jet streams lessons 25/26 jet streams defined as a narrow ribbon of fast moving air : –1000’s of...

Jet StreamsLessons 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.– This is caused by marked horizontal thermal

gradients in the atmosphere.– The most marked thermal gradients are usually

associated with frontal systems.

Polar Front/Subtropical Jet Streams

Cross Section of a Frontal System

Cold Front Jet Stream

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.

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).

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.

Position of Jet Streams Relative to Fronts

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.

Typical Jet Stream

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.

Newton Laws

2. Force = Mass x Acceleration

Newton Laws

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

STJ Development

• Air accelerates to the east

• Forms strong westerly jet stream at 30°N/S

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PG F

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Sub -Tro p ic a l J e t

Sub -Tro p ic a l J e t

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.

EASTERLY EQUATORIAL JET

Only occurs in summer in northern hemisphere

Easterly Jet Stream

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).

Easterly Equatorial Jet

• Found between 200 and 100 Mb levels.

• Core of Easterly jet is – 15ºN over India and– 5 ºN over West Africa.

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.