EAS 486 Lecture Content for Day 7: Jet Streaks and Coupled Jet Circulations

The lecture content included:

  1. Exam 1 Thursday Feb 26
    1. Material on Instabilities, fronts and jets
    2. 8 1/2 x 11 inch equation card can be used
      1. Physical meaning and application of equations to weather maps are far more likely questions than derivation
  2. Jet Streaks
    1. Development
      1. Form in frontogenetical when two flows come together.
      2. Laplacian of ageostrophic wind yields increase of jet core velocity when ageostrophic circulation crosses region of stretching deformation from warm to cold air. => Jet-front system
    2. Key point: air parcels are moving through jet streak more rapidly than the streak is propagating
    3. Vertical circulation about jet streak
      1. See Moore and VanKnowe, 1992, MWR about straight vs. curved jet streaks and how poorly QG theory handles the dynamics.
      2. See Bluestein and Thomas, 1984, MWR about inability of QG dynamics to handle straight jet streak involving large acceleration and deceleration
      3. Circulation can be diagnosed using the Vorticity Equation when neglecting the tilting term.
        1. Rate of change of absolute vorticity in a parcel = - (absolute vorticity)(divergence)
          1. Trick: View air parcels streaming through jet streak, not look at local advection.
        2. Left Entrance region: Parcels experience increasing vorticity as they approach the jet core => convergence to give positive sign to right-hand side of equation
        3. Left Exit region: Parcels experience decreasing vorticity as they leave the jet core => divergence to give negative sign to right-hand side of equation
        4. Right Entrance region: Parcels experience decreasing vorticity as they enter the jet core => divergence to give negative sign to right-hand side of equation
        5. Right Exit region: Parcels experience increasing vorticity as they leave the jet core => convergence to give negative sign to right-hand side of equation
        6. Entire right side of jet circulation will reverse if inertial instability present (absolute vorticity < 0)
      4. Secondary circulation described by equation of Va
        1. This form subject to approximations for QG motion (Vg replaces V on RHS; no omega)
        2. This form subject to approximations for SG motion (Va, omega allowed to advect momentum, but not allowed in derivatives; Reference Bluestein and Thomas, 1984, MWR)
        3. When reexpressed, this equation shows that a component of the isallobaric wind can force a secondary circulation that intensifies jet streak
        4. Book covers QG approach of secondary circulation
      5. Propagation
        1. QG Height tendency equation shows that the 3-D Laplacian of the height tendency is the result of
          1. Advection of geostrophic vorticity
          2. Vertical differential geostrophic temperature advection
            1. Upward increase of warm-air advection produces height falls (since actual height tendency would be the opposite sign as the 3-D Laplacian)
          3. If the upper troposphere is considered near barotropic, the geostrophic advection of vorticity forces the the confluent pattern to move downwind.
        2. Pettersen (1936) shows the propagation speed c depends on:
          1. The change of height tendency downwind (numerator)
          2. Laplacian of height change along jet (denominator)
          3. Larger height tendencies (stronger jet streak) produces more negative (slower versus basic current) propagation speed
          4. Stronger isotach gradient along jet produces less negative propagation speed (denominator bigger)
          5. Questionable application due to limited use of QG theory in jet/front systems.
    4. Low-level jet streaks
      1. Forced by:
        1. Upper-level jet streak (Beebe and Bates 1955; Uccelini and Johnson 1979)
          1. Lower tropospheric portion of thermally indirect circulation in the exit region of straight jet streak (coupled jet circulation)
          2. Some form of accelerated low-level warm air flow under divergent portion of curved jet streak circulation
          3. Coupling may not reach the surface in all cases
            1. Small static stability assists deeper coupling.
          4. Bluestein (Fig. 2.105) has diagram of crossing ULJ and LLJ
            1. Ascent portion of each reinforces one another, leading to the often-seen convection in this pattern.
            2. Produces katafront (clouds in warm sector ahead of front)
          5. Bluestein has diagram of parallel ULJ and LLJ
            1. Small lift due to convergence ahead of LLJ is suppressed due to sinking induced by ULJ.
            2. In warm sector, convection suppressed with hot surface temperatures.
            3. Produces anafront (clouds in cold sector behind front)
        2. Inertial oscillation forced by diurnally varying (Bonner 1968)
          1. Wind speed at the top of the boundary layer
            1. Stronger at night due to inversion creating near-frictionless layer
            2. Weaker during the night as mixed layer extends upward, mixing momentum into the friction layer, reducing wind speed.
          2. Thermal wind component
            1. Day: warmer over Rockies, cooler over Plains forces northerly component of Vth, opposing mean wind
            2. Night: cooler over Rockies, warmer over Plains forces southerly component of Vth, accelerating mean wind
          3. Inertial oscillation about mean southerly wind during the warm season includes:
            1. Acceleration of wind speed overnight
            2. Veering of LLJ from southerly in evening to southwest late at night
        3. Low-level blocking and deflection by mountain ranges
          1. Barrier LLJ:
            1. Oriented perpendicular to isobars/contours down the local pressure gradient.
            2. Propels cold air southward
              1. east of Rockies: "Blue Norther" or "Texas Norther"
              2. east of Appalachians: assists cold air damming in the strongest storms.
      2. Low-level jet streaks are not lifting mechanisms or directly proportional to buoyant instability.
        1. When Gulf of Mexico has mT easily available, they can advect moisture efficiently into the Plains to the east of the Rockies.
        2. Moist axis and warm axis not coincident with position of LLJ (often to the east)
        3. Moisture convergence poleward of LLJ, but not that strong a lifting mechanism.
        4. LLJs associated with severe weather as a rule of thumb, but do not, in of themselves, cause ascent nor destabilize the atmosphere.

Last updated: February 19, 2009

Return to EAS 486 Page

Send comments to Bob Weisman