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

Jet Streams and Jet Streaks


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


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


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)


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


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


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


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


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)


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


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


An Example:


Divergence = yellow contoursRegions of expected upward vertical

motion


An Example:


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?


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


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


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


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


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.



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

Right Entrance

LeftExit

RightExit


Composite Structure:



Composite Structure:



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.

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Jet Streams and Jet Streaks