Transcript Jet stream and jet streak

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)
Jet Stream (NH)
Jet Stream
From the Coriolis Force point of view:
Westerly Wind
Cold air
North
West
East
Trade Wind
Equator
Warm air
Westerly wind (upper levels):
Wind blows from west
Trade wind (low levels):
Wind blows from northeast
Jet Stream
From the thermal wind balance point of view:
-10 oC
cold
-5 oC
VT
Vg2
VT=Vg2-Vg1
0 oC
Vg1
5 oC
warm
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.
in d
west w
X
500 mb
z
500 mb
z
n
warm
cold
no wind
1000 mb
1000 mb
N
S
Thermal Wind
Vertical cross-section (x-z plane)
z
Vg2
VT
Vg2: geostrophic wind at upper level
Vg1: geostrophic wind at lower level
Vg1=0
E
W
Top view
W
N
Cold air
VT
Warm air
S
VT=Vg2-Vg1
E
Since Vg1=0,
VT=Vg2
The difference in wind between the
two levels (the thermal wind) relates
to the temperature pattern (cold
temperature on the left) the same
way that the geostrophic wind
relates to the pressure pattern (low
pressure on the left).
Jet Stream
From the thermal wind balance point of view:
z
S-N temperature gradient results
from the energy 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
(geostrophic + hydrostatic balances)
N
E
Cold air
Warm air
Equator
W
Maximum wind occurs below the level
where temperature gradient changes
the sign (tropopause)
Jet Stream (J)
Temperature field: isotherms
clear-air
turbulence!
Jet Stream (NH)
Jet Stream (SH)
Jet Streak
A region of accelerated wind speed along the axis of a jet stream.
Entrance
Exit
Dashed lines: isotachs
Jet Streak
Jet Streak
Jet Streak (Quasi-Geostrophic theory)
Low pressure
High pressure
Pressure gradient force
Coriolis force
Low pressure
IC
D III
II D
C IV
High pressure
Ageostrophic wind
I : Convergence
II : Divergence
III: Divergence
IV : Convergence
Jet Streak (Entrance)
Direct thermal circulation
Jet Streak (Exit)
Indirect thermal circulation
Jet Streak and low-level cyclones
III
Direct thermal
circulation
IV
Indirect thermal
circulation
I
II
Jet Streak and low-level cyclones
Adiabatic
III
cooling
Direct thermal
circulation
Adiabatic
warming
IV
Indirect thermal
circulation
Adiabatic
warming
I
II
Adiabatic
cooling
Reduce horizontal
T gradient
Enhance horizontal
T gradient
Region III is better than Region II for
low-level cyclones to develop!
Tropopause Folding
Troposphere and stratosphere exchange air
Folding of the tropopause
with direct ageostrophic
circulation around the jet
stream (entrance of a jet
streak)
Global Circulation
Polar Front
Polar Front
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