Satellites and RADAR

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Satellites and RADAR

• AOS 101 Discussion 304
• Feb 26th, 2009

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Review

• Hand in forecasts / hw 4
• Last week Saturday high 25, low 19, precip.
  .37 inches (snow 6.7 inches) (18 to 1 ratio)
• A little more snow due to more northerly track of
  surface cyclone
• Weekly discussions
• Hw 3 discussion

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Review

• ?U Q W
• U internal energy
• Q heat added (taken away) to the system
• W work done by the system
• What law does this statement obey?
• The First Law of Thermodynamics

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More Review

• E hv, h is a constant
• Higher frequency equals higher energy
• ? c / v , c 3 x 108 m/s (speed of light)
• Shorter wavelength higher energy E 1/?
• Why do you need protection when taking an X-ray
  but not using a cell phone?
• Radiation (EM waves) can be absorbed, reflected,
  scattered, and transmitted.
• What is difference between reflection and
  scattering?

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• Ignoring scattering, a r t 1

ATMOSPHERE
REFLECTED
ABSORBED
CLOUD
TRANSMITTED
SCATTERED
GROUND
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At the earths surface

• No radiation is transmitted through the ground
  (i.e. t 0)
• So a r 1 meaning the surface absorbs whatever
  radiation is not reflected.
• Surfaces with high albedos (like snow) will not
  absorb as much energy as surfaces with low
  albedos (like asphalt).

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BIG PICTUREheat budget of earth and atmosphere
SPACE
ATMOSPHERE
Shortwave
Longwave
Conduction Convection
Latent Heat
GROUND
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Why is the sky blue?

• The atmosphere scatters visible light, so we
  receive direct light (from the direction of the
  sun) and diffuse light (from all directions).
• The atmosphere more effectively scatters shorter
  wavelengths (blue) than longer wavelengths (red).
• Also, human eyes are sensitive to blue light (a
  likely evolutionary characteristic).
• Thus when the sun is high in the sky, away from
  the sun the sky appears blue.
• At sunset, sunlight must pass through a larger
  amount of atmosphere, enough so that red light is
  also scattered resulting in a red sky/clouds near
  the horizon.

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What do the colors black and white imply about
visible light?

• Black- no visible transmitted
• White- combo of all colors in the visible
  spectrum, all light transmitted, scattered,
  reflected equally. Using a prism exemplifies
  this. Prism uses refraction since glass is a
  different medium than air. Also, refracts
  differently based on color (known as dispersion)

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Why is the sky on the moon Black?

• Surface pressure on moon is one trillionth that
  of surface pressure on Earth so there is very
  little gas atmosphere above you and thus no
  scattering of light, only the sun is visible.

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Cloud color
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More scattering

• Many small objects scatter more then one big
  object.
• Examples- shattered ice cube pieces scatter
  better then original big ice cube
• Two clouds with same amount of water- one with
  smaller drops scatters better.
• Climate implication if we add more nuclei to
  air cloud droplets will become smaller scattering
  more incoming solar

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Ceilometers
Measures cloud height but time it take for light
to be scattered back.
Visibility similar but in horizontal direction,
This weeks topic
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Satellites- History

• October 4, 1957 The former Soviet Union
  launched the first satellite, Sputnik 1.
• 1959 Scientists, primarily at UW Space Science
  Engineering Center, began to conduct major
  meteorological satellite research.

This proved to Americans that the Soviets had
incredible scientific capabilities. It led
Americans to fear that the Soviets would launch
missiles containing nuclear weapons. As a
result, space science boomed in the United States.
www.wikipedia.com
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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 6-8
  thousand 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

• Measure 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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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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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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Interpreting Visible vs. IR
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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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IR Pros/Cons

• Pros
• Distinguishing higher clouds from lower ones
• Observing storms at night
• Distinguishing clouds from snow cover
• Cons
• Sometimes hard to distinguish between a thick
  cirrus and thunderstorms
• Makes clouds appear blurred with less defined
  edges than visible images

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RADAR

• RADAR Radio Detection and Ranging
• During World War II, this technique was
  developed to track enemy ships and aircraft.
• It was noted, however, that snow and rain would
  obstruct the viewing of enemy ships and aircraft.
• During the war, this was seen as a major problem
• Eventually, the benefits of weather RADAR were
  noted, and meteorological RADAR research began.

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How does RADAR work?

• RADAR uses electromagnetic radiation to sense
  precipitation location and intensity
• Sends out a microwave pulse (4-10 cm wavelength)
  and waits for a return echo.
• If the pulse encounters precipitation, it gets
  scattered.
• 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!)
• An image showing precipitation intensity is
  called a reflectivity image
• Intensity measured in decibels (dBZ)

http//www.wrh.noaa.gov/images/mso/radar.jpg
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U.S. RADAR Sites
http//radar.weather.gov/
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Types of RADAR Conventional

• Simply displays echoes on a RADAR screen
• 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!

http//www.crh.noaa.gov/images/sgf/em/spotter_trai
ning/image002.gif
http//www.crh.noaa.gov/pah/storm/May.6.2003/radar
pix/radar.1.jpg
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RADAR Problems
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Doppler RADAR

• Does everything a conventional RADAR can do, but
  utilizes the Doppler effect.
• Doppler effect 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 traveling 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!
• Red Winds away from RADAR site, Green Winds
  toward RADAR site
• This is exactly 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.
• Gives instantaneous profile of atmosphere for
  winds and precipitation intensity.

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Examples

• Some Real-time examples
• Make sure to check the website as I will be
  putting some informative and helpful tutorials
  for analyzing radar and satellite images which
  may be a little hard to understand in a few
  instances