Transcript Jet Streams - meteonaa.esy.es

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.
– 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
Low pressure aloft
Sub-Tropical Jet
PGF
High pressure aloft
Sub-Tropical Jet
Low pressure aloft
Warm moist
rising air(Cb/Ts)
PGF
N
Equatorial Trough
Low Surface Pressure (ITCZ)
W
S
E
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
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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.)
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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.