly)

11. Southerly flow speeds up as it moves poleward Coriolis force deflects flow to right in NH (westerly) (Ahrens 1994) (http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/subt_jet_form.html) If r , then v must  to keep M constant

12. Jet Streaks • Areas of maximum wind speed embedded within jet stream • Move (propagate) through larger jet stream pattern • Meso- to Meso- in scale • ~1000-3000 km long • ~100-400 km wide • ~2-3 km deep

13. Four-Quadrant Model 1 • Assumes straight-line jet streak (no curvature) • Two divergent regions • left exit • right entrance • Two convergent regions • left entrance • right exit • Divergence and convergence are created by ageostrophic portion of wind • geostrophic wind is essentially nondivergent

14. Four-Quadrant Model 2 • Dark red  height contours • Black/shading  isotachs/jet streak • Black arrows  ageo wind vectors (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

15. Ageostrophic Wind 1 • Portion of real wind that departs from geostrophy • Three components • isallobaric (pressure changes) • inertial-advective (horizontal advection) • inertial-convective (vertical advection)

16. Ageostrophic Wind 2 • Ageostrophic wind is perpendicular and to left of acceleration vector • Pay attention to the du/dt term

17. Ageostrophic Wind 3 • Westerly wind accelerates (decelerates) in entrance (exit) region • Entrance region: • u increases, so vag is positive (strongest along axis) • strongly positive vag (southerly) at axis, weaker on either side • convergence (divergence) in left (right) entrance region • Exit region: • u decreases, so vag is negative (‘strongest’ along axis) • strongly negative vag (northerly) at axis, weaker on either side • divergence (convergence) in left (right) exit region • Ageostrophic wind is perpendicular and to left of acceleration vector • Pay attention to the du/dt term

18. Ageostrophic Wind 4 • Entrance region: • ageostrophic wind ‘blows’ from higher to lower heights (warm to cold air) • convergence in left entrance region • divergence in right entrance region (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

19. Ageostrophic Wind 5 • Exit region: • ageostrophic wind ‘blows’ from lower to higher heights (cold to warm air) • divergence in left exit region • convergence in right exit region (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

20. Ageostrophic Wind 6 Point A PGF > CF (Z increases) vag > 0 Point C CF > PGF (Z decreases) vag < 0 Point B (New) PGF = (New) CF

21. Four-Quadrant Model 2 • Entrance Region: Direct Thermal Circulation • Cold air sinks in left entrance region (warms adiabatically) • Warm air rises in right entrance region (cools adiabatically) • Converts potential to kinetic energy • Frontolytic (weakens T) • Exit Region: Indirect Thermal Circulation • Cold air rises in left exit region (cools adiabatically) • Warm air sinks in right exit region (warms adiabatically) • Converts kinetic to potential energy • Frontogenetic (strengthens T) (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

22. Relative vorticity (dark red solid lines) generated by shear only (no curvature) • Westerly wind (u > 0, v = 0) • Positive vorticity advection (PVA) in left exit and right entrance regions • Negative vorticity advection (NVA) in left entrance and right exit regions (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

23. Vertical motion forced only by differential vorticity advection (no thermal advection; watch out) • Winds increase with height below jet level (as do vorticity and vorticity advection) • PVA increases with height in left exit and right entrance regions ( UVM) • NVA increases with height in left entrance and right exit regions ( DVM) (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

24. Q-G height tendency equation (no thermal adv.): • PVA  height falls (left exit/right entrance regions) • NVA  height rises (left entrance/right exit regions) • Z weakens upstream of jet core (entrance) • Z strengthens downstream of jet core (exit) • Jet streak should propagate toward tightening Z (i.e., downstream) (http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

25. Rising Motion • Dines compensation principle: • upper-level divergence must be compensated by low-level convergence cyclone deepens cyclone fills (http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jetstrk.rxml)

26. Jet Exit Regions • Lower branch of ITC follows isentropes • Not exactly a ‘box’ as shown previously • “Think escalators, not elevators.” (Uccellini 1999) (Uccellini and Johnson 1979)

27. Effects of Curvature • Black arrows  ageostrophic wind vectors • Ridge: Actual wind is supergeostrophic • Trough: Actual wind is subgeostrophic (Shapiro and Kennedy 1981)

28. Anticyclonic Jet Streaks • Half-wavelength (trough-ridge) decreases with time • Supergeostrophic parcels cannot follow height field • Parcels cut toward lower heights and accelerate • Divergence and UVM increase

29. Cyclonic Jet Streaks • Jet streak enters base of trough (cyclonic curvature) • Upstream ageostrophic flow increases • Divergence and UVM increase • New jet streak develops downstream • ‘Old’ jet never really makes it ‘around the bend’

30. Curved Jet Streaks 1 • A: straight-line jet streak • Four-cell divergence/convergence pattern • B: cyclonically-curved jet streak • Enhanced convergence/divergence on poleward side • Equatorward side questionable • C: anticyclonically-curved jet streak • Enhanced convergence/divergence on equatorward side • Poleward side questionable (Beebe and Bates 1955)

31. cyclonic straight-line anticyclonic • 600-hPa  • A: Straight-line • UVM in left exit/ right entrance regions • DVM in left entrance/right exit regions • B: Cyclonic • Enhanced UVM in left exit region • Enhanced DVM in left entrance region • Muddled VM on right side • C: Anticyclonic • Enhanced UVM in right entrance region • Enhanced DVM in right exit region • Muddled VM on left side (Moore and VanKnowe 1992)

32. Cyclonic Jets and Cyclogenesis • Solid contours  mean spring 300-hPa isotachs • Stars  areas of cyclogenesis • Left exit region favorable for cyclogenesis (Hovanec and Horn 1975)

33. Satellite Detection of Jet Streams • Jet streams evident in: • Visible imagery • Infrared (IR) imagery • Water Vapor (WV) imagery

34. Polar Front Jet Detection 1 Schematic Visible + 250-hPa Isotachs (Bader et al. 1995)

35. Polar Front Jet Detection 2 Infrared + 500-hPa Z/ Schematic (Bader et al. 1995)

36. Polar Front Jet Detection 3 Schematic Water Vapor + 250-hPa Isotachs (Bader et al. 1995)

37. Sub-Tropical Jet Detection 1 Schematic Infrared + 250-hPa Isotachs (Bader et al. 1995)

38. Sub-Tropical Jet Detection 2 Schematic Water Vapor + 250-hPa Isotachs (Bader et al. 1995)

39. Jet Axis Detection 1 (unstable) (unstable) (stable) (stable) Over Water Over Land (Bader et al. 1995)

40. Jet Axis Detection 2 jet axis (unstable) (unstable) (stable) (stable) Over Water Visible (Bader et al. 1995) (Conway 1997)

41. Coupled Upper/Lower Jet Streaks • Low-level jets (LLJs) linked to ULJs • Found under both entrance and exit regions of upper-level jets • Associated with lower branches of secondary (ageostrophic) circulations • Result of pressure changes associated with UL convergence and divergence

42. upper (Uccellini and Johnson 1979) • Note southerly ageo component underneath exit region lower entrance lower exit

43. Note southerly LL branch of ITC (Carlson 1991)

44. Dashed contours  isallobars • LLJ forms in response to pressure changes via convergence/divergence from ITC • Favored with strong ULJ and unstable conditions (Carlson 1991)

45. Favorable conditions for severe thunderstorm development • Note crossing of ULJ and LLJ (Newton 1967)

46. Vertically ‘uncoupled’ upper/lower jet- front system • ULJ’s exit region west of sfc front/LLJ • Front’s DTC and UL jet’s ITC destructively interact • Limited UVM ahead of front (Shapiro 1982)

47. Vertically ‘coupled’ upper/lower jet- front system • ULJ’s exit region above sfc front/LLJ • Front’s DTC and UL jet’s ITC constructively interact • Enhanced UVM ahead of surface front (Shapiro 1982)

48. Entrance Region LLJ • Similar to exit region LLJ (divergence) • ‘Escalator’ (follows isentropic surfaces) (http://www.eas.slu.edu/CIPS/Presentations/Isentropic/jetstreaks/sld017.htm)

49. Coupled LLJs • Entrance region LLJ displaced south of jet axis (cf. exit region LLJ) • Sloped ascent along isentropic surfaces (‘escalators’) • Response to ULJ’s divergent regions (http://www.eas.slu.edu/CIPS/Presentations/Others/AMSseminarJan2003/sld019.htm)

50. Coupled Upper-Level Jets 1 • Juxtaposition of divergent regions • right entrance region of ‘northern’ jet (DTC) • left exit region of ‘southern’ jet (ITC) • Divergent regions interact • Enhanced divergence in region between the two jets