EAS 486 Lecture Content for Day 2: Thursday, January 15, 2009

The lecture covered the following:

  1. Instabilities in the Atmosphere (Ray, Chapter 11)
    1. Buoyant Instability
      1. Depends on Brunt-Vaisala Frequency, N (see equations from class)
        1. Function of theta (dry) or theta-E (moist)
        2. Assumptions: atmospheric hydrostatic and incompressible
      2. Release of instability results in saturated buoyant convection (thunderstorms) or unsaturated buoyant convection (thermals)
      3. Net effect: heat and moisture mixed upward; momentum mixed downward
      4. Effects in the real atmosphere complicated by:
        1. Friction and diffusion
          1. Slow down thermals
          2. Expand region influenced by thermals
        2. Entrainment
          1. Reduces effect of thermals (although can enhance if very dry air entrained into cloud)
        3. Environmental pressure is affected by thermal buoyancy
          1. Pressure gradient develops about updraft
          2. Example: MCC
      5. Buoyant Energy
        1. Sum of area on thermodynamic diagram between sounding and parcel trace
        2. Convective Inhibition (CIN)
        3. Convective Available Potential Energy (CAPE)
          1. Net Buoyant Energy doesn't work.
          2. Need to put in CIN to get CAPE released.
        4. Parcel trace crucial (have to lift the right parcel)
      6. Other Terms Applied to Soundings
        1. Lifting Condensation Level (LCL)
        2. Convective Condensation Level (CCL)
        3. Level of Free Convection
        4. Equilibrium Level
    2. Inertial Instability
      1. Instability due to gradients of momentum
      2. Absolute Momentum M = u - fy in a cross-section perpendicular to the jet
      3. Stability refers to a tube of fluid perpendicular to the flow
      4. Stability condition: Absolute vorticity is positive (see class derivation)
      5. Rare occurrence where absolute vorticity is negative
        1. Anticyclonic shear side of intense jet streak (most frequently over ocean in 90+ m/s jet)
        2. Severe clear air turbulence
      6. Inertial stability (constant M) maintained by:
        1. air flow towards lower heights into jet streak ("downhill acceleration")
        2. air flow towards higher heights into jet streak ("uphill deceleration ") Example: 200 mb US chart from 20-Feb-2007

Last updated: January 20, 2009

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