advanced synopticm. d. eastin jet streams and jet streaks
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Advanced Synoptic M. D. Eastin
Jet Streams and Jet Streaks
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Advanced Synoptic M. D. Eastin
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
Jet Streams and Jet Streaks
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Advanced Synoptic M. D. Eastin
Jet StreamsBasic 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
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Advanced Synoptic M. D. Eastin
Jet StreamsBasic Forcing Mechanism:
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
y
T
pf
R
p
udg
J Mean ZonalWind
MeanTemperature
J
Maximum N-STemperature and
Pressure Gradient
H LPGF
Equator
North Pole
H
L
PGF
Coriolis forceturns wind
Jet
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Advanced Synoptic M. D. Eastin
Jet StreamsBasic 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)
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Advanced Synoptic M. D. Eastin
Common Jet StreamsPolar-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
• 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
Isentropic Mean Meridional Cross Section
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Advanced Synoptic M. D. Eastin
Common Jet StreamsSubtropical 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
• 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
Isentropic Mean Meridional Cross Section
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Advanced Synoptic M. D. Eastin
Common Jet StreamsSubtropical Jet (STJ):
• Often very difficult to distinguish from the polar jet on daily weather maps
• Since the subtropical jet is located further south (where f is smaller), a strong jet can still develop from a moderate temperature gradient
• Let’s do a simple analysis assuming
the following are held constant:
∂p ~ 800 mb p ~ 500 mb Rd ~ 300 J/kg/K ∂y ~ 1000 km
Polar-Front Jet
Subtropical Jet
Polar Jet: Φ ~ 40ºN ∂T ~ 20 K
Subtropical Jet: Φ ~ 20ºN ∂T ~ 10 K
∂ug ~ 98 m/s
∂ug ~ 92 m/s
y
T
pf
R
p
udg
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Advanced Synoptic M. D. Eastin
Common Jet StreamsLow-level Jets (LLJ):
• Located 500-2000 m AGL• Winds rarely exceed 25 m/s
• Associated with weak horizontal temperature gradients confined to lower levels
• 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
ColdAir
WarmAir
LLJ
WarmAir
ColdAir
J
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Advanced Synoptic M. D. Eastin
Common Jet Streams2. 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
Nocturnal Boundary Layer – Radiational Cooling
WarmAirJ
LLJ
East-West Cross Section 9 June 2002 at 12ZPotential temperature (red contours)
Wind speed (shading)
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Advanced Synoptic M. D. Eastin
Jet StreaksBasic Characteristics:
• Faster moving “pockets” of air embedded within the jet stream• ~250-1000 km in length• ~50-200 km in width
• 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
Jet Stre
am
Jet Streaks
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Advanced Synoptic M. D. Eastin
Physical Interpretation of the Basic Pattern:
• Using a simplified vorticity equation:
• 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
Recall: QG theory provides an alternativeexplanation (with the same result)
Divergence / convergence patterns result from ageostrophic motions
y
v
Dt
D
x
u +
_VortMin
VortMax
JET
VorticityDecrease
VorticityIncrease
VorticityIncrease
VorticityDecrease
JET
Descent
AscentDescent
Ascent
LeftExit
LeftExit
RightExit
RightExit
LeftEntrance
LeftEntrance
RightEntrance
RightEntrance
Jet Streak Vertical Motions
VorticityChange
Divergence
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Advanced Synoptic M. D. Eastin
An Example:
Jet Streak Vertical Motions
Divergence = yellow contoursRegions of expected upward vertical
motion
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Advanced Synoptic M. D. Eastin
An Example:
Jet Streak Vertical Motions
Important Considerations!!!
1. Forcing is at upper-levels2. Forcing is on the synoptic scale3. Is there a mechanism for low-level lift?4. Is the low-level environment moist?
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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
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Advanced Synoptic M. D. Eastin
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
Coupling between Jet Streaks and Fronts
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Advanced Synoptic M. D. Eastin
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
Coupling between Jet Streaks and Fronts
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Advanced Synoptic M. D. Eastin
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
Jet Streaks and Severe Weather
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Advanced Synoptic M. D. Eastin
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.
Jet Streaks and Severe Weather
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Advanced Synoptic M. D. Eastin
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
Jet Streaks and Severe Weather
Left Entrance
Right Entrance
LeftExit
RightExit
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Advanced Synoptic M. D. Eastin
Composite Structure:
Jet Streaks and Severe Weather
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Advanced Synoptic M. D. Eastin
Composite Structure:
Jet Streaks and Severe Weather
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Advanced Synoptic M. D. Eastin
ReferencesBeebe, 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 WeatherSystems. 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 tocoherent 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 forthe development of severe convective storms. Mon. Wea. Rev., 107, 682-703.