Jet Stream

1. Jet Stream An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. It is marked by a concentration of isotherms and strong vertical shear. (from AMS)

2. Jet Stream (NH)

3. Westerly Wind Trade Wind Jet Stream From the Coriolis Force point of view: Cold air North Westerly wind (upper levels): Wind blows from west West East Trade wind (low levels): Wind blows from northeast Equator Warm air

4. V V g1 g2 V T Jet Stream From the thermal wind balance point of view: o o - - 10 10 C C cold cold o o - - 5 5 C C VT=Vg2-Vg1 o o 0 0 C C o o 5 5 C C warm warm

5. Thermal Wind The (vector) difference between the wind at two levels (pressure levels). Consider, the case of no wind near the surface (the 1000 mb surface is horizontal) and a temperature pattern with a uniform north-to-south gradient up to the 500 mb level. X

6. Thermal Wind Vertical cross-section (x-z plane) VT VT=Vg2-Vg1 z Vg2 Vg2: geostrophic wind at upper level Vg1: geostrophic wind at lower level Vg1=0 E VT=Vg2 W Since Vg1=0, The difference in wind between the two levels (the thermal wind) relates to the temperature pattern (cold temperature on the left) the same way that the geostrophic wind relates to the pressure pattern (low pressure on the left). Top view N Cold air W E VT Warm air S

7. N Cold air W E Warm air Equator Jet Stream From the thermal wind balance point of view: z S-N temperature gradient results from the energy unbalance between the long-wave and short-wave radiation at different latitudes. E W In midlatitude, westerly wind has to increase with height in order to maintain the thermal wind balance (geostrophic + hydrostatic balances) Maximum wind occurs below the level where temperature gradient changes the sign (tropopause)

8. Jet Stream (J) Temperature field: isotherms clear-air turbulence!

9. Jet Stream (NH)

10. Jet Stream (SH)

11. Jet Streak A region of accelerated wind speed along the axis of a jet stream. Entrance Exit Dashed lines: isotachs

12. Jet Streak

13. Jet Streak

14. Ageostrophic wind Pressure gradient force Coriolis force Jet Streak (Quasi-Geostrophic theory) Low pressure Low pressure III D C I D C II IV High pressure High pressure I : Convergence II : Divergence III: Divergence IV : Convergence

15. Exit Div Con Con Div Entrance Jet Streak (Entrance) Cold Air Warm Air Direct thermal circulation

16. Exit Div Con Con Div Entrance Jet Streak (Exit) Cold Air Warm Air Indirect thermal circulation

17. L L Jet Streak and low-level cyclones III Exit Div Div Cold Air Direct thermal circulation IV Con Con Indirect thermal circulation Con I Div II Warm Air Entrance

18. Enhance horizontal T gradient L L Reduce horizontal T gradient Jet Streak and low-level cyclones Adiabatic cooling III Exit Div Cold Air Direct thermal circulation IV Con Indirect thermal circulation Con I Adiabatic warming Div Adiabatic warming II Warm Air Entrance Adiabatic cooling Region III is better than Region II for low-level cyclones to develop!

19. Tropopause Folding Troposphere and stratosphere exchange air Folding of the tropopause with direct ageostrophic circulation around the jet stream (entrance of a jet streak)

20. Global Circulation

21. Polar Front

22. Polar Front

23. Polar Front Polar jet stream and polar front Solid lines : isotachs Dashed lines: isotherms The break in the tropopause is associated with the vertical frontal layer