Satellite and Radar

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Satellite and Radar

• Lecture 5
• October 7, 2009

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Review from last week

• Pressure Gradient Force
• PGF CHANGE IN PRESSURE / DISTANCE
• Direction of PGF always pointed from HIGH
  pressure toward LOW pressure, directly
  perpendicular to an isobar
• Magnitude of PGF- strength is directly related to
  how closely packed the isobars are at the
  surface.

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Review from last week

• Coriolis Force
• The CF is an apparent force that results from the
  constant rotation of the Earth.
• In N. Hemisphere, acts at a 90 angle to the
  right of the object in motion (such as the wind)
• We cannot see the planet rotating, so when
  something is moving, we perceive it as being
  deflected to the right of its intended trajectory
  in the N. Hemisphere

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Since friction is directed opposite of the wind,
it slows the wind. When it slows the wind, the
magnitude of the CF is affected and the CF no
longer balances the PGF. Remember, CF is always
90 right of the wind in the northern hemisphere.
As a result the PGF is the dominate force driving
the wind and the wind turns in the direction of
the PGF. This allows the wind to cross the
isobars toward low pressure. 
L
996 mb
Pressure Gradient Force
Wind
1000 mb
Frictional Force
Coriolis Force
1004 mb
H
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Review from last week

• Geostrophic Balance
• A balance between the pressure gradient force and
  the Coriolis force
• Balance allows PGF to be equal and opposite the
  CF. This balance will tell use the magnitude of
  the geostrophic wind
• The geostrophic wind moves parallel to lines of
  constant pressure, with low pressure on the left
• Frictional Force
• Friction affects geostrophic balance by putting a
  drag-force on the air friction always acts in
  the direction opposite the direction of the wind

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Satellites

• October 4, 1957 Russia launched Sputnik 1, the
  first satellite in history
• As a result, space science boomed in America as
  it led Americans to fear that the Soviets would
  launch missiles containing nuclear weapons.
• 1959 Scientists at the Space Science and
  Engineering Center (SSEC) at UW-Madison conducted
  pioneering meteorological satellite research,
  revealing the vast benefits of meteorological
  satellites.

http//burro.astr.cwru.edu/stu/advanced/20th_sovie
t_sputnik.html
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Evolution Until Today

• First weather satellite lasted 79 days
• Now many years
• Two distinct types of weather satellites
• GOES - Geostationary Operational Environmental
  Satellites
• POES - Polar Operational Environmental Satellites
  (also referred to as LEO Low Earth Orbit)
• They are defined by their orbital characteristics
• There are also many other satellites in orbit,
  some of which are not functioning and those are
  referred to as space debris.

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Geostationary Vs. Polar Orbiting
http//cimss.ssec.wisc.edu/satmet/modules/sat_basi
cs/images/orbits.jpg
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GOES

• GOES Geostationary Operational Environmental
  Satellites
• Orbit as fast as the earth spins
• Maintain constant altitudes (36,000 km, or
  22,300 miles) and momentum over a single point,
  always over the equator
• Imagery is obtained approximately every 15
  minutes unless there happens to be an important
  meteorological phenomenon worth higher temporal
  resolution
• Generally has poor spatial resolution- sees large
  fixed area and covers polar regions poorly.
• But, good for viewing large scale meteorological
  phenomena (cyclones, hurricanes, etc.) at lower
  and middle latitudes

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GOES
GOES- EAST (GOES- 12)
GOES- WEST (GOES 11)
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GOES COVERAGE
http//goes.gsfc.nasa.gov/pub/goes/global_geosynch
_coverage.gif
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Sample Composite
http//www.ssec.wisc.edu/data/comp/latest_moll.gif
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POES

• POES Polar Operational Environmental
  Satellites
• Rotates around the earth from pole to pole
• Significantly closer to the Earth than
  geostationary satellites (879 km above the
  surface)
• Sees the entire planet twice in a 24 hour period
• Lower altitude gives it a good spatial
  resolution Very high resolution images of the
  atmosphere and Earth
• Poor temporal resolution Over any point on
  Earth, the satellite only captures two images per
  day!
• Best resolution over the poles

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POES COVERAGE
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POES

• More then a few in orbit currently
• Two examples are TERRA and AQUA
• Have different viewing instruments on them
• One example is MODIS Moderate Resolution Imaging
  Spectroradiometer
• Acquires data in 36 spectral bands (groups of
  wavelengths)
• As a result, MODIS can create a true color
  visible image, which can
• Show changes in vegetation during fall/spring
• Show smoke plumes, dust plumes, etc.

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Example MODIS image
http//www.ssec.wisc.edu/modis-today/images/terra/
true_color/2008_02_24_055/t1.08055.USA_Composite.1
43.4000m.jpg
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Wildfires Near Los Angeles Using MODIS
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Types of Satellite Imagery

• VISIBLE

• Measures visible light (solar radiation, 0.6 ?m)
  which is reflected back to the satellite by cloud
  tops, land, and sea surfaces.
• Thus, visible images can only be seen during
  daylight hours!
• Dark areas Regions where small amounts of
  visible light are reflected back to space, such
  as forests and oceans
• Light areas Regions where large amounts of
  visible light are reflected back to space, such
  as snow or clouds

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Visible Pros/Cons

• Pros
• Seeing basic cloud patterns and storm systems
• Monitoring snow cover
• Shows nice shadows of taller clouds (has a 3-D
  look to it)
• Cons
• Only useful during the daylight hours
• Difficult to distinguish low clouds from high
  clouds since all clouds have a similar albedo
  (reflect a similar amount of light)
• Hard to distinguish snow from clouds in winter

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Types of Satellite Imagery

• WATER VAPOR (WV)

• Displays infrared radiation emitted by the water
  vapor (6.5 to 6.7 ?m) in the atmosphere
• Bright, white shades represent radiation from a
  moist layer or cloud in the upper troposphere
• Dark, grey or black shades represent radiation
  from the Earth or a dry layer in the middle
  troposphere

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Types of Satellite Imagery

• INFRARED (IR)

• Displays infrared radiation (10 to 12 ?m)
  emitted directly by cloud tops, land, or ocean
  surfaces
• Wavelength of IR depends solely on the
  temperature of the object emitting the radiation
• Cooler temperatures (like high cloud tops) are
  shown as light gray, or white tones
• Warmer temperatures (low clouds, ocean/lake
  surfaces) are shown dark gray
• Advantage You can always see the IR satellite
  image

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Interpreting Visible vs. IR
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RADAR

• What does Radar mean?
• Radio Detection and Ranging
• During World War II, this Radio Detection and
  Ranging technique was developed to track enemy
  ship and aircraft. However, it was soon noted
  that precipitation, of any kind, would obstruct
  this remote detection. At first this was a
  problem, but the potential benefits were soon
  seen. This was the birth of weather Radar. 

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How does RADAR work?

• Radar uses electromagnetic radiation to sense
  precipitation.
• Sends out a microwave pulse (wavelength of 4-10
  cm) and listens for a return echo.
• If the radiation pulse hits precipitation
  particles, the energy is scattered in all
  directions
• The RADAR has a listening period. When it
  detects radiation scattered back, the radiation
  is called an echo.

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How does RADAR work?

• The RADAR beam is typically 0.5o above the
  horizon and 1.5o wide.
• It rotates in a full circle, with a radius of
  200 miles
• Time difference between transmission and return
  of signal distance to the storm
• The intensity of precipitation is measured by the
  strength of the echo, in units of decibels (just
  like intensity of sound waves!)

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• An image showing precipitation intensity is
  called a reflectivity image
• Intensity measured in decibels (dBZ)

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• www.radar.weather.gov/graphics/ridge_sitemap.gif

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Types of RADAR

• Conventional Radar
• Echoes are simply displayed on radar screen.
• Only produces reflectivity images.
• Circular sweeps and vertical sweeps, to attempt
  to reconstruct the precipitation type and
  intensity throughout the atmosphere
• Can identify storm structure, locations of
  tornadoes, and even non-meteorological objects!

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Good/Bad of Conventional Radar

• Good for
• Seeing bands/location of precip and their
  intensity
• Hook echoes
• Bow echoes
• Bad for
• Ground clutter, bouncing off things other than
  precipitation
• Overestimation/Underestimation of precip
• Cannot tell type of precipitation by radar alone
  (Have to use temperatures, actual observations,
  etc.

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Doppler Radar

• One of the most advanced versions of radar
• Does everything a conventional radar can do, PLUS
  more...
• In addition to conventional techniques, the
  Doppler Radar has a scan that operates on
  principle of the Doppler Effect
• Usually described using sound waves
• Definition The change in the observed frequency
  of waves produced by the motion of the wave source

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Doppler Radar in Meteorology

• Measures changes in wavelength of the RADAR beam
  after it is scattered from a travelling object
• Wavelength of the beam changes after it
  strikes the object
• Thus, wind direction AND speed can be measured
  by RADAR

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Doppler RADAR in Meteorology

• This is VERY useful in detecting tornado
  signatures!
• Doppler can measure wind speed and direction in a
  storm and can be viewed in a storm-relative
  velocity image
• Red Winds away from RADAR site, Green Winds
  toward RADAR site
• This is how the National Weather Service issues
  tornado warnings

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Phased-array radar

• Next generation of radar.
• Can scan multiple levels at once using multiple
  radar beams sent out at one time.
• Scanning only takes 30 secs compared to 6
  minutes for the Doppler
• Gives instantaneous profile of atmosphere for
  winds and precipitation intensity.

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Examples

• Birds on radar
• http//www.crh.noaa.gov/images/mkx/radar/birdanima
  tion.gif
• http//www.crh.noaa.gov/mkx/?nusing-radar