486 Lecture Content for Day 1: Tuesday, January 13, 2009

The lecture covered the following:

  1. Introduction to the Course (see Syllabus)
    1. Important: This is an experiment, so much may change.
    2. I will announce if the Iowa NWA Conference is required.
      1. If so, those who can't attend will have a substitute assignment.
    3. Ray is becoming outdated rapidly.
      1. Strongly suggest buying Bluestein's book
      2. (about $80 on Amazon.com last time I looked)
      3. Subscribe to the AMS Journals!
  2. Organization of the Course
    1. Tuesday is Lecture Day.
      1. This is not a "severe and hazardous weather" storm-chaser-drooling class.
      2. We will be using theory (what there is of it) whenever possible.
      3. We will cover both warm season and cold season mesoscale systems
        1. Since this is the cold season, we'll start there.
    2. Thursday is Lab Day.
  3. Introduction to the Mesoscale (Reference: Ray, Chapter 2)
    1. Mesoscale has become a catch-all for scales of motion between the lower friction layer (small) and mid-latitude cyclones.
    2. What they tend to have in common
      1. Not good theoretical basis
        1. Not enough observations at the mesoscale to make a good theory. (See Fig. 29.7, page 697)
        2. Resolution of any phenomena is "two delta x" (twice the spacing of the obs.)
          1. Polar Front Theory (1919) used nearly 50 years of European observations transmitted by telegraph
          2. Surface data only recently getting close to mesoscale on land.
            1. every 10-20 minutes on land
            2. denser network of surface observations to define some phenomena
            3. still not on the scale of a single thunderstorm.
          3. Radiosonde data is barely synoptic scale in space over land and planetary scale in time.
            1. Information has degraded from the 1950's (4 launches a day at more stations; stationary weather ships and buoys launching radiosondes)
            2. Relying on remote sensing, but technology has not advanced enough to fill in gaps.
        3. Theories are based on unique data sets and methods
          1. Wakimoto's photogrammetry
          2. Fujita's aerial damage surveys
          3. Storm chaser logs converted to numerical models
          4. Original severe storm forecast rules development at Air Force Global Weather Central in 1947-63 by meteorologists looking for reoccuring synoptic scale weather patterns without understanding dynamics.
        4. Conceptual models are key.
      2. Scale of motion is on smaller space and time scales, so geostrophic, hydrostatic balance no longer apply.
        1. Key scales defined by (see class notes for actual terms):
          1. Rossby number (mean wind)/(coriolis force) < 1
          2. Richardson number (thermodynamic stability)/(vertical wind shear) < 1/4
      3. Release of instability is involved.
        1. Perturbation theory only works when the disturbance is small.
        2. Instabilities can grow as large if not larger than the basic state.
        3. Results of instability release can feed back on other scales, altering the synoptic or large mesoscale systems.
        4. Numerical models, even if they have small enough resolution, tend to parameterize mesoscale processes.
          1. Average effect of convection over an area.
          2. Tend not to be correct at a given point once mesoscale process takes over.
          3. Parameterization does not allow sufficient feedback.
          4. Result difficult to separate from computational errors.
  4. Instabilities (Reference: Ray, Chapter 11)
    1. Buoyant
      1. Release produces: cumulus and dry air convection
    2. Inertial
      1. Release produces: clear air turbulence in areas of strong horizontal wind shear
    3. Symmetric
      1. Release produces: intense bands of cold season precipitation.
    4. Kelvin-Helmholtz
      1. Release produces: clear air turbulence in areas of strong vertical wind shear
  5. 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

Last updated: 14-Jan-2009

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