Radar and Satellites

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Radar and Satellites

• EAS 211
• Spring 2005
• 04/04/05

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Weather Satellites

• Weather satellites are a very important tool for
  the meteorologist. Since over 70 of the earths
  surface is covered with water, satellites provide
  invaluable observations of the sky in places
  where there are no surface based observers.
  Before weather satellites were in use, severe
  storms, such as typhoons and hurricanes often
  went undetected until they came dangerously close
  to inhabited areas.

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Two Types of wx satellites

• Geostationary (geo-synchronous)
• Polar-Orbiting (Sun-synchronous)

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Geostationary Satellites

• Orbit the equator at the same rate the earth
  spins
• Altitude of 36,000 km above a fixed point
• Continuous monitoring of a specific region
• Real-time data system (transmit pictures as soon
  as they are taken)
• Allows for looping of images to get idea of cloud
  movementwhich allows us to estimate wind
  direction and speed.

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Polar-Orbiting Satellites

• Orbit closely follows earths meridian lines
• Pass very near the north and south poles
• As the earth rotates to the east, each pass of
  the satellite observes an area to the west of the
  previous pass
• Eventually the satellite covers the entire earth
  (passes same pt twice a day)
• P-O satellites provide exceptionally sharp
  pictures as they are examining clouds directly
  below them
• They circle the earth at 850 km

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Satellite technology since the 1960s

• The first satellites used television cameras to
  photograph clouds.
• Contemporary satellites use radiometers which
  detect radiation that emanates from the clouds,
  allowing use 24 hours a day.
• New generation Geostationary Operational
  Environmental Satellite (GOES) series has the
  ability to obtain cloud images, and at the same
  time, provide vertical profiles of the
  temperature and moisture content of the
  atmosphere below it.

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Satellites also provide information on cloud
height and thickness

• Visible photographs show the sunlight reflected
  by the clouds upper surface. B/c thick clouds
  have higher albedo (reflectivity) than thin
  clouds, they appear brighter on a visible image.
  However, high, middle, and low clouds have just
  the about the same albedo, making them nearly
  indistinguishable using only visible imagery.

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Infrared Imagery

• Distinguish b/w high, middle and low clouds
• Better images of the actual radiating sfc
• Warm clouds radiate higher levels, higher
  temperatures appear darker on IR images than cold
  clouds
• Can distinguish b/w warm (dark) and cold (bright)
  clouds
• Cloud temp can be converted by a computer into a
  3D image of a cloud. (Seen on TV)
• Computer enhancement enhances the contrast b/w
  the colors assigned to represent each
  temperature, specifically, they are assigned
  colors between black (warm) and white (cold).

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• In regions where the are no clouds, it is
  difficult to examine the movement of air. To
  help with this situation, the newest satellites
  are equipped with water vapor sensors that can
  profile the distribution of atmospheric moisture
  in the middle and upper troposphere.
• WV images are extremely helpful when examined as
  a time-lapse loop.
• Identify wet and dry regions (DCB, WCB)
• Identify middle and upper tropospheric swirling
  flow patterns and jet streams.

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RADAR

• Radio Detection and Ranging
• Radar is another essential tool for
  meteorologists, as it also gathers information
  about storms and precipitation in previously
  inaccessible regions. Meteorologists use wx
  radar to examine the inside of a cloud much like
  Dr. uses X-rays to examine the inside of the
  human body.

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• Radar is used to detect precip (and greater)
  sized particles (targets) and (more recently)
  determine the speed of the target.
• The radar unit consists of a transmitter that
  emits quick, but powerful microwave pulses having
  a wavelength b/w 1.0 and 20 cm.
• When this energy encounters a foreign object
  (called a target), energy is scattered in all
  directions.

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• A small portion of this energy is scattered back
  (backscattered) toward the radar and is detected
  by the receiver
• Transmitted power 105 Watts
• Backscatter power 10-10 Watts
• Backscattered energy is amplified and displayed
  as an echo
• Echo intensity is proportional to the amount of
  backscattered energy received
• Elapse time between transmission and reception
  indicates the targets range from the radar.

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We can vary the wavelength of the emitted pulse

• Smaller targets require detection by shorter
  wavelengths
• Cloud droplets are detected by radars utilizing a
  1 cm wavelength
• Longer wavelengths (3-10 cm) are only mildly
  scattered by cloud droplets, but are strongly
  scattered by larger precipitation particles.
• Attenuationenergy loss en route from absorption
  and scattering as wavelength dec., attenuation
  increases drastically
• The brightness of an echo is directly related to
  the intensity of the precipitation, so Radar
  indicates location and intensity of precipitation.

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Doppler Radar (WSR-88D)

• Utilizes the Doppler Shift
• As the transmitted pulse encounters a target,
  some of the pulse is backscattered
• If the target is moving toward or away from the
  radar, the backscattered radiation will return to
  the receiver with a different frequency than the
  initially transmitted pulse
• Measure this small shift in frequency, called the
  Doppler Shift Frequency (DSF)
• DSF fd fr-ft
• Fr receiver frequency
• Ft transmitted frequency

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DSF is related to Vr (radial velocity)

• When fd lt0, Vr is positive (away from the radar
  red colors)
• fd 0 (stationary targets)
• fd gt 0, Vr is negative (toward the radar green
  colors)

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Doppler Dilemma

• VmaxRmax (cwavelength)/8
• Speed of light c 300,000,000 m/s
• Wavelength is the emitted microwave radiation
  from the radar
• Higher speeds smaller ranges
• Lower speeds larger ranges

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Pulse Repetition Frequency (PRF)

• Higher the PRF, the shorter the time b/w pulses
  (during which the radar listens for
  backscattered energy)
• This means we can detect larger objects at a
  shorter range
• So we can resolve a higher Vmax
• Current Doppler Radars
• Vmax 25 m/s
• Rmax 120 nm

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What do we measure?

• Radial velocity (towardgreen or awayred)
• Reflectivity (dBz)
• Values greater than or equal to 30 mean storm is
  convective (thunderstorm)
• Values greater or equal to 60 probably hail, most
  likely severe.

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How do we examine what has been measured?

• PPI (Plan Position Indicator)
• A circular display which indicates a targets
  direction and distance from the radar.
• RHI (Range Height Indicator)
• A square display which illustrates the vertical
  structure (cross-section) of the target at
  different ranges from the radar

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