Jet stream

Jet stream and other upper air winds

• Jet stream formation

• Jet stream position

• Why the jet stream is important– Cyclones

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?

The Jet Stream

Recall the horizontal temperature effectson the pressure:

500 mb

700 mb

850 mb

Warm Cold Psurface

The Jet StreamConsider the balance of forces at each level:

500 mb

700 mb

850 mb

Warm ColdPs

PGFCo

Polar Jet FormationPolar Jet Formation

Steep gradients of Steep gradients of temperature temperature change at the change at the Polar front Polar front trigger steep trigger steep pressure pressure gradients, which gradients, which then forces then forces higher velocity higher velocity geostrophic geostrophic winds.winds.

This is the trigger This is the trigger for jet stream for jet stream flow.flow.

Figure 11.13AFigure 11.13A

Jet StreamJet Stream

Figure 11.9Figure 11.9

High velocity Polar and subtropical jet stream winds are located High velocity Polar and subtropical jet stream winds are located in the lower tropopause, and they oscillate along planetary ridges in the lower tropopause, and they oscillate along planetary ridges and troughs.and troughs.

Figure 11.10Figure 11.10

300 mb Winds & Jets300 mb Winds & Jets

Figure 11.11Figure 11.11

300 mb pressure 300 mb pressure surface maps surface maps illustrate lines of illustrate lines of equal wind speed equal wind speed (isotachs) as the (isotachs) as the jets meander.jets meander.

Jet streaks are Jet streaks are the maximum the maximum winds, exceeding winds, exceeding 100 knots.100 knots.

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.

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.

Generation of Divergence Aloft

Supergeostrophic

Geostrophic

Subgeostrophic

Geostrophic

Supergeostrophic

© 1997 Prentice-Hall Inc., From: Moran and Morgan, Meteorology

Generation of Divergence Aloft andResulting Vertical Motion

© 1997 Prentice-Hall Inc., From: Moran and Morgan, Meteorology

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

Speed Divergence

Slow Speed Fast Speed

The faster speed wind will “pull away” from the slowerspeed wind thereby “spreading out” the air.

Direction Divergence

The air spreadsout as the twostreams flowaway from eachother.

Divergence Aloft and Cyclogenesis

Initially

No UpperDivergence

No LowerDivergence

Trough Approachesfrom the West

Upper Divergenceforms due to the trough

No LowerDivergence

Divergence Aloft and Cyclogenesis

Trough Approachesfrom the West

No LowerDivergence

The divergence aloftinitiates an upward verticalmotion beneath the upperlevel divergence.

Divergence Aloft and Cyclogenesis

Surface convergence developsin response to the rising motion.

Divergence Aloft and Cyclogenesis

If more mass is being removed from the column by the upper

level divergence than is replacedby surface convergence, then a

low pressure center at the surfacewill either develop of deepen.

Cyclogenesis!!!!!L

Jet Convergence & DivergenceJet Convergence & Divergence

The polar jet forces air convergence aloft upstream of the deepening The polar jet forces air convergence aloft upstream of the deepening open wave cyclone, and then divergence downstream.open wave cyclone, and then divergence downstream.

When these winds are gone, the cyclone degrades.When these winds are gone, the cyclone degrades.

Figure 13.10AFigure 13.10A Figure 13.10BFigure 13.10B

Summary of Cyclone WeatherSummary of Cyclone Weather

Figure 13.11Figure 13.11

Upper and Upper and surface maps surface maps illustrate the role illustrate the role of convergence of convergence and divergence and divergence aloft, and the aloft, and the pattern of pattern of clouds, clouds, precipitation, precipitation, and and temperatures on temperatures on the ground.the ground.