EAS 486 Lecture Content for Day 3: Conditional Symmetric Instability and Kelvin-Helmholtz Instability

The lecture covered the following:

  1. Instabilities in the Atmosphere (Ray, Chapter 11) Continued
    1. Symmetric Instability
      1. Combination of inertial and buoyant instabilities when neither stability criterion satisfied.
      2. Middle ground for a "tube of fluid" displaced between momemtum surfaces and theta surfaces. (Bluestein, Fig. 2.70)
      3. Unstable tube of fluid slowly accelerates when theta surfaces slope more than momentum surfaces.
      4. In practice, never happens for unsaturated atmosphere
      5. Bluestein (Chap. Ray) says favorable conditions include: high vertical wind shear (M-surfaces more horizontal), low static stability (theta surfaces vertical), and large anticyclonic wind shear (weak inertial instability)
        1. Wouldn't happen concurrently
      6. Conditional Symmetric Instability
        1. Instability condition is when theta-E surfaces slope more than momentum (often replaced by geostrophic momentum) surfaces. (Moore and Blakely, Nov. 1988, MWR)
        2. Restructure sounding to be taken along an M-surface, then it would look just like buoyant stability assessment (Emanuel, 1983)
        3. Alternate formulations
          1. Richardson number > 1 for stability
          2. Equivalent Potential Vorticity (EPV) negative if buoyantly stable (Moore and Lambert, 1993, WAF)
            1. See this calculation on the NWS HPC Model Diagnostics Page
            2. If EPV < 0 for any reason, some kind of convective is possible
        4. Produces narrow bands of intense cold-season precipitation
          1. Frontogenesis produces single band; CSI produces multiple bands
        5. Very rare, but occurs in association with frontogenesis
    2. Kelvin-Helmholtz Instability
      1. Source: Vertical wind Shear in area of low static stability
        1. Instability condition: Ri < 1/4
        2. Vortex sheet displaced displaced to get constructive reinforcement of ascent and descent areas (diagram in class)
      2. Net result: Dangerous turbulence for aircraft
        1. Most likely to occur near jet streak or in lee of mountains (mountain waves)
        2. Complicated interaction between Kelvin-Helmholtz Waves and stable gravity waves reflecting off boundaries, e.g., surface, tropopause
      3. Lifetime of waves: a few minutes

Last updated: January 26, 2009

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