EAS 486 Lecture Content for Day 4: Definition and Slope of a Front

The lecture covered the following:

  1. Fronts and Jet Streaks
    1. Problem: Mesoscale in one dimension; synoptic-scale in another dimension
    2. Definition of a front
      1. Elongated area of strong temperature gradient (or other parameter such as q, density, theta, theta-E) and relatively large static stability and cyclonic vorticity
        1. Strong =>At least one order of magnitude greater than synoptic-scale (10°K/1000 km)
        2. Elongated => Length at least one-half order of magnitude greater than width
    3. Definition of a jet
      1. Intense, narrow, quasi-horizontal channel of wind associated with strong vertical shear
        1. Intense => At least 30 m/s for upper-troposphere; at least 15 m/s for lower troposphere
        2. Narrow => Width one-half to one order of magnitude less than length
        3. Strong vertical shear => At least 5-10 m s-1 km-1; one-half to one order of magnitude greater than synoptic-scale shear
      2. Jet Streak - Above definition with isotach maximum
    4. Both features cross over scales
      1. Synoptic scale length O(1000 km)
      2. Mesoscale width O(100 km)
      3. Need synoptic-scale, quasi-geostrophic, mesoscale dynamics to analyze
      4. High definition resolution of fronts may indicate even smaller scale.
        1. Reference: Shapiro (1984, MWR) found the scale of front contrast to be on the order of 100 m
      1. Methodology to handle situation: assume geostrophic/thermal wind balance across front; ignore along front differences
    5. Frontal Slope
      1. Coordinate system
        1. x parallel to front
        2. y perpendicular to the front
      2. Derivation of Frontal Slope (see class notes) implies :
        1. Have to have a cross front break in the pressure gradient with a stronger pressure gradient on the cold side
        2. Cyclonic wind shear across front.
        3. Vorticity at front approaches infinity in this model
      3. Margules' Formula (see class notes) implies:
        1. Would seem to mean that stronger fronts (larger temperature gradients) would have shallower slope (more horizontal)
        2. Not always likely since stronger vorticity likely across front
        3. Do see that southern extent of arctic surge often marked by shallow frontal slope (most of cold air confined to lowest 150-300 mb)
        4. Front must slope towards cold air with height
          1. Theory says can't have a negative slope since front is defined as being statically stable.
          2. Occasional mesoscale front can briefly have a slope over warm air.
          3. Cold Frontogenesis Aloft (CFA)
            1. Key idea in STORM model adjustment to Norwegian polar front model (References: Locatelli et al., June 2002, MWR; Locatelli et al., Feb. 2002, WAF; and many earlier articles)
            2. "Tipped Forward" cold front: lower-tropospheric cold front leads surface cold front
            3. Unstable lapse rate leads to convective development ahead of cold front
            4. I've never seen it in observations, including in cases cited. (Locatelli et al., June 2002 could only find it in model output)
            5. Other elements of STORM model (References: Martin et al., Wong et al., Locatelli et al., all in 1995 MWR)
              1. Drytrough
              2. Arctic Front

Last updated: February 22, 2007

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