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					                    Lecture 7



Homework Due Friday, October 5, 2012
        Microphysics Problem set (at the end of the lecture 6 )
        TYU Ch 6: 1,2,5,7,11,14,17,18,20;     TYPSS Ch 6: 2
        TYU Ch 7: 1,5,10,11,13,14,15,18,20; TYPSS Ch 7: 1
        TYU Ch 8: 1,2,6,7,12,14,15,18,20;     TYPSS Ch 8: 2
What does “stability” mean?
                           “Air Parcel”
•   Number of molecules remains
    constant
•   Expands as it rises
•   Pressure equilibrates with
    surroundings
•   Density may or may not equal
    surroundings
     – Depends on temperature
     – Larger temperature than
       environment will make
       molecules move faster and
       the pressure higher
     – higher pressure will make
       parcel volume expand more,
       lowering density relative to
       the environment
                      Basic Principle
• Warm air is less dense than colder air at the same pressure
• Weight of air parcel is proportional to its density (more dense air
  weights more for equal volume)
• The decrease in pressure with height creates a net force upward
  that balances the force of an air parcels weight downward.
• Consider a given pressure change with height, in balance with air
  parcels at some environmental temperature T, then
    – A parcel with temperature greater than T, will be less dense than environmental
      parcels and so its weight or downward force will be less than than of
      environmental parcels, and so there will be a net force upward on than parcel
      causing it to rise!
    – The opposite is true for a cold parcel
• Bottom line: warm air parcels (relative to air parcels surrounding it at
  the same pressure) will rise and cold air will sink!
         Adiabatic Process
• A thermodynamic process, where there is
  no flux of energy or mass across the walls
  of the parcel. Therefore, the parcel can
  exchange energy with the surroundings
  only through performing work on the
  surroundings or vise versa.
      Dry Adiabatic Process
• Adiabatic process where no phase
  changes of water take place
     Moist Adiabatic Process
• Adiabatic process where phase changes
  of water take place, causing the internal
  energy (temperature) to rise as the vapor
  condenses or vise versa
Lapse Rate
 • Lapse rate is the rate at which temperature
   decreases with height
 • Environmental lapse rate is that measured
   by a thermometer at different pressures (or
   height)
 • Parcel lapse rate is that occurring within a
   parcel as it rises, expands and cools
     –   Pressure change causes air to expand and cool,
         conserving its total energy
     –   This causes temperature of a dry air parcel to decrease at
         the dry lapse rate of 9.8K/km
     –   If the parcel is saturated, than latent heat released as
         water condenses and so the moist lapse rate is < 9.8K km
     –   Actual moist lapse decreases with temperature, but a
         typical (in cloud) moist lapse rate is ~ 5K/km
     –   Average (tropospheric) environmental lapse rate is
         ~5.5K/km
     –   Typical stratospheric environmental lapse rate is ~0K/km
 For stability we compare parcel
lapse rate to environmental lapse
                rate
                         UNSTABLE   STABLE


                            DRY         DRY
Stability depends on
a parcel’s lapse rate
    compared to
environmental lapse
         rate


                        UNSTABLE    STABLE


                           DRY          MOIST
Conditional
 Instability
 Stuve Diagram

• Compare parcel lapse
  rate to environmental
  lapse rate
          LCL and LFC


                            UNSTABLE




 Positive area (parcel
 warmer than environment)
                                       LFC
                                                  Negative area (parcel
                                                  colder than environment)


                                        Conditionally
LCL                                     Unstable parcel
                                        (after being forced up to
                                        LFC, parcel becimes
                                        unstable)
 Mechanisms
to lift a parcel
     to LFC

				
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posted:5/24/2013
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