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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?

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jet stream and other upper air winds
Jet stream and other upper air winds
  • Jet stream formation
  • Jet stream position
  • Why the jet stream is important
    • Cyclones
the jet stream
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 stream4
The Jet Stream

Recall the horizontal temperature effects

on the pressure:

500 mb

700 mb

850 mb

Psurface

Warm

Cold

the jet stream5
The Jet Stream

Consider the balance of forces at each level:

Co

PGF

500 mb

700 mb

850 mb

Ps

Warm

Cold

slide6

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

slide8

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.

slide9

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

the jet stream10
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 stream11
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
Generation of Divergence Aloft

Supergeostrophic

Supergeostrophic

Geostrophic

Geostrophic

Subgeostrophic

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

generation of divergence aloft and resulting vertical motion
Generation of Divergence Aloft andResulting Vertical Motion

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

divergence
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
Speed Divergence

Slow Speed

Fast Speed

The faster speed wind will “pull away” from the slower

speed wind thereby “spreading out” the air.

direction divergence
Direction Divergence

The air spreads

out as the two

streams flow

away from each

other.

divergence aloft and cyclogenesis
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

divergence aloft and cyclogenesis18
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

divergence aloft and cyclogenesis19
Divergence Aloft and Cyclogenesis

Surface convergence develops

in response to the rising motion.

divergence aloft and cyclogenesis20
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

slide21

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.

slide22

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