EAS 486 Lecture Content for Day 6: Frontogenesis: QG and SG

The lecture covered the following:

  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. Fronts and Jet Streaks (cont)
  3. Quasi-geostrophic frontogenesis
    1. Assumptions
      1. No diabatic effects
      2. Level surface (omega is zero at the lower boundary)
      3. v is replaced by vg in confluence term
      4. Confluence expressed by (-dvg/dy) (all partial derivatives) and is held fixed
    2. Produces odd looking front
      1. Isentropes like "Japanese fan"
      2. Static instability on warm side of front
      3. "e-folding time" for frontal strengthing = 10**5 seconds or 1 day
      4. To get typical frontal gradient takes 2.5 days
        1. In nature, frontal collapse takes a matter of hours
    3. Is positive if Q-vector has a component (Qn) pointing from cold side to warm side of front
    4. So, QG Theory fails for frontogenesis
      1. Any process involving strong frontogenesis would throw off QG views of dynamics (differential vorticity advection, Laplacian of thermal advection; Q-vector analysis)
      2. Especially bad during early frontal development stages.
  4. Semi-geostrophic Theory
    1. Comparison of terms retained in differential equation of change following the motion
      1. QG only allows ageostrophic wind in secondary circulation
      2. SG allows ageostrophic advection of geostrophic winds
      3. Still, Vag cannot appear to the right of a differential term
    2. Sawyer-Eliassen Equation
      1. Forcing Terms
        1. Result of changes in cross-front temperature gradients produced by geostrophic stretching deformation along the front.
        2. Result of changes in cross-front temperatures gradients as geostrophic shearing deformation tilts along front isotherms into the cross-front direction
          1. Usually show frontolysis along warm front; frontogenesis along cold front
        3. Result of differential diabatic heating
      2. Transform to "geostrophic coordinates"
        1. Better resolution near troughs than near ridges (weakness of approximation)
      3. Equation breaks down for
        1. Buoyant Convection
        2. Sharp curvature in parcel trajectories
        3. Straight flow with rapid parcel accelerations (reference: Bluestein and Thomas, Dec. 1984, MWR)

Last updated: February 5, 2009

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