Jet stream

1. Jet stream

2. Jet stream and other upper air winds • Jet stream formation • Jet stream position • Why the jet stream is important • Cyclones

3. The Jet Stream • Puzzling Questions • Why does it take longer to fly from New York to Los Angeles than it takes to fly from Los Angeles to New York? • Why do storms (low pressure systems) usually move from west to east? • Why are the 500 mb winds very different from those at the surface? • Why are the upper level winds much faster than those at the surface?

4. The Jet Stream Recall the horizontal temperature effects on the pressure: 500 mb 700 mb 850 mb Psurface Warm Cold

5. The Jet Stream Consider the balance of forces at each level: Co PGF 500 mb 700 mb 850 mb Ps Warm Cold

6. Polar Jet Formation Steep gradients of temperature change at the Polar front trigger steep pressure gradients, which then forces higher velocity geostrophic winds. This is the trigger for jet stream flow. Figure 11.13A

8. Jet Stream Figure 11.10 Figure 11.9 High velocity Polar and subtropical jet stream winds are located in the lower tropopause, and they oscillate along planetary ridges and troughs.

9. 300 mb Winds & Jets 300 mb pressure surface maps illustrate lines of equal wind speed (isotachs) as the jets meander. Jet streaks are the maximum winds, exceeding 100 knots. Figure 11.11

10. The Jet Stream • What have we found? • A horizontal temperature difference causes a horizontal pressure difference aloft. • The isobars tilt, being higher in the warm air. • Because the tilt increases with height, the horizontal PGF increases with height. • The geostrophic winds increase with height.

11. The Jet Stream • What does this tell us about the real winds? • The winds blow from the west aloft. • Faster air trip from L.A. to New York than New York to L.A. • The winds aloft can change direction if the horizontal temperature gradient changes direction. • The winds aloft are strongest near the largest horizontal temperature gradient. • The strongest band of winds aloft is called the jet stream.

12. Generation of Divergence Aloft Supergeostrophic Supergeostrophic Geostrophic Geostrophic Subgeostrophic © 1997 Prentice-Hall Inc., From: Moran and Morgan, Meteorology

13. Generation of Divergence Aloft andResulting Vertical Motion © 1997 Prentice-Hall Inc., From: Moran and Morgan, Meteorology

14. Divergence • Divergence -- The spreading out of air. • Convergence -- The piling up of air. Equal to “negative divergence.” • Two types of divergence: • Speed Divergence • Direction Divergence

15. Speed Divergence Slow Speed Fast Speed The faster speed wind will “pull away” from the slower speed wind thereby “spreading out” the air.

16. Direction Divergence The air spreads out as the two streams flow away from each other.

17. Divergence Aloft and Cyclogenesis No Upper Divergence Upper Divergence forms due to the trough No Lower Divergence No Lower Divergence Initially Trough Approaches from the West

18. Divergence Aloft and Cyclogenesis The divergence aloft initiates an upward vertical motion beneath the upper level divergence. No Lower Divergence Trough Approaches from the West

19. Divergence Aloft and Cyclogenesis Surface convergence develops in response to the rising motion.

20. Divergence Aloft and Cyclogenesis If more mass is being removed from the column by the upper level divergence than is replaced by surface convergence, then a low pressure center at the surface will either develop of deepen. Cyclogenesis!!!!! L

21. Jet Convergence & Divergence Figure 13.10A Figure 13.10B The polar jet forces air convergence aloft upstream of the deepening open wave cyclone, and then divergence downstream. When these winds are gone, the cyclone degrades.

22. Summary of Cyclone Weather Upper and surface maps illustrate the role of convergence and divergence aloft, and the pattern of clouds, precipitation, and temperatures on the ground. Figure 13.11