Jets DynamicsWeather 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

subtropical jet

SJ:upper

branch of Hadley

circulation

polar jetPJ:

boundary between air

masses

Jet Streams – General Characteristics

Figures created at:http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl

• 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

Jet Streams – Subtropical Jet

DLA Fig. 10.28

Associated with the Hadley Circulation and largely due to

conservation of angular momentum

(see Module 7)

DLA Fig. 10.27

• 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

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

300 mb GFS 06Z March 8, 2010

IR: 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

300 mb GFS 06Z March 8, 2010

WV: 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

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

SLP, 1000-500 hPa Thickness: 06Z March 8, 2010

06Z March 8, 2010 – Polar Jet

300 mb GFS 06Z March 8, 2010

IR: 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

300 mb GFS 06Z March 8, 2010

WV: 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

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

SLP, 1000-500 hPa Thickness: 06Z March 8, 2010

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 00Z March 10, 201018Z March 9, 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:

Ageostrophic, Divergent, and Vertical Motions Associated With Jets

Entrance Region:

• winds are accelerating va is positive

• winds are accelerating most along the jet axis, hence, largest va occurs there

Exit Region:

• 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

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 contoursThermally DIRECT / INDIRECT CIRCULATION indicated

at jet entrance at jet exit

Ageostrophic, Divergent, and Vertical Motions Associated With Jets

Circulation based on:

1. jet level divergence and convergence2. mass continuity3. stratospheric cap on vertical motion

at jet entrance at jet exit

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

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?

Jets, Transverse Ageostrophic Circulations, and Snow Storms