jet streak dynamics

Jet Streak Dynamics

Dr. Scott M. RochetteSUNY Brockport16 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

• QuasiQuasi-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 Associated with polar front, separating polar cell and Ferrel cellcell 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

• 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

Zkf

RVVV dgLgUT

ˆ

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

• 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)

constantmvrM

(Ahrens 1994)

(http://apollo.lsc.vsc.edu/classes/met130/notes/chapter11/subt_jet_form.html)

-Southerly flow speeds up as it moves poleward-Coriolis force deflects flow to right in NH (westerly)

-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


• Dark red height contours• Black/shading isotachs/jet streak• Black arrows ageo wind vectors

(http://courses.ncsu.edu/classes/mea444-sekoch/Jets_Basics/straightjet.html)

Ageostrophic Wind 1

• Portion of real wind that departs from geostrophy

• Three components– isallobaric (pressure changes)– inertial-advective (horizontal advection)– inertial-convective (vertical advection)

gag VVV

Ageostrophic Wind 2

• Ageostrophic wind is perpendicular and to left of acceleration vector

• Pay attention to the du/dt term

dt

Vd

f

kVag

ˆ

agfvdt

du

agfudt

dv

Ageostrophic Wind 3

• 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 is perpendicular and to left of acceleration vector• Pay attention to the du/dt term

agfvdt

du

Ageostrophic Wind 4

• 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

• 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 APGF > CF

(Z increases)vag > 0

Point CCF > PGF

(Z decreases)vag < 0

Point B(New) PGF = (New) CF


• 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)


• 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

y

u

y

u

x

v


• 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)

fVp

f

p

fgg

2

2202


• 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)

ggVfp

f

02

2202


Rising Motion

• Dines compensation principle:– upper-level divergence must be

compensated by low-level convergence

(http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jetstrk.rxml)

cyclone deepens

cyclone fills

Jet Exit Regions

• Lower branch of ITC follows isentropes• Not exactly a ‘box’ as shown previously• “Think escalators, not elevators.” (Uccellini 1999)

(Uccellini and Johnson 1979)

Effects of Curvature

• Black arrows ageostrophic wind vectors• Ridge: Actual wind is supergeostrophic• Trough: Actual wind is subgeostrophic

(Shapiro and Kennedy 1981)

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

– UVM in left exit/ right entrance regions

– DVM in left entrance/right exit regions

• B: Cyclonic– Enhanced UVM in left exit

region– Enhanced DVM in left entrance

region– Muddled VM on right side

• C: Anticyclonic – Enhanced UVM in right entrance

region– Enhanced DVM in right exit

region– Muddled VM on left side

straight-line

cyclonic

anticyclonic

(Moore and VanKnowe 1992)

Cyclonic Jets and Cyclogenesis

• Solid contours mean spring 300-hPa isotachs• Stars areas of cyclogenesis• Left exit region favorable for cyclogenesis

(Hovanec and Horn 1975)

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)


Schematic Infrared + 500-hPa Z/

(Bader et al. 1995)


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

Over Land

(Bader et al. 1995)

Over Water

(unstable)

(stable)

(unstable)

(stable)

Jet Axis Detection 2

Visible

(Bader et al. 1995)

Over Water

(unstable)

(stable)

(unstable)

(stable)

jet axis

(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

• Note southerly ageo component underneath exit region

upper

lower entrance lower exit

(Uccellini and Johnson 1979)

• 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

(Carlson 1991)

• Favorable conditions for severe thunderstorm development

• Note crossing of ULJ and LLJ

(Newton 1967)

• 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

(Shapiro 1982)

Entrance Region LLJ

• Similar to exit region LLJ (divergence)

• ‘Escalator’ (follows isentropic surfaces)

(http://www.eas.slu.edu/CIPS/Presentations/Isentropic/jetstreaks/sld017.htm)

Coupled LLJs

• Entrance region LLJ displaced south of jet axis (cf. exit region LLJ)

• Sloped ascent along isentropic surfaces (‘escalators’)• Response to ULJ’s divergent regions

(http://www.eas.slu.edu/CIPS/Presentations/Others/AMSseminarJan2003/sld019.htm)

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

• ‘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

• Later coupling• Diabatic heating enhances

UVM• Lower (upper) branch of

ITC (DTC) enhanced • Efficient northward/

poleward transport of heat/moisture

• Downward branches weak

(Hakim and Uccellini 1992)

• Surface low not particularly deep• Note extensive precipitation shield


• Note position of surface low relative to UL jets• Heavy dashed line cross-section axis


L

• Heavy arrows 300-hPa ageostrophic streamlines• Hatching 300-hPa divergence• Note super(sub)geostrophic streamlines in ridge

(trough)


• 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)


• Note rapid motion of surface low (relative to jets)• Note 11-hPa deepening over 12 hours!

L


• Note motion of surface low (relative to jets)• Note 10-hPa deepening over 12 hours [21 hPa (24

h-1)]!

L

L

• 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

jet streak dynamics

Documents

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portion of real wind

jet streammove

straightline jet

jet streak dynamicsdr

larger jet stream patternmeso

right southerly flow