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					                         SST Anomalies




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                                                         SST anomalies from 1984-2010




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SST Anomalies




                0.4                                                                                                                  2002-2003
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                0.3
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                0.2
                                                                                                                                     1982-1983

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                 0
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                                                                                                                                      1998-1999
SST anomalies




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                -0.2
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Convection and Atmospheric
         Stability

      AOS 101 Dis. 305
             Atmospheric Stability
• What is stability?
• Stability refers to a condition of equilibrium
  If we apply some perturbation to a system, how will
  that system be affected?

   – Stable: System returns to original state
   – Unstable: System continues to move away from original
     state
   – Neutral: System remains steady after perturbed
                  Stability Example

      Stable: Marble returns to its original position




Unstable: Marble rapidly moves away from initial position
                                     Stability
How does a bowl and marble relate to the atmosphere??

• When the atmosphere is stable, a parcel of air that is lifted will want to return
back to its original position:




                    http://www.maltaweather.info/cumulus.jpg
                                        Stability

 • When the atmosphere is unstable (with respect to a lifted parcel of air), a
 parcel will want to continue to rise if lifted:




http://blogs.trb.com/news/weather/weblog/wgnweather/archives/051906_cumulus_clouds.jpg
What do we mean by an air parcel?
– Imaginary small body of air a few meters wide
   • Can expand and contract freely
   • Does not break apart
   • Only considered with adiabatic processes - External air
     and heat cannot mix with the air inside the parcel. The
     parcel warms or cools purely due to pressure changes.
   • Space occupied by air molecules inside parcel defines
     the air density
   • Average speed of molecules directly related to air
     temperature
   • Molecules colliding against parcel walls define the air
     pressure inside
            Buoyancy and Stability
• Since Pressure is the same, the only other variable changing is
  temperature. But remember they are on the same side of the
  equation so they are inversely proportional

• So if ρparcel < ρenv. Then the parcel floats or in other words is
  positively buoyant

• In terms of temperature that would mean
If
T of parcel > T of environment the parcel is positively buoyant
   (less dense and will rise) (unstable) (or less stable)
T of parcel < T of environment the parcel is negatively buoyant
   (more dense and will sink) (stable)
T of parcel = T of environment the parcel is neutrally buoyant
   (will not rise or sink) (neutral)
Atmospheric Stability
               Atmospheric Soundings
• Vertical “profiles” of the atmosphere are taken at 0000 UTC (7 AM CDT) and 1200 UTC
(7 PM CDT) at ~80 stations across the country, and many more around the world.
Sometimes also launched at other times when there is weather of interest in the area.
• Weather balloons rise to over 50,000 feet and take measurements of several
meteorological variables using a “radiosonde.”




                                                             http://www.ua.nws.noaa.gov/n
                                                             ws_upper.htm
• Temperature
• Dew point temperature
• Wind Direction and Speed
• Pressure



From these variables we
can find the following:
•Saturation Mixing Ratio
•Mixing Ratio


                             http://www2.ljworld.com/photos/2006/may/24/98598/
                                                           Temp.
                                                           Sensor

              Radiosonde Package
Balloon Attachment point
                                                           Humidity
                                             GPS antenna   Sensor




           Unwinder




  www.chmi.cz/meteo/ oap/eoap_basic.html


                           Reciever/Processo
                           r

                                           Calibration        Transmitter
                                           check box          Antenna
                  Inflating the balloon




http://www.wrh.noaa.gov/re
v/tour/UA/inflation.php
          Baselining the Radiosonde




  Sensor Inspection and Battery Materials,         Radiosonde being Baselined and
  write down the radiosonde calibration            Acquiring GPS Information
  information

http://www.wrh.noaa.gov/rev/tour/UA/baseline.php
                           Balloon Launch


        airport




Reno-Tahoe International Airport
view from Launch Site

 Before the launch, obtain
 measurements of local T,          Wait to get clearance from Federal Aviation
 humidity, and P                   Administration (FAA)


   (http://www.wrh.noaa.gov/rev/tour/UA/launch.php)
                      Balloon Launch

                                           After release the balloon,
                                           activate the tracking system
                                           and monitor the data during
                                           the ascent.




                                           Pre-launch takes about 30 mins, while
                                           sounding may take about 90 mins.




(http://www.wrh.noaa.gov/rev/tour/UA/launch.php)
Vertical Profile of Atmospheric Temperature,
  Allow us to assess Atmospheric Stability

                          We must compare the parcel's
                          temerature Tp with the
                          temperature of the
                          surrounding environment Te.

                          Tp > Te The parcel is positively
                          buoyant, it is less dense and
                          will rise.
                          Tp < Te The parcel is negatively
                          buoyant, it is more dense and
                          will sink.
                          Tp = Te The parcel is neutrally
                          buoyant, it will not rise or
                          sink.
                        Lapse Rates
Lapse Rate: The rate at which temperature decreases with height
(Remember the inherent negative wording to it)


Environmental Lapse Rate: Lapse rates associated with an
observed atmospheric sounding (negative for an inversion layer)


Parcel Lapse Rate: Lapse rate of a parcel of air as it rises or falls
(either saturated or not)
MALR - Moist Adiabatic Lapse Rate: Saturated air parcel
DALR - Dry Adiabatic Lapse Rate: Dry air parcel
                     DALR
• Air in parcel must be unsaturated (Relative
  Humidity < 100%)
• Rate of adiabatic heating or cooling = 9.8°C
  for every 1000 meter (1 kilometer) change in
  elevation
  – Parcel temperature decreases by about 10°C if
    parcel is raised by 1km, and increases about
    10°C if it is lowered by 1km
                    MALR
• As rising air cools, its RH increases because
  the temperature approaches the dew point
  temperature, Td
• If T = Td at some elevation, the air in the
  parcel will be saturated (RH = 100%)
• If parcel is raised further, condensation will
  occur and the temperature of the parcel will
  cool at the rate of about 6°C per 1km in the
  mid-latitudes
                  DALR vs. MALR
• The MALR is less
  than the DALR
  because of latent
  heating
   – As water vapor
     condenses into
     liquid water for a
     saturated parcel,
     LH is released,
     lessening the
     adiabatic cooling    Remember no heat exchanged with environment
DALR vs. MALR
                 Absolute Stability
• The atmosphere is
  absolutely stable when
  the environmental lapse
  rate (ELR) is less than the
  MALR
   – ELR < MALR < DALR
   – A saturated OR
     unsaturated parcel will be
     cooler than the
     surrounding environment
     and will sink, if raised
               Absolute Stability
• Inversion layers are
  always absolutely
  stable
   – Temperature
     increases with height
   – Warm air above cold
     air = very stable
               Absolute Instability
• The atmosphere is
  absolutely unstable
  when the ELR is greater
  than the DALR
   – ELR > DALR > MALR
   – An unsaturated OR
     saturated parcel will
     always be warmer than
     the surrounding
     environment and will
     continue to ascend, if
     raised
             Conditional Instability
• The atmosphere is
  conditionally unstable
  when the ELR is greater
  than the MALR but less
  than the DALR
   – MALR < ELR < DALR
   – An unsaturated parcel will
     be cooler and will sink, if
     raised
   – A saturated parcel will be
     warmer and will continue
     to ascend, if raised
             Conditional Instability
• Example: parcel at
  surface
   – T(p) = 30°C, Td(p) =
     14°C (unsaturated)
   – ELR = 8°C/km for first
     8km
       • Parcel is forced upward,
         following DALR
       • Parcel saturated at 2km,
         begins to rise at MALR
       • At 4km, T(p) = T(e)…this
         is the level of free
         convection (LFC)
              Conditional Instability
• Example continued…
   – Now, parcel will rise on
     its own because T(p) >
     T(e) after 4km
   – The parcel will freely rise
     until T(p) = T(e), again
       • This is the equilibrium
         level (EL)
       • In this case, this point
         is reached at 9km
   – Thus, parcel is stable
     from 0 – 4km and               LCL
     unstable from 4 – 9km          - Lifting
                                    condensation
                                    level
             Lifting by Convection
• As the earth is heated by
  the sun, thermals (bubbles
  of hot air) rise upward from
  the surface
• The thermal cools as it rises,
  losing some of its buoyancy
  (its ability to rise)
• The vertical extent of the
  cloud is largely determined
  by the stability of the
  environment
          Lifting by Convection
• A deep stable layer
  restricts continued
  vertical growth
• A deep unstable layer
  will likely lead to
  development of rain-
  producing clouds
• These clouds are more
  vertically developed
  than clouds developed
  by convergence lifting
          Lifting by Convergence
• Convergence exists
  when there is a
  horizontal net inflow
  into a region
• When air converges
  along the surface, it is
  forced to rise
        Lifting by Convergence
• Large scale convergence can lift air hundreds
  of kilometers across
• Vertical motions associated with convergence
  are generally much weaker than ones due to
  convection
• Generally, clouds developed by convergence
  are less vertically developed
         Lifting due to Topography
• This type of lifting occurs
  when air is confronted by a
  sudden increase in the
  vertical topography of the
  Earth
   – When air comes across a
     mountain, it is lifted up and
     over, cooling as it is rising
• The type of cloud formed is
  dependent upon the
  moisture content and
  stability of the air
  Lifting Along Frontal Boundaries
• Front – The transition zone between two air
  masses of different densities
• Lifting occurs along two different types of
  fronts
  – Cold Front
  – Warm Front
          Lifting Along Cold Fronts




• A colder,denser air mass lifts the warm, moist air ahead of it
• As the air rises, it cools and condenses, producing clouds and
  precipitation
• The steep slope of the cold front leads to more vigorous rising motion
• Hence, cold fronts are often associated with thunderstorms
           Lifting Along Warm Fronts




• A warmer, less dense air mass rises up and over the cold air ahead
  of the warm front
• Air rises, cools and condenses
• Warm fronts have gentler slopes and move slower than cold fronts
• Generally, precipitation is more steady and widespread
         Skew T Log P Diagram
• It is called Skew-T Log-P because the
  temperature axis (x axis) is skewed and the
  pressure axis (the vertical coordinate) is a
  logorithmic axis.
• Area α Energy/Work
• There are several set of lines on a Skew-T Log-
  P diagram and they will be shown in the next
  couple slides.
Moist Adiabatic Lines
Dry Adiabatic Lines
Mixing Ratio Lines
Temperature Lines (Isotherms)



        Isotherms
Lifting Condensation Level (LCL)
 Level at which a parcel lifted from the surface
 would reach saturation (i.e. level at which
 temperature = dewpoint temperature)

 LCL will refer to the height above the ground
 at which the cloud base is located
Level of Free Convection (LFC)
Level at which a parcel lifted from the surface would
be warmer than its environment

Above this level, air is able to freely convect, or
ascend without resistance to the tropopause

        Equilibrium Level (EL)
 Level at which a parcel is no longer warmer than its
 environment, usually is the tropopause

 Above the tropopause, environmental temperature
 increases with height in the stratosphere
 -Corresponds to cloud top height
         12


                        EL
         10

                                     Tparcel
Height (km)



              8

                        Tenv
              6
                                                     LFC
              4

                                               LCL
              2

                                                           Td

                  -40          -30       -20   -10     0    10   20   30

                                 Temperature (C)
         How to read from the diagram?
•   Each point on the plot
    corresponds to a temperature and
    pressure value. Temperature scale
    in blue can be read from the
    bottom of the plot.
•   For example:
• A:                                        A
     – Pressure : 250 mb
     – Temperature : -20 C
• B:
     – Pressure : 600 mb                B
     – Temperature : -10 C
• C:                                        C
     – Pressure : 900 mb
     – Temperature : 25C (Since it is
       exactly in between the line
       when T =20 and T=30 C)
Environmental Temperature Profile
        -80C     -70C -60C -50C -40C   Here are some
                                       measurements:
                                       Pressure   Temp.   Dew.pt

                                       1000       30      10
                                       500        -10     -10
                         Temperatur    200        -65     -80
                         e profile
                                       Notice on each
                                       pressure level, there is
                                       a set of temperature
                                       and dew point
                                       correspond to it. Start
    Dew point
                                       by plotting the
    Temperatur
                                       temperature and dew
    e profile
                                       points at that same
                                       pressure level, and
                                       then connect the
                                       points on the plot.
                                 •   Let’s lift a parcel from 1000mb

Parcel Path                      •
                                     up to the top.
                                     And the parcel at 1000mb has
                                     temperature of 30C and dew
                                     point of 10C.
    -80C   -70C -60C -50C -40C   •   The parcel is unsaturated at
                                     1000mb, therefore when it
                                     rises, it cools at dry adiabatic
                                     lapse rate.
                                 •   At 1000mb, we could read the
                                     mixing ratio value of the parcel
                                     is about 8 g/kg from the plot
                                     (purple lines) at the parcel’s
                                     dew point (10C).
                                 •   Since water vapor inside the
                                     parcel are not used at all to
                                     form any condensates yet when
                                     the parcel is still unsaturated as
                                     it is lifted up, the mixing ratio of
                                     the parcel stays constant.
    LCL                          •   So, from the dew point at
                                     1000mb, follow the MIXING
                                     RATIO line upward, until the dry
                                     adiabatic line crosses with the
                                     mixing ratio line.
Parcel Path
    -80C   -70C -60C -50C -40C
                                 •   Once the parcel is lifted up and
                                     cooled dry adiabatically to be
                                     the same as its dew point
                                     temperature, it reaches
                                     saturation.
                                 •   The pressure level at which the
                                     parcel get saturated is called the
                                     lifting condensation level (LCL).
                                     It also marks the where the
                                     cloud base would be if the
                                     cloud is formed by lifting parcels
                                     from 1000mb in this case.
                                 •   As the SATURATED parcel
                                     continue to rise beyond LCL, it
    LCL                              cools at moist adiabatic lapse
                                     rate.
                                 •   The dotted line represents the
                                     parcel path.

				
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