Jet Streak Dynamics
Dr. Scott M. Rochette
SUNY Brockport
16 October 2003
Basis of Presentation
• Jet Stream/Streak Basics
• Four-Quadrant Model
• Role of the Ageostrophic Wind
• Effect of Curvature
• Satellite Imagery
• Coupled Jet Streaks (both kinds!)
• Summary
Jet Streams
• Quasi-horizontal, intense, narrow stream of
air, associated with strong vertical wind shear
– Intense: > 25 m s-1 (> 15 m s-1 for lower trop.)
– Narrow: width ~0.5-1 order of magnitude less
than length
– Strong VWS: at least 5-10 m s-1 km-1 (at least 0.5-1
order of magnitude greater than synoptic-scale
shear)
• Typically found at/near tropopause
• Two types in mid-latitudes
– Polar Front Jet (PFJ)
– Sub-Tropical Jet (STJ)
Polar Front Jet
• Associated with polar front, separating polar
cell and Ferrel cell
• Best defined at 250-300 hPa
• During cold season:
– Stronger (can reach/exceed 100 m s-1)
– Farthest south (can approach 30° N)
• During warm season:
– Weaker (~50 m s-1 or less)
– Confined mainly to northern latitudes (~50° N)
• Strong horizontal/vertical temperature
gradients (thermal wind argument)
Thermal Wind Argument



 Rd  ˆ
VT  VgU  VgL   k  Z 
 f 
• Difference in geostrophic wind between two levels
(i.e., vertical wind shear)
• Analogous to geostrophic wind, except parallel to
thickness contours with cold air (low Z) to left
• Proportional to thickness (mean temperature)
gradient
• Westerly wind should strengthen (weaken) with
height below (above) tropopause
• This leads to a relative wind maximum near
tropopause
cold
warm
(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polar_jet_form.html)
(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polar_jet_form2.html)
(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/polarjet_plan.html)
Sub-Tropical Jet
• Found mainly between 20° and 35° N (south
of PFJ) during cold season
• Best defined at 200-250 hPa
• Separates Ferrel cell and Hadley cell
• Speeds approach 70 m s-1
• Relatively steady wrt intensity (cf. PFJ)
• Weak temperature gradients (cf. PFJ)
• Primarily the result of conservation of angular
momentum (spinning skater)
Conservation of Angular
Momentum
M  mvr  constant
• Angular momentum = mass x velocity x radial
distance (distance between object and rotation axis)
• Ice skater pulls arms in close to body  spins faster
• Air near tropopause flows north in upper branch of
Hadley cell
• r decreases  v increases (to hold M constant)
• Coriolis force deflects flow to right (southerly flow
becomes westerly)
-Southerly flow speeds up as it
moves poleward
-Coriolis force deflects flow to
right in NH (westerly)
(Ahrens 1994)
(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/subt_jet_form.html)
-If r , then v must  to keep M
constant
Jet Streaks
• Areas of maximum wind speed
embedded within jet stream
• Move (propagate) through larger jet
stream pattern
• Meso- to Meso- in scale
– ~1000-3000 km long
– ~100-400 km wide
– ~2-3 km deep
Four-Quadrant Model 1
• Assumes straight-line jet streak
(no curvature)
• Two divergent regions
– left exit
– right entrance
• Two convergent regions
– left entrance
– right exit
• Divergence and convergence are created by
ageostrophic portion of wind
– geostrophic wind is essentially nondivergent
Four-Quadrant Model 2
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)
• Dark red  height contours
• Black/shading  isotachs/jet streak
• Black arrows  ageo wind vectors
Ageostrophic Wind 1

 
Vag  V  Vg
• Portion of real wind that departs from
geostrophy
• Three components
– isallobaric (pressure changes)
– inertial-advective (horizontal advection)
– inertial-convective (vertical advection)
Ageostrophic Wind 2


kˆ dV
Vag  
f dt
du
 fvag
dt
dv
  fuag
dt
• Ageostrophic wind is perpendicular and to
left of acceleration vector
• Pay attention to the du/dt term
Ageostrophic Wind 3
du
 fvag
dt
• Westerly wind accelerates (decelerates) in entrance
(exit) region
• Entrance region:
– u increases, so vag is positive (strongest along axis)
– strongly positive vag (southerly) at axis, weaker on either
side
– convergence (divergence) in left (right) entrance region
• Exit region:
– u decreases, so vag is negative (‘strongest’ along axis)
– strongly negative vag (northerly) at axis, weaker on either
side
– divergence (convergence) in left (right) exit region
Ageostrophic Wind 4
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)
• Entrance region:
– ageostrophic wind ‘blows’ from higher to lower
heights (warm to cold air)
– convergence in left entrance region
– divergence in right entrance region
Ageostrophic Wind 5
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)
• Exit region:
– ageostrophic wind ‘blows’ from lower to higher
heights (cold to warm air)
– divergence in left exit region
– convergence in right exit region
Ageostrophic Wind 6
Point A
PGF > CF
(Z increases)
vag > 0
Point C
CF > PGF
(Z decreases)
vag < 0
Point B
(New) PGF = (New) CF
Four-Quadrant Model 2
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)
• Entrance Region: Direct Thermal Circulation
•
•
•
•
Cold air sinks in left entrance region (warms adiabatically)
Warm air rises in right entrance region (cools adiabatically)
Converts potential to kinetic energy
Frontolytic (weakens T)
• Exit Region: Indirect Thermal Circulation
•
•
•
•
Cold air rises in left exit region (cools adiabatically)
Warm air sinks in right exit region (warms adiabatically)
Converts kinetic to potential energy
Frontogenetic (strengthens T)
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)
v u
u
     
x y
y
• Relative vorticity (dark red solid lines)
generated by shear only (no curvature)
• Westerly wind (u > 0, v = 0)
• Positive vorticity advection (PVA) in left exit
and right entrance regions
• Negative vorticity advection (NVA) in left
entrance and right exit regions
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)



 2 f 02  2 
f 
  
 
 Vg   g  f 
2 
 p 
 p

• Vertical motion forced only by differential vorticity
advection (no thermal advection; watch out)
• Winds increase with height below jet level (as do
vorticity and vorticity advection)
• PVA increases with height in left exit and right
entrance regions ( UVM)
• NVA increases with height in left entrance and right
exit regions ( DVM)
(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

 2 f 02  2 
  
    f 0Vg   g
2 
 p 

• Q-G height tendency equation (no thermal adv.):
– PVA  height falls (left exit/right entrance regions)
– NVA  height rises (left entrance/right exit regions)
• Z weakens upstream of jet core (entrance)
• Z strengthens downstream of jet core (exit)
• Jet streak should propagate toward tightening Z
(i.e., downstream)
Rising Motion
• Dines compensation principle:
– upper-level divergence must be
compensated by low-level convergence
cyclone
deepens
cyclone
fills
(http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jetstrk.rxml)
Jet Exit Regions
(Uccellini and Johnson 1979)
• Lower branch of ITC follows isentropes
• Not exactly a ‘box’ as shown previously
• “Think escalators, not elevators.” (Uccellini 1999)
Effects of Curvature
(Shapiro and Kennedy 1981)
• Black arrows  ageostrophic wind vectors
• Ridge: Actual wind is supergeostrophic
• Trough: Actual wind is subgeostrophic
Anticyclonic Jet Streaks
•
•
•
•
Half-wavelength (trough-ridge) decreases with time
Supergeostrophic parcels cannot follow height field
Parcels cut toward lower heights and accelerate
Divergence and UVM increase
Cyclonic Jet Streaks
•
•
•
•
•
Jet streak enters base of trough (cyclonic curvature)
Upstream ageostrophic flow increases
Divergence and UVM increase
New jet streak develops downstream
‘Old’ jet never really makes it ‘around the bend’
Curved Jet Streaks 1
• A: straight-line jet streak
– Four-cell divergence/convergence
pattern
• B: cyclonically-curved jet streak
– Enhanced convergence/divergence on
poleward side
– Equatorward side questionable
• C: anticyclonically-curved jet streak
– Enhanced convergence/divergence on
equatorward side
– Poleward side questionable
(Beebe and Bates 1955)
• 600-hPa 
• A: Straight-line
cyclonic
– UVM in left exit/
right entrance
regions
– DVM in left
entrance/right exit
regions
• B: Cyclonic
straight-line
– Enhanced UVM in
left exit region
– Enhanced DVM in
left entrance region
– Muddled VM on
right side
anticyclonic
• C: Anticyclonic
(Moore and VanKnowe 1992)
– Enhanced UVM in
right entrance
region
– Enhanced DVM in
right exit region
– Muddled VM on
left side
Cyclonic Jets and
Cyclogenesis
(Hovanec and Horn 1975)
• Solid contours  mean spring 300-hPa isotachs
• Stars  areas of cyclogenesis
• Left exit region favorable for cyclogenesis
Satellite Detection of Jet
Streams
• Jet streams evident in:
– Visible imagery
– Infrared (IR) imagery
– Water Vapor (WV) imagery
Polar Front Jet Detection 1
Schematic
Visible + 250-hPa Isotachs
(Bader et al. 1995)
Polar Front Jet Detection 2
Infrared + 500-hPa Z/
Schematic
(Bader et al. 1995)
Polar Front Jet Detection 3
Schematic
Water Vapor + 250-hPa Isotachs
(Bader et al. 1995)
Sub-Tropical Jet Detection 1
Schematic
Infrared + 250-hPa Isotachs
(Bader et al. 1995)
Sub-Tropical Jet Detection 2
Schematic
Water Vapor + 250-hPa Isotachs
(Bader et al. 1995)
Jet Axis Detection 1
(unstable)
(unstable)
(stable)
(stable)
Over Water
Over Land
(Bader et al. 1995)
Jet Axis Detection 2
(unstable)
jet axis
(unstable)
(stable)
(stable)
Over Water
Visible
(Bader et al. 1995)
(Conway 1997)
Coupled Upper/Lower
Jet Streaks
• Low-level jets (LLJs) linked to ULJs
• Found under both entrance and exit
regions of upper-level jets
• Associated with lower branches of
secondary (ageostrophic) circulations
• Result of pressure changes associated
with UL convergence and divergence
upper
(Uccellini and Johnson 1979)
lower entrance
lower exit
• Note southerly ageo component underneath exit
region
(Carlson 1991)
• Note southerly LL branch of ITC
(Carlson 1991)
• Dashed contours  isallobars
• LLJ forms in response to pressure changes via
convergence/divergence from ITC
• Favored with strong ULJ and unstable conditions
(Newton 1967)
• Favorable conditions for severe thunderstorm
development
• Note crossing of ULJ and LLJ
(Shapiro 1982)
•
•
•
•
Vertically ‘uncoupled’ upper/lower jet- front system
ULJ’s exit region west of sfc front/LLJ
Front’s DTC and UL jet’s ITC destructively interact
Limited UVM ahead of front
(Shapiro 1982)
•
•
•
•
Vertically ‘coupled’ upper/lower jet- front system
ULJ’s exit region above sfc front/LLJ
Front’s DTC and UL jet’s ITC constructively interact
Enhanced UVM ahead of surface front
Entrance Region LLJ
(http://www.eas.slu.edu/CIPS/Presentations/Isentropic/jetstreaks/sld017.htm)
• Similar to exit region LLJ (divergence)
• ‘Escalator’ (follows isentropic surfaces)
Coupled LLJs
(http://www.eas.slu.edu/CIPS/Presentations/Others/AMSseminarJan2003/sld019.htm)
• Entrance region LLJ displaced south of jet axis (cf. exit region LLJ)
• Sloped ascent along isentropic surfaces (‘escalators’)
• Response to ULJ’s divergent regions
Coupled Upper-Level Jets 1
• Juxtaposition of divergent regions
– right entrance region of ‘northern’ jet
(DTC)
– left exit region of ‘southern’ jet (ITC)
• Divergent regions interact
• Enhanced divergence in region between
the two jets
(Uccellini and Kocin 1987)
• ‘Typical’ East Coast Heavy Snow Event
• Note position of surface low relative to UL jets
(Uccellini and Kocin 1987)
• Northern DTC  northerly LL ageo wind
• Southerly ITC  southerly LL ageo wind
• Result: enhanced LL frontogenesis and UL divergence
• ‘Early’ coupling
• Enhanced divergence/UVM
(Hakim and Uccellini 1992)
• Later coupling
• Diabatic heating enhances
UVM
• Lower (upper) branch of ITC
(DTC) enhanced
• Efficient northward/
poleward transport of
heat/moisture
• Downward branches weak
(Uccellini and Kocin 1987)
• Surface low not particularly deep
• Note extensive precipitation shield
L
(Uccellini and Kocin 1987)
• Note position of surface low relative to UL jets
• Heavy dashed line  cross-section axis
(Uccellini and Kocin 1987)
• Heavy arrows  300-hPa ageostrophic streamlines
• Hatching  300-hPa divergence
• Note super(sub)geostrophic streamlines in ridge (trough)
(Uccellini and Kocin 1987)
•
•
•
•
Arrows  vertical/ageostrophic circulations
Shading  UVM < - 5 b s-1
Note sloped ascent region over MD/VA
Maximum UVM corresponds to +SN over MD/VA
• 1200 UTC 14 October 2003 Isobars/250-hPa Isotachs
• Note coupled jet structure over Illinois
• Red line denotes cross-section axis
– St. Cloud, MN (STC) to Beckley, WV (BKW)
L
• 0000 UTC 15 October 2003 Isobars/250-hPa Isotachs
• Note rapid motion of surface low (relative to jets)
• Note 11-hPa deepening over 12 hours!
L
L
• 1200 UTC 15 October 2003 Isobars/250-hPa Isotachs
• Note motion of surface low (relative to jets)
• Note 10-hPa deepening over 12 hours [21 hPa (24 h-1)]!
• 1200 UTC 14 October 2003 Isobars/250-hPa Isotachs/
Ageostrophic Wind Barbs
• Note strong ageo flow from cold to warm (warm to cold)
in jet exit (entrance) regions
• 1200 UTC 14 October 2003 Isobars/250-hPa Isotachs/Div.
• Northern AC jet fits conceptual model of con/div
• Southern (broad CYC) jet’s left exit region exhibits strong
divergence, but does not fit conceptual model as well
•
•
•
•
1200 UTC 14 October 2003 Cross-Section (STC-BKW)
Captures entrance region of northern jet only
Note sloped ascent region (similar to Uccellini and Kocin)
Strong UVM, weak DVM
• 1200 UTC 14 October 2003 IR/isobars/thickness contours
• Note comma-cloud shield wrt low pressure center
– well-defined cold front
– warm front not as distinct, but cloud shield visible
• 1200 UTC 14 October 2003 IR/250-hPa Isotachs
• Note sharp edge along axis of northern jet
• 1200 UTC 14 October 2003 IR/250-hPa Isotachs/Divergence
• Note enhanced divergence from entrance/exit interaction
– correlates well to cloudy regions
– some cloud-top divergence possible
– convergent regions correlate well to cloud-free areas
• 1200 UTC 14 October 2003 WV/250-hPa Isotachs
• Note sharp moisture gradient along axis of northern jet
Summary 1
• Jet streaks affect sensible weather
– enhance cyclogenesis
– provide forcing for LLJs
– strengthen/suppress deep UVM
• Rawinsondes/profilers somewhat limited
– rawinsondes: coarse spatial/temporal resolution
– profilers: better spatial/temporal resolution, but
over limited geographic area
Summary 2
• Use satellite imagery/conceptual models/
gridded model data to fill in the ‘gaps’
– visible/IR/WV imagery useful for detection
– conceptual models helpful, but are not universally
applicable
– Rapid Update Cycle (RUC) initializations can be
useful, but caution should be exercised
• as is the case with any model
• conceptual or numerical