Transcript Lecture #16: Jet Streams and Streaks

Jet Streams and Jet Streaks
Advanced Synoptic
M. D. Eastin
Jet Streams and Jet Streaks
Jet Streams
• Definition and Basic Characteristics
• Basic Forcing Mechanism
• Common Jets in the Mid-latitudes
Jet Streaks
• Definition and Basic Characteristics
• Vertical Motion Pattern
• Coupling with Surface Fronts
• Relationship to Severe Weather
Advanced Synoptic
M. D. Eastin
Jet Streams
Basic Characteristics:
• Long narrow band of strong winds
• ~500-6000 km in length
• ~100-400 km in width
• Not a continuous band
• Maximum winds ~50-250 knots
• Can be located at any altitude
• Common mid-latitude types
include the polar, subtropical,
and low-level jets
• Migrate and evolve over times
scales from a few hours to
seasonally
• Primarily influence the motion
and evolution of synoptic-scale
systems
• Contribute to the initiation and
evolution of mesoscale systems
and deep convection
Advanced Synoptic
M. D. Eastin
Jet Streams
Basic Forcing Mechanism:
J
Mean Zonal
Wind
J
Mean
Temperature
 All jets are a response to flow down strong
large-scale pressure gradients (produced
by temperature gradients) that is then turned
by the Coriolis force
 All long-lived jets are in thermal wind balance
u g
p

Rd  T 


f p  y 
North Pole
L
Jet
H
PGF
H
Equator
Advanced Synoptic
Coriolis force
turns wind
PGF
L
Maximum N-S
Temperature and
Pressure Gradient
M. D. Eastin
Jet Streams
Basic Forcing Mechanism: Thermal Wind Balance
 Notice how all of the strong upper-level jets (at 300 mb) are located directly above
a strong low-level temperature gradient (at 850 mb)
Advanced Synoptic
M. D. Eastin
Common Jet Streams
Polar-Front Jet (PFJ):
• Often located near 300 mb just below the mid-latitude tropopause
• Winds are westerly (blow west to east) and often exceed 75 m/s
• Associated with strong quasi-horizontal temperature gradients at low-levels
(Note: Jet migration is a response to the strong temperature gradient moving)
• Present year round
Isentropic Mean Meridional
Cross Section
• Furthest north (~50ºN)
and weakest during
the summer months
• Furthest south (~35ºN)
and strongest during
the winter months
• Most deep convection
develops equatorward
of the polar jet
Advanced Synoptic
M. D. Eastin
Common Jet Streams
Subtropical Jet (STJ):
• Often located near 200 mb just below the tropical tropopause
• Winds are westerly but rarely exceed 50 m/s
• Associated with a moderate quasi-horizontal temperature gradients at mid-levels
• Primarily a wintertime
phenomenon
Isentropic Mean Meridional
Cross Section
• Meanders between 20ºN
and 35ºN
• Often is oriented from
the southwest to the
northeast across the
Pacific and southern
or western U.S.
(“pineapple express”)
• Most deep convection
develops poleward of
the subtropical jet
Advanced Synoptic
M. D. Eastin
Common Jet Streams
Subtropical Jet (STJ):
• Often very difficult to distinguish
from the polar jet on daily weather
maps
Polar-Front Jet
• Since the subtropical jet is located
further south (where f is smaller),
a strong jet can still develop from
a moderate temperature gradient
u g
p

Rd  T 


f p  y 
Subtropical Jet
• Let’s do a simple analysis assuming
the following are held constant:
∂p
p
Rd
∂y
~ 800 mb
~ 500 mb
~ 300 J/kg/K
~ 1000 km
Advanced Synoptic
Polar Jet: Φ ~ 40ºN
∂T ~ 20 K
∂ug ~ 98 m/s
Subtropical Jet: Φ ~ 20ºN
∂T ~ 10 K
∂ug ~ 92 m/s
M. D. Eastin
Common Jet Streams
Low-level Jets (LLJ):
• Located 500-2000 m AGL
• Winds rarely exceed 25 m/s
LLJ
Cold
Air
• Associated with weak horizontal
temperature gradients confined
to lower levels
Warm
Air
• Can occur year round
1. Pre-frontal LLJ:
• Located just ahead (east) of
strong cold fronts
• Responsible for the rapid
advection of warm moist air
that can help “feed” deep
convection along the front
Advanced Synoptic
Cold
Air
J
Warm
Air
M. D. Eastin
Common Jet Streams
2. Nocturnal LLJ:
• Primarily oriented north-south
• Maximum intensity at night in the summer
• Increased nocturnal thunderstorm activity
is partially a result of the LLJ providing a
continuous supply of warm, moist air to
the storm cloud bases
• Intensity fluctuations are linked to diurnal
changes in the low-level temperature
gradients along the gradually sloping
(east-west) topography
East-West Cross Section
9 June 2002 at 12Z
Potential temperature (red contours)
Wind speed (shading)
J
Warm
Air
LLJ
Advanced Synoptic
M. D. Eastin
Jet Streaks
Basic Characteristics:
• Faster moving “pockets” of air
embedded within the jet stream
• ~250-1000 km in length
• ~50-200 km in width
Jet Streaks
• Migrate and evolve over times
scales from a few hours to a
few days
• Motion is often much slower
than the speed of the wind
within the jet stream or streak
• Primarily influence the initiation
and evolution of mesoscale
systems and deep convection
• Contribute to the evolution of
synoptic-scale systems since
most contain strong PVA
Advanced Synoptic
M. D. Eastin
Jet Streak Vertical Motions
Physical Interpretation of the Basic Pattern:
Vort
Max
• Using a simplified vorticity equation:
 u v 
D
  
 
Dt

x
y 

Vorticity
Change
Left
Entrance
Vorticity
Increase
+
Left
Exit
Vorticity
Decrease
JET
Divergence
• Thus, the vorticity change experienced by
an air parcel moving through the jet streak:
Vorticity decrease → Divergence aloft
→ Upward motion
Vorticity increase → Convergence aloft
→ Downward motion
Vorticity
Decrease
Right
Entrance
_
Vorticity
Increase
Right
Exit
Vort
Min
Left
Entrance
Left
Exit
Descent
Ascent
JET
Recall:
QG theory provides an alternative
explanation (with the same result)
Divergence / convergence patterns
result from ageostrophic motions
Advanced Synoptic
Ascent
Right
Entrance
Descent
Right
Exit
M. D. Eastin
Jet Streak Vertical Motions
An Example:
Divergence = yellow contours
Regions of expected upward vertical motion
Advanced Synoptic
M. D. Eastin
Jet Streak Vertical Motions
An Example:
Important Considerations!!!
1.
2.
3.
4.
Forcing is at upper-levels
Forcing is on the synoptic scale
Is there a mechanism for low-level lift?
Is the low-level environment moist?
Advanced Synoptic
M. D. Eastin
Coupling between Jet Streaks and Fronts
 The orientation of a surface front and an upper-level jet streak can lead to either
enhanced (deep) convection or suppressed (shallow) convection along the front
Enhanced Convection → Left exit or right entrance region is above the front
→ Helps destabilize the potentially unstable low-level air
→ Increases the likelihood of deep convection
Advanced Synoptic
M. D. Eastin
Coupling between Jet Streaks and Fronts
 The orientation of a surface front and an upper-level jet streak can lead to either
enhanced (deep) convection or suppressed (shallow) convection along the front
Suppressed Convection → Left entrance or right exit region is above the front
→ Prevents destabilization of the potentially unstable air
→ Decreases the likelihood of deep convection
Advanced Synoptic
M. D. Eastin
Coupling between Jet Streaks and Fronts
 The orientation of a surface front , an upper-level jet streak, and a low-level jet streak
can further enhance deep convection along the front
More Enhanced Convection → A “favorable” combination of ageostrophic circulations
from each jet streak and the front can create strong
lift along the warm (unstable) side of the front
→ Often the location of the most severe deep convection
Advanced Synoptic
M. D. Eastin
Jet Streaks and Severe Weather
Is severe weather often associated with jet streaks?
• Recent climatology conducted by Clark et al. (2009)
• Examined the location all severe weather reports (tornado, hail, winds) relative to any
upper-level jet streaks during the warm season (March-September) of 1994-2004
Expectations:
• Most severe weather is associated with jet streaks
→ Increased vertical shear
→ Enhanced storm longevity
• More severe weather in the right entrance and left-exit regions
→ Enhanced vertical motion
→ Greater likelihood of surface parcels being lifted to LFC
→ Greater near-surface moisture convergence
Advanced Synoptic
M. D. Eastin
Jet Streaks and Severe Weather
Answer: Yes - severe weather is regularly associated with jet streaks
Results:
• A total of 126,864 storm reports occurred during the period of study
• 84% were associated with an upper-level jet streak.
Advanced Synoptic
M. D. Eastin
Jet Streaks and Severe Weather
Where is the severe weather located?
Results:
• Majority of reports
are located in the
right-entrance
region and along
the jet streak axis
in the exit region
Left
Entrance
Left
Exit
Right
Entrance
Right
Exit
Advanced Synoptic
M. D. Eastin
Jet Streaks and Severe Weather
Composite Structure:
Advanced Synoptic
M. D. Eastin
Jet Streaks and Severe Weather
Composite Structure:
Advanced Synoptic
M. D. Eastin
References
Beebe, R. G., and F. C. Bates, 1955: A mechanism for the assisting in the release of convective instability. Mon. Wea.
Rev., 83, 1-10.
Bluestein, H. B., 1986: Fronts and jet streaks: A theoretical perspective. Mesoscale Meteorology and Forecasting,
Amer. Meteor. Soc., Boston, 173-215.
Bluestein, H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather
Systems. Oxford University Press, New York, 594 pp.
Bonner, W. D., 1968: Climatology of the low level jet. Mon. Wea. Rev., 96, 833-850.
Browning, K. A., and C. W. Pardoe, 1973: Structure of low-level jet stream ahead of mid-latitude cold fronts. Quart. J. Roy.
Meteor. Soc., 99, 619-638.
Clark, A. J., C. J. Schaffer, W. A. Gallus, and K. Johnson-Omara, 2009: Climatology of storm reports relative to
upper-level jet streaks. Wea. Forecasting, 24, 1032-1051.
Keyser, D., M. J. Reeder, and R. J. Reed, 1988: A generalization of Pettersen’s frontogenesis function and its relation to
the forcing of vertical motion. Mon. Wea. Rev., 116, 762-780.
Krisnamurti, T. N., 1961: The subtropical jet stream in winter. J. Meteor., 18, 172-191.
Murray, R., and S. M. Daniels, 1953: Transverse flow at the entrance and exit to jet streams. Quart. J. Roy. Meteor. Soc.,
99, 619-638.
Pyle, M. E., D. Keyser, and L. F. Bosart, 2004: A diagnostic study of jet streaks: Kinematic signatures and relationship to
coherent tropopause disturbances. Mon. Wea. Rev., 132, 297-319.
Uccellini, L. W., and D. J. Johnson, 1979: The coupling of upper and lower tropospheric jets streaks and implications for
the development of severe convective storms. Mon. Wea. Rev., 107, 682-703.
Advanced Synoptic
M. D. Eastin