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									Meteorology 2603

     Lecture 10
  3 February 1999



                    1
                  Today

 Exam 1: Wednesday 10 February
 Review Session:
    – Monday 8 February 1999
    – 7:00 PM
    – EC 1410
 Thermodynamic Diagram
 Stability


                                  2
        Moist Adiabatic Lapse Rate
     In a moist adiabatic process, assume that
      the relative humidity of the air is equal to
      or greater than 100 %.
     Recall: The dry adiabatic lapse rate is
        – -10oC km-1
        – For every 1 km you lift a parcel, the
          temperature of that parcel will fall 10oC.

Example: An unsaturated parcel with a temperature of
5oC is lifted 4.5 km. What is the new temperature of the
parcel?
      5oC + (-10oC km-1)(4.5 km) = 5oC - 45oC = -40oC
                                                           3
    Moist Adiabatic Lapse Rate
 With a moist parcel, the RH100% so
  we assume condensation.
 With condensation there is a release of
  latent heat.
 This release of latent heat, warms the
  parcel.
 The parcel does not cool with height as
  fast as does the dry parcel.

                                            4
    Moist Adiabatic Lapse Rate
   The moist (or wet) adiabatic lapse rate
    is
    – -6oC km-1
    – Note: This is an average value!!!
 Since the concentration of water vapor
  decreases with height, the release of
  latent heat decreases with hight.
 The moist adiabatic lapse rate is not
  constant with height!!! It is curved.

                                              5
            Thermodynamic Diagram
         Moist (pseudo) adiabats (red):
                                p Moist adiabats tell you what happens to the
                                   temperature of moist air (RH=100%) as it
                                   rises. If there is no condensation, the
                                   process is reversible.

      T
          Any time you are going up a moist adiabat, you are making a cloud.
          If you condense out all the moisture, you go down dry adiabat.

Problem:        (a) Moist air rising from the surface (T=12oC) will have a
temperature of _________ at 1 km. (b) If dry, the temperature will be? Why?
       (a)      T = 12oC + (-6oC km-1) x (1 km) = 6oC
       (b)       T = 12oC + (-10oC km-1) x (1 km) = 2oC
                                                                              6
                    Stability
    Stable - Lift an unsaturated parcel and the
     parcel cools dry adiabatically. In a stable
     environment, the parcel will be cooler
     than the environment so the parcel will
     sink and return to it’s original level.
                                 Parcel
Environment

     13C         10C                     1 km




     20C         20C
                                                   7
                  Stability
    Neutral - Lift an unsaturated parcel and
     the parcel cools dry adiabatically. In a
     neutral environment, the parcel will be the
     same temperature as the environment so
     the parcel will stay at the new level.
                                  Parcel
Environment

     10C          10C                    1 km




     20C          20C
                                                   8
                  Stability
   Unstable - Lift an unsaturated parcel and
    the parcel cools dry adiabatically. In an
    unstable environment, the parcel will be
    warmer than the environment so the parcel
    will move away from the original level.
                                Parcel
Environment

      8C         10C                   1 km




      20C        20C
                                                9
           Thermodynamic Diagram
         Stability: To determine the stability you
          must plot a sounding. A sounding is the
          temperature at various heights as
          measured by a balloon-borne radiosonde.

                                 p
                                           The sounding is also
     COLD             WARM                 called the environmental
                                           lapse rate (ELR).
     T

Note: We also plot dew point on the chart -- we’ll get to that later.   10
      Thermodynamic Diagram
   Stability:
    – We can evaluate the stability of an
      atmospheric layer by comparing the
      sounding (blue) to the dry (green) and
      moist (red) adiabats.




                                    CONDITIONALLY
STABLE            UNSTABLE            UNSTABLE      11
           Absolute Stability
   Special Kinds of Absolute Stability
    – Lapse -- The sounding is absolutely stable.
      The lapse rate is just the environmental
      sounding.
    – Isothermal -- The sounding is absolutely
      stable. The temperature is constant with
      height.
    – Inversion -- The sounding is absolutely
      stable. The temperature increases with
      height.

                                                    12
                   Absolute Stability
                    Lapse    Isothermal



    Pressure



                                          Inversion
Dry Adiabatic
 Lapse Rate




 Moist Adiabatic                   Temperature
  Lapse Rate                                          13
    Factors to Increase Stability
   Warming Aloft
    – Radiational absorption
    – Latent heat release
    – Sensible heat transport into a region
   Cooling Below
    – Radiational Cooling
    – Latent heat “absorption”
    – Sensible heat transfer out of a region
    – Air mass moving over a cold surface

                                               14
    Factors to Increase Stability

   Compression of a Layer of Air
    – Top of the layer will heat (by compression)
      more quickly than the bottom of the layer.
    – The lapse rate of the layer decreases.




                                                    15
Factors to Increase Instability
   Warming Below
    – Radiational absorption
    – Latent heat release
    – Sensible heat transport into a region
    – Air mass moving over a warm surface
   Cooling Aloft
    – Radiational Cooling
    – Latent heat “absorption”
    – Sensible heat transfer out of a region

                                               16
Factors to Increase Instability

   Lifting of a Layer of Air
    – Top of the layer will cool (by expansion)
      more quickly than the bottom of the layer.
    – The lapse rate of the layer increases.




                                                   17
            Neutral Stability
   Displace a parcel and it remains where you
    put it.
    – It doesn’t return to it’s original position.
    – It doesn’t fly away from it’s original position.
   Dry Neutral
    – Environmental lapse rate equals the dry
      adiabatic lapse rate.
    – Can result from thoroughly mixing air.
   Moist Neutral
    – Environmental lapse rate equals the dry
      adiabatic lapse rate.
                                                         18
        Convective Instability
   Lift layer with a dry top and a moist
    bottom.
    – The top layer cools dry adiabatically.
    – The bottom layer cools moist adiabatically.
    – Since the top layer cools faster than the
      bottom layer, we see a net cooling aloft.
      This tends to destabilize the atmosphere.




                                                    19
              Thermodynamics

   Thermodynamic Diagram:
    – Handy little diagram -- Has on it:
       » pressure (P), altitude (Z)
       » temperature (T)
       » dry adiabatic lapse rates
             (10C/100 m)
       » moist pseudoadiabatic lapse rates
             (~0.60C/100 m)
       » saturation mixing ratio lines (for RH stuff)



                                                        20
       Thermodynamics
 Thermodynamic      Diagram (cont.):
 – Is used everyday in forecasting
   offices
   »clouds: type, time of formation, bases,
    tops
   »thunderstorms: hail, wind gusts,
    tornadoes
   »precipitation: type, intensity, amount
   »turbulence and icing for aircraft
   »temperatures: max, min
   »fog, freezing rain, rain vs. snow, etc.   21
Norman                   Temperature
                          Sounding
Sounding

3 February   Dew Point
             Sounding
   1999




                                       22
23
      Thermodynamic Diagram

   Construction:

                                     Altitude in Km or
                                     1,000’s of feet


                                     Pressure levels
                                         in mb.

     -400 C                 +300 C
              Temperature                   How high is the
                                            500 mb level?
                                                          24
         Thermodynamic Diagram
       Saturation mixing ratio line (yellow):
                      p
                          The saturation mixing ratio line tells
                          how much H2Ov is in the parcel at
                          a particular P, T if RH = 100%.
T
What is ws at 1000 mb and -100 C?

What is the RH at 1000 mb when T=240 C and Td=130 C?

If T=200 C and RH = 70%, what is Td at 1000 mb?

                                                                   25
         Thermodynamic Diagram
    Dry adiabats (green):
                            p

                             The dry adiabats tell us what happens
                             to the temperature of a dry air parcel
                             as it rises or sinks. (RH<100%)
 T

What is the temperature of a parcel at 1000 mb and T=200C if lifted to
900 mb? to 600 mb?

What will the temperature of a parcel at 600 mb and T= -200C be if it
sinks to 1000 mb?



                                                                         26
Temperature of a parcel
lifted dry adiabatically to
600 mb. Tparcel = -20C




                              Temperature of a parcel at 1000 mb
                                       Tparcel = 20C           27

								
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