1. Jets Dynamics Weather Systems – Fall 2017
Outline:
a. Why, when and where?
b. What is a jet streak?
c. Ageostrophic flow associated with jet streaks

2. 2 jet streams come up frequently when discussing weather:
Jet Stream Definitions
Jet Stream: relatively strong winds concentrated within a narrow stream in the atmosphere. While this term may be applied to any such stream regardless of direction (including vertical) or altitude, it generally refers to a quasi-horizontal region of maximum winds embedded in the midlatitude westerlies and concentrated in the upper-atmosphere
2 jet streams come up frequently when discussing weather:
subtropical jet stream (subtropical jet)
polar front jet stream (polar jet)

3. upper branch of Hadley circulation
Jet Streams – General Circulation
subtropical jet
SJ:
upper branch of Hadley circulation
polar jet
PJ: boundary between air masses

4. can meander significantly, generally not continuous around the globe
Jet Streams – General Characteristics
can meander significantly, generally not continuous around the globe
speeds vary significantly
large seasonal variation
vertical and horizontal wind shear often large in and on the flanks of jet streams
there are unique temperature, vertical motion, turbulence, clouds and precipitation patters associated with jets
300 mb GFS 06Z March 8, 2010
Figures created at:

5. found at approximately 30° latitude, in both hemispheres
Jet Streams – Subtropical Jet
found at approximately 30° latitude, in both hemispheres
wind speeds are generally knots (40-75 m/s), but can be much greater
generally NOT associated with surface frontal features
often marked by high-level transverse bands of cirrus clouds that form on the warm side of the subtropical jet
Associated with the Hadley Circulation and largely due to conservation of angular momentum
(see Module 7)
DLA Fig
DLA Fig

6. usually found between 45-65° latitude
Jet Stream Definitions – Polar Jet
usually found between 45-65° latitude
wind speeds are about knots (40-65 m/s), but may be as high as 180 knots (90 m/s)
large vertical extent
associated with the large temperature (p, density, thickness) gradients associated with the transition zone between 2 air masses
can be associated with surface frontal boundaries
DLA Fig. 7.24

7. 06Z March 8, 2010

8. IR: 06Z March 8, 2010 300 mb GFS 06Z March 8, 2010
06Z March 8, 2010 – Subtropical Jet
IR: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
found at approximately 30° latitude, in both hemispheres
wind speeds are generally knots (40-75 m/s), but can be much greater
generally NOT associated with surface frontal features
often marked by high-level transverse bands of cirrus clouds that form on the warm side of the subtropical jet

9. WV: 06Z March 8, 2010 300 mb GFS 06Z March 8, 2010
06Z March 8, 2010 – Subtropical Jet
WV: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
found at approximately 30° latitude, in both hemispheres
wind speeds are generally knots (40-75 m/s), but can be much greater
generally NOT associated with surface frontal features
often marked by high-level transverse bands of cirrus clouds that form on the warm side of the subtropical jet

10. SLP, 1000-500 hPa Thickness: 06Z March 8, 2010
06Z March 8, 2010 – Subtropical Jet
SLP, hPa Thickness: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
found at approximately 30° latitude, in both hemispheres
wind speeds are generally knots (40-75 m/s), but can be much greater
generally NOT associated with surface frontal features
often marked by high-level transverse bands of cirrus clouds that form on the warm side of the subtropical jet

11. IR: 06Z March 8, 2010 300 mb GFS 06Z March 8, 2010
06Z March 8, 2010 – Polar Jet
IR: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
usually found between 45-65° latitude
wind speeds are about knots (40-65 m/s), but may be as high as 180 knots (90 m/s)
large vertical extent
associated with the large temperature (p, density, thickness) gradients associated with the transition zone between 2 air masses
can be associated with surface frontal boundaries

12. WV: 06Z March 8, 2010 300 mb GFS 06Z March 8, 2010
06Z March 8, 2010 – Polar Jet
WV: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
usually found between 45-65° latitude
wind speeds are about knots (40-65 m/s), but may be as high as 180 knots (90 m/s)
large vertical extent
associated with the large temperature (p, density, thickness) gradients associated with the transition zone between 2 air masses
can be associated with surface frontal boundaries

13. SLP, 1000-500 hPa Thickness: 06Z March 8, 2010
06Z March 8, 2010 – Polar Jet
SLP, hPa Thickness: 06Z March 8, 2010
300 mb GFS 06Z March 8, 2010
usually found between 45-65° latitude
wind speeds are about knots (40-65 m/s), but may be as high as 180 knots (90 m/s)
large vertical extent
associated with the large temperature (p, density, thickness) gradients associated with the transition zone between 2 air masses
can be associated with surface frontal boundaries

14. Jet Stream Definitions
Previous slides have been describing the subtropical and polar jets as if they are always distinctly different entities. In truth, they can and often do merge.
300 mb GFS 06Z March 8, 2010

15. Why Jet Streams Exist
We have already qualitatively discussed how T / density / pressure /wind relationships can explain the presence of a jet stream  large horizontal temperature / density gradients produce large height and pressure gradients and, thus strong winds. We now have the tools to do so quantitatively:
Hyspometric Equation: relates the mean T of a layer to the thickness and GPE. Helps to explain the creation of large height/pressure gradients in the upper-troposphere
Thermal Wind Equation: relates the horizontal T structure to the vertical wind shear of the geostrophic wind. Helps to explain the increase in wind speed with height in the troposphere, the jet max at the tropopause, and the subsequent decrease above the tropopause
Geostrophic Balance: relates the PGF to wind speed and direction. Helps to explain the jet max at tropopause level and the tendency for westerlies in the midlatitudes

16. Jet Streak: zone of extra-strong winds within a jet stream
Isotach Analysis of Jets
Jet Streak: zone of extra-strong winds within a jet stream
Jet streaks move eastward at speeds:
slower than the actual wind speeds
faster than longwave propagation
through the longwave troughs and ridges
12Z March 9, 2010
18Z March 9, 2010
00Z March 10, 2010

17. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
In this x-y (map) figure:
isotachs = black contours
jet streak = blue area and arrow
Z = dark red contours, and H and L
cross stream (or transverse) ageostrophic winds
= thick black vectors, with speed given by arrow length
5. upper level divergence = DIV and CONV

18. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
Jet core is composed of 4 quadrants
(names based on facing downwind):
upstream left (or left entrance)
upstream right (or right entrance)
downstream left (or left exit)
downstream right (or right exit)

19. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
Parcels undergo positive (negative) acceleration as they approach (leave) the jet core.
Greatest positive (negative) acceleration is along jet axis.
Acceleration means parcels are not in geostrophic balance.

20. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
assuming no friction:

21. winds are accelerating  va is positive
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
Entrance Region:
Exit Region:
winds are accelerating  va is positive
winds are accelerating most along the jet axis, hence, largest va occurs there
winds are decelerating  va is negative
winds are decelerating most along the jet axis, hence, largest va occurs there

22. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
at jet entrance
at jet exit
Vertical cross sections perpendicular to the jet:
left panel: jet entrance (A-A’ = poleward-equatorward)
right panel: jet exit (B-B’ = poleward-equatorward)
J: jet core
air mass boundaries: brown contours
Thermally DIRECT / INDIRECT CIRCULATION indicated

23. Ageostrophic, Divergent, and Vertical Motions Associated With Jets
at jet entrance
at jet exit
Circulation based on:
jet level divergence and convergence
mass continuity
stratospheric cap on vertical motion

24. thermally direct circulation:  warm air rising/cold air sinking
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
at jet entrance
at jet exit
thermally direct circulation:
 warm air rising/cold air sinking
 conversion of APE to KE
thermally indirect circulation:
 warm air sinking/cold air rising
 conversion of KE to APE

25. Jets, Transverse Ageostrophic Circulations, and Snow Storms
Two jet cores and their associated Vag circulations. Can you see why this combination of Vag circulations from the two jets is favorable for snowfall in between the jets?

26. MEPS Probability of
10 m winds > 21 m/s

27. MSLP & 200 hPa winds
00 UTC Saturday

28. MSLP & 12 hour precipitation (mm)
00 UTC Saturday

29. MSLP & 200 hPa winds
00 UTC Sunday

30. MSLP & 200 hPa winds
00 UTC Monday

32. 10 m winds > 21 m/s
10 m winds > 24 m/s
10 m winds > 28 m/s

33. Member 1
Member 2