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ATOC 1050 by wuxiangyu

VIEWS: 30 PAGES: 93

									Weather and the
 Atmosphere

NSAP Short Course
 for SEs and SAs
      About the course

• Selected topics and lectures from a CU
  course – ATOC 1050 (45, 50-min.
  classes).
• Because of varied backgrounds – no
  significant memory of calculus or
  differential equations is assumed
• Please interrupt with short questions when
  necessary.
• Course content goes beyond what is
  applicable for any individual RAL project,
  but most is related to some project.
• Terminology is important as well as
  concepts.
• Course web site with notes at
  http://www.rap.ucar.edu/general/events.h
  tml
                    Content
SESSION 1
• Part 1 – Composition, mean structure, variables,
    diurnal and annual cycles
•   Part 2 – Water in the atmosphere, cloud and
    precip formation
SESSION 2
• Part 3 – Winds and their cause; air-masses,
    fronts and cyclones
•   Part 4 – Mesoscale processes, severe weather
     Part 1
Composition of the
  Atmosphere
The atmosphere – A molecular cocktail
 COMPOSITION OF THE ATMOSPHERE
     NEAR EARTH’S SURFACE

“Permanent” gases (percent is about the same
  everywhere)
• Nitrogen – 78 %
• Oxygen – 21 %
• Hydrogen - .00006 %
• Others (less than 1 %) – ozone, methane, helium, etc.
Variable gases (amount depends on place and time)
• Water vapor – 0-4 %
• Carbon dioxide – about .037 %
Water vapor – dark-dry, light-humid
            CO2 varies with time
       Upward trend from 1958-2000, plus seasonal
                       variational




Not
zero
   Content of the atmosphere
      in addition to gases

• Mineral particles – dust from ground,
  man-made pollution
• Water droplets – clouds, fog
• Water drops – rain
• Ice – small crystals, snow flakes, hail
Structure of the atmosphere

• Varies depending on the meteorological
  conditions
• But we can talk about average conditions
  - horizontal structure (as shown on a
     weather map)
  - vertical structure
     Vertical structure of the
           atmosphere
• First – how deep is it?
                    White line is the thickness
                    of the atmosphere (99%
                          of molecules)




                    Earth
  Within this thin atmosphere,
  there are different ways of
         defining layers
• How temperature changes with height
  (increase versus decrease with height)
• Composition (mixture of gases)
• Electrical structure
• Turbulence intensity
    Different Layers of the Atmosphere




   Planetary
boundary layer –
   more later
       A Glossary of Weather
             Variables
• Air temperature
• Air pressure
• Humidity – amount of water vapor
• Clouds
• Precipitation – rate, type, distribution
• Wind – direction and speed, turbulence
  intensity
• Visibility
Air Temperature
               Temperature
• Related to rate of motion of molecules: The
    warmer the air, the more rapidly the molecules
    move.
•   Primary temperature scales
    – Fahrenheit (F): freezing point of water = 32 F,
      boiling point = 212 F
    – Celsius (C): freezing = 0 C, boiling =100 C
    – Absolute (A) or Kelvin (K): absolute zero = 0 A =
      -273 C (all molecular motion stops)
• Temperature observations near the surface (2
    m) are reported in degrees F, and above that
    are reported in degrees C.
      Vertical Temperature Structure
             of the Atmosphere




Commercial
  Aircraft


  All the
 Weather
   Denver temperature profile at 5:00 AM
               28 Feb 03


Stratosphere



Tropopause
                                   Up

                                    Dry adiabatic
Troposphere                          lapse rate
                                     reference
                                        lines
Air Pressure
Why do we care about
     pressure?
   Air Pressure - Definition
• Pressure = force/area (e.g., pounds per
 square inch)
 1) Hydrostatic pressure – weight of the
 atmosphere above the surface
 2) Dynamic pressure – force of the wind
 (e.g., against a door)
          Air Pressure - Units
• In this course, pressure is expressed in millibars
    (mb) – average sea-level pressure = 1013 mb
•   In the science literature, it is expresses in hecta
    Pascals (hPa = 1 mb)
•   Sometimes (e.g., in public weather forecasts) it
    is given in inches of mercury – average = 29.92
    in. mercury
•   Surface pressure values are “adjusted” to sea
    level, so that values show high and low pressure
    patterns rather than the elevation of the
    observation
How is sea-level pressure estimated
  for plotting on weather maps?
Vertical Pressure Variation in the
           Atmosphere
Air Density
• General definition of density - Mass per
  unit volume
• Air density – Mass (e.g., kilograms) of air
  molecules per unit volume (cubic meter)

      More dense           Less Dense

       ***********         *   *   *   *   *
       ***********         *   *   *   *   *
       ***********         *   *   *   *   *
       ***********         *   *   *   *   *
Density Variation With Height
                     Humidity
• A measure of the amount of water vapor in the
    air
•   There are many different measures of humidity
    –   Relative humidity
    –   Vapor pressure
    –   Dew point temperature
    –   Specific humidity
    –   Absolute humidity
                    Clouds

• Percent of sky covered by clouds
• Types of clouds
  – Cumulus
  – Cirrus
  – Stratus, etc.
                       Wind
• Speed – miles per hour or knots (nautical miles
    per hour), meters per second
•   Direction - Given in terms of the direction from
    which the wind is blowing. A northwesterly wind
    is blowing from the northwest.
•   Gustiness – turbulent component of wind
•   Horizontal wind is measured, but vertical wind is
    also very important
Turbulence near the ground
               Visibility

• The maximum distance at which an
  observer can distinguish an object against
  the sky as a background
• May be limited by fog, air pollution, etc.
            Precipitation

• Type of precipitation
  - rain, snow, sleet, hail, etc.
• Rate at which it is falling
  (inches/hour)
  Scales of atmospheric motion -
        Storms of all sizes


• Mid-latitude cyclones – 1000+ mile
• Hurricanes – 500+ miles
• Thunderstorms – 1-100 miles
• Tornadoes – few hundred yards
• Turbulence – centimeters-meters
 Measuring the atmosphere

• Weather balloons, or radiosondes make
 “upper-air” measurements of horizontal
 wind, temperature and humidity.
Radiosonde –
helium
balloon with
measurement
instruments
 Measuring the atmosphere
• Weather balloons, or radiosondes make
  “upper-air” measurements of horizontal
  wind, temperature and humidity.
• Surface weather stations (land and seas)
  measure horizontal wind, temperature,
  humidity, cloud cover
• Satellites can estimate winds, temperature
  and humidity
Radiosonde launch locations
“Surface” observations - NWS
Zooming in to see even more
          Northern Utah
        Other data sources

• Cloud-track winds
• Water-vapor-track winds
• NEXRAD radar winds and reflectivity
• Doppler lidar winds
• Wind profilers
• Commercial aircraft – TAMDAR, AMDAR
The Planetary Boundary Layer –
more later, but here’s a preview
 • BL - the region of the troposphere immediately
   above Earth’s surface where vertical turbulent
   transfers of heat, moisture and momentum are
   large compared to the troposphere above.

 • Daytime temperature lapse rate is nearly dry
   adiabatic, or neutral (9.8 C/km).

 • Nighttime temperature lapse rate is typically
   stable (inversion).
Why care about boundary layers
• We live there.
• Transport and diffusion of plumes takes place
    primarily in the boundary layer.
•   Some of our meteorological conditions at the
    surface originate higher in the troposphere
    (precip, large scale T changes, etc.), but many
    develop within the BL.
    – Air pollution (public health) –most originates at
      surface and stays within BL
    – Diurnal temperature fluctuations
    – Mesoscale wind circulations
          Jaxonville,
         Florida
         sounding


         7 PM local



1.5 km
Jaxonville,
Florida
sounding


7 AM local
         Part 2
  Warming Earth and its
Atmosphere – The Diurnal
   and Seasonal Cycles
   Or, what happens to the
energy received from the sun?
First – We Need to Understand The
    Ways in Which Heat Can be
  Transferred in the Atmosphere

       •Convection
       •Latent heat
       •Conduction
       •Advection
       •Radiation
    1. Convection

Heat transfer in the vertical
  through buoyant motion
Atmospheric convection
                 Cumulus
                  Cloud

  Upward
    Heat
  Transfer
2. Latent heat – the
      concept
      Personal Experiences –
    Latent Heat Consumption by
            Evaporation
• Chill stepping out of a shower –
  evaporation consumes heat
• Use of “swamp coolers” instead of air
  conditioners in dry climates
• It is cooler over green grass than over a
  stone surface – evaporation from leaves
• Cool breeze with thunderstorms in vicinity
  – evaporation of rain below the cloud
       Personal Experiences –
       Latent Heat Release by
      Condensation or Freezing

• Spraying fruit trees with water when frost
  damage is possible – freezing of water
  releases heat
• The “bubbly” appearance of cumulus
  clouds – released latent heat in cloud
  makes them buoyant
The Transport of Latent Heat
                             Condensation
                               Vapor to
                                Cloud
 Water Vapor     Air Moves     Droplets
    *****
    *****


                               Heat Released
                                  In Cloud
 Evaporation –
  Heat Taken
  From Ocean
          3. Conduction
• Transfer of heat from molecule to
  molecule
• Think of heat in terms of how fast the
  molecules are moving
• Fast moving molecules where something
  is hot collide with adjacent slow moving
  molecules where it is cooler, causing the
  slower molecules to speed (heat) up.
  Heat conductivity – ability of
    material to conduct heat
• Air -        .02 (Watts per meter per oC)
• Wood -       .08
• Water -      .60
• Wet soil -   2.1
• Stone -      2.7
• Iron -       80.
 Thus, Air is a Poor
 Conductor of Heat
   (ever think about why
thermopane windows work?)
   4. Advection


       Wind direction
Warm                     Cold
 air                      air
               X


        YOU ARE HERE
        (Temperature
        will increase)
             5. Radiation

• electromagnetic radiation
• Different names are used for different
 wavelengths of electromagnetic radiation.
Everything with a Temperature
above absolute zero emits EMR
Intensity of Light Versus Wavelength




      (µm)

       Temperature of the sun = 6000° K (10,500° F)
The Sun’s Emission Spectrum
       Versus Earth’s
    What Interferes with
    Transmission of Light
  Through the Atmosphere?
• Some gases – called optically active gases.
  Different gases absorb different
  wavelengths
• Particles of dust – natural and man-made
• Clouds and fog
Now That We Know How Heat
    is Transferred in the
         Atmosphere!
     ----------------------

   Putting it All Together
The Atmospheric Energy Checkbook – Balance
     Gains and Losses (On the Average)
Putting the Balance Under a
         Microscope
    How the Warm Ground
Heats the Air In Contact With It




                          Convection



                            Conduction in
         Ground           lowest millimeter
The Diurnal and Seasonal
   Cycles of Heating –

Exist Because of the Earth-
       Sun Geometry
            The Cycles

• Diurnal – Rotation of Earth on axis
  every 24 h
• Annual or seasonal – Revolution of
  Earth around the sun every 365.25
  days
Equinoxes: Points in Earth’s orbit around the sun where the sun is
directly over the equator (12 hrs light & 12 hrs of darkness).
Solstice: Points in orbit where the sun is “displaced” farthest N or S.
        Seasonal Cycle
              -Causes-
• The sun heats the ground less
  intensely when it is at a low angle in
  the sky (winter), and more intensely
  when it is at a high angle above the
  horizon (summer).
• The suns heat the ground less when
  the daylight is shorter (winter), etc.
Factors That Cause Temperature
 Variation From Place to Place
  •Latitude – warmer to south
  •Land and water distribution
    winter → warmer near water
    summer → cooler near water
  •Ocean currents
    Warm currents generally on east side of
    continents, cold currents on the west side
  •Elevation of the surface above sea level
    Temperature decreases with elevation
• Surface type – e.g., urban versus
  rural
• Surface wetness
 Cooler of
Higher Elev.


  Warmer
Over Ocean
 in Winter

   Warmer
  to South -
     More
  Radiation
Two cities
with the
same mean
annual
temperature
but with very
different
climates
 Daily Temperature Cycle

Balance of Incoming Solar and
      Outgoing Infrared
Max
 Temperature Profile Near The Ground
                 DAY




                          profile




Note: Temperatures almost always decrease with height
      NIGHT




      FROST



Inversion: Temperatures increase with height
 What Can Effect The Day-
Night Temperature Change?
• Cloud cover reduces diurnal variability
• Terrain: surface type, mountains,
    canyons,…
•   Wind: causes turbulent mixing of the
    atmosphere which reduces diurnal
    variability
•   Approaching weather systems: cold
    fronts, warm fronts, squall lines,
    hurricanes
•   Humidity
   Topographic Effects on
   Nighttime Temperature




Cold air pooling

								
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