Jet Stream

1
Jet Stream
An area of strong winds that are concentrated in
a relatively narrow band in the upper troposphere
of the middle latitudes and subtropical regions
of the Northern and Southern Hemispheres. Flowing
in a semi-continuous band around the globe from
west to east, it is caused by the changes in air
temperature where the cold polar air moving
towards the equator meets the warmer equatorial
air moving polarward. It is marked by a
concentration of isotherms and strong vertical
shear. (from AMS)
2
Jet Stream
From the Coriolis Force point of view
Westerly wind Wind blows from west
Cold air
North
West
Trade wind Wind blows from northeast
East
Equator
Warm air
3
Thermal Wind
-
10
o
C
-
10
o
C
cold
cold
o
o
-
5
C
-
5
C
VTVg2-Vg1
o
o
0
C
0
C
o
o
5
C
5
C
warm
warm
4
Thermal Wind
The (vector) difference between the wind at two
levels (pressure levels).
Consider, the case of no wind near the surface
(the 1000 mb surface is horizontal) and a
temperature pattern with a uniform north-to-south
gradient up to the 500 mb level.
X
5
Thermal Wind
VT
VTVg2-Vg1
z
Vg2
Vg2 geostrophic wind at upper level
Vg1 geostrophic wind at lower level
Vg10
E
VTVg2
Since Vg10,
W
N
The difference in wind between the two levels
(the thermal wind) relates to the temperature
pattern (low temperature on the left) the same
way that the geostrophic wind relates to the
pressure pattern (low pressure on the left).
Cold air
E
W
VT
Warm air
S
6
Jet Stream
From the thermal wind balance point of view
z
S-N temperature gradient results from the
unbalance between the long-wave and short-wave
radiation at different latitudes.
E
W
In midlatitude, westerly wind has to increase
with height in order to maintain the thermal
wind balance
Maximum wind occurs below the level where
temperature gradient changes the sign (tropopause)
7
Jet Stream (J)
Temperature field isotherms
clear-air turbulence!
8
Jet Stream (NH)
9
Jet Stream (SH)
10
Jet Streak
A region of accelerated wind speed along the axis
of a jet stream.
Entrance
Exit
Dashed lines isotachs
11
Jet Streak
12
Jet Streak
13
Jet Streak (Quasi-Geostrophic theory)
Low pressure
Low pressure
III
D
C
I
D
C
II
IV
High pressure
High pressure
I Convergence II Divergence III
Divergence IV Convergence
14
Jet Streak (Entrance)
Cold Air
Warm Air
Direct thermal circulation
15
Jet Streak (Exit)
Cold Air
Warm Air
Indirect thermal circulation
16
Jet Streak and low-level cyclones
III
Exit
Div
Div
Cold Air
Direct thermal circulation
IV
Con
Con
Indirect thermal circulation
Con
I
Div
II
Warm Air
Entrance
17
Jet Streak and low-level cyclones
Adiabatic cooling
III
Exit
Div
Cold Air
Direct thermal circulation
IV
Con
Indirect thermal circulation
Con
I
Adiabatic warming
Div
Adiabatic warming
II
Warm Air
Entrance
Adiabatic cooling
Region III is better than Region II for low-level
cyclones to develop!
18
Tropopause Folding
Troposphere and stratosphere exchange air
Folding of the tropopause with direct
ageostrophic circulation around the jet stream
(entrance to a jet streak)
19
Polar Front
20
Polar Front
Polar jet stream and polar front
Solid lines isotachs Dashed lines isotherms
The break in the tropopause is associated with
the vertical frontal layer