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Jet Streams Lessons 25/26

Jet Streams Lessons 25/26. Jet Streams. Defined as a narrow ribbon of fast moving air : 1000’s of miles in length, up to 200 miles wide, approx.. 2 miles deep. Jet Streams (cont..). Cause: The main constituent of a jet stream is the thermal wind component.

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Jet Streams Lessons 25/26

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  1. Jet StreamsLessons 25/26

  2. Jet Streams • Defined as a narrow ribbon of fast moving air : • 1000’s of miles in length, • up to 200 miles wide, • approx.. 2 miles deep.

  3. Jet Streams (cont..) • Cause: • The main constituent of a jet stream is the thermal wind component. • This is caused by marked horizontal thermal gradients in the atmosphere. • The most marked thermal gradients are usually associated with frontal systems.

  4. Polar Front/Subtropical Jet Streams

  5. Cross Section of a Frontal System

  6. Cold Front Jet Stream

  7. Jet Stream Turbulence (CAT) • Turbulence in jet streams arises from internal friction in the shear zone leading to a breakdown in the smooth flow. • The windshear associated with jet streams if in excess of the following values will produce clear air turbulence (CAT): • 5 Kt. per 1000 feet vertically, • 20 Kt. per 60 N.M. horizontally. • Horizontal shear is usually strongest on the cold air side.

  8. Location of Clear Air Turbulence • Preferred region • near or below jet axis on the low-pressure side, (cold air or polar air side). • with a secondary maximum above the axis on the anticylonic side (above the tropopause).

  9. Location of Polar Frontal Jet. • Located in the warm air mass. • Core of the Jet is about 1km below the tropopause. • Approximately 200 nm behind surface position of the cold front; • and 400 - 500 nm ahead of the surface position of the warm front.

  10. Position of Jet Streams Relative to Fronts

  11. Polar Frontal Jet Streams • Very variable in position and speed. • Located anywhere from between 35º and 70º in both hemispheres. • Positioned at lower latitudes in winter. • Core height usually between 300 to 250 mb. • Speeds frequently 100-150 Kt in winter and occasionally 200 Kt. • Usually westerly direction between 190º to 350º. • Associated with surface fronts. • Jet weakens in summer and moves poleward.

  12. Typical Jet Stream

  13. Newton Laws 1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

  14. Newton Laws 2. Force = Mass x Acceleration

  15. Newton Laws 3. For every action there is an equal and opposite reaction.

  16. STJ Development • Air accelerates to the east • Forms strong westerly jet stream at 30°N/S

  17. Subtropical Jet Stream • Usually found at an average latitude of 30º in both hemispheres. • May vary from between 25º to 40º • Core lies at 200 Mb level; • Below the tropical tropopause. • Position less variable than PFJ. • Jet weakens considerably in the summer hemisphere and moves poleward.

  18. EASTERLY EQUATORIAL JET Only occurs in summer in northern hemisphere

  19. Easterly Jet Stream

  20. Easterly Equatorial Jet • Usually found from mid-June to September. • Follows movement of thermal equator. • Band of easterly winds from equator to about 25º N. • Jet speeds reached in only 2 zones. • Philippines to Sudan (max mean speed 100 Kt) and • Over W. Africa (max mean speed 60-70 Kt).

  21. Easterly Equatorial Jet • Found between 200 and 100 Mb levels. • Core of Easterly jet is • 15ºN over India and • 5 ºN over West Africa.

  22. EEJ (cont.) • Jet formed due to intense heating over Asian plateau. • Cool zone exists over the ocean. • Thermal gradient is reversed. • Over W Africa similar conditions exist. • No Easterly jets appear in the Southern hemisphere.

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