Transcript lecture09_jets

Jets Dynamics
Weather Systems – Fall 2015
Outline:
a. Why, when and where?
b. What is a jet streak?
c. Ageostrophic flow associated with jet streaks
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:
1. subtropical jet stream (subtropical jet)
2. polar front jet stream (polar jet)
Jet Streams – General Circulation
SJ:
upper
branch of
Hadley
circulation
PJ:
boundary
between air
masses
polar jet
subtropical
jet
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
Figures created at:
http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl
300 mb GFS 06Z March 8, 2010
Jet Streams – Subtropical Jet
• found at approximately 30° latitude, in both hemispheres
• wind speeds are generally 80-150 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. 10.27
DLA Fig. 10.28
Jet Stream Definitions – Polar Jet
• usually found between 45-65° latitude
• wind speeds are about 80-130 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
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 80-150 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
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 80-150 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
06Z March 8, 2010 – Subtropical Jet
SLP, 1000-500 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 80-150 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
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 80-130 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
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 80-130 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
06Z March 8, 2010 – Polar Jet
SLP, 1000-500 hPa Thickness:
06Z March 8, 2010
•
•
•
•
•
300 mb GFS 06Z March 8, 2010
usually found between 45-65° latitude
wind speeds are about 80-130 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
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
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
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
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
In this x-y (map) figure:
1. isotachs = black contours
2. jet streak = blue area and arrow
3. Z = dark red contours, and H and L
4. cross stream (or transverse) ageostrophic winds
= thick black vectors, with speed given by arrow length
5. upper level divergence = DIV and CONV
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
Jet core is composed of 4 quadrants
(names based on facing downwind):
1. upstream left (or left entrance)
2. upstream right (or right entrance)
3. downstream left (or left exit)
4. downstream right (or right exit)
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
1. Parcels undergo positive (negative) acceleration as they
approach (leave) the jet core.
2. Greatest positive (negative) acceleration is along jet axis.
3. Acceleration means parcels are not in geostrophic balance.
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
assuming no friction:
du
1 ¶p
- fv = = - fvg
dt
r ¶x
du
= f v - vg = fva
dt
where va = v - vg
(
)
dv
1 ¶p
+ fu = = fug
dt
r ¶y
dv
= f ug - u = - fua
dt
where ua = u - ug
(
)
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
du
= fva
dt
dv
= - fua
dt
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
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
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
at jet entrance
at jet exit
Circulation based on:
1. jet level divergence and convergence
2. mass continuity
3. stratospheric cap on vertical motion
Ageostrophic, Divergent, and Vertical Motions Associated With Jets
at jet entrance
thermally direct circulation:
warm air rising/cold air sinking
conversion of APE to KE
at jet exit
thermally indirect circulation:
warm air sinking/cold air rising
conversion of KE to APE
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?