Lecture 8 (10/28) METR 1111

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Lecture 8 (10/28)METR 1111

• Radars

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Radar its History

• RADAR is an acronym
• Stands for RAdio Detecting And Ranging
• In 1930s, radar used to monitor shipping traffic
• In WWII, used to detect aircraft
• Precipitation would cause interference
• After WWII, radar used for weather too

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How Does it Work?

• Radar creates a very strong (1 megawatt) pulse
  of electromagnetic energy (or radiation)
• Antenna (or dish) focuses beam and it travels
  through the atmosphere
• Anything the beam hits is called a target
• Target scatters radiation back (some straight
  back at the antenna)
• This reflected radiation is measured
• You can figure out distance with speed beam
  travels at and the time the process takes.

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Properties of Targets

• Amount of energy reflected back is called
  reflectivity.
• Reflectivity depends on the number of targets in
  an area and their size
• Problem a few large drops can reflect the same
  radiation as a bunch of small drops
• Thats why radar is not an accurate tool for
  measuring rainfall

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Radar Images

• Reflectivity images show the location and amount
  of reflectivity on a display.
• This is what you see on TV
• Velocity Images - product of Doppler radar
• Shows component of the wind towards or away from
  the radar
• This is useful for detecting rotation in storms
  (tornadoes), damaging winds

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Reflectivity Image

• Typical reflectivity image on next slide
• Color scale tells you what the image means
• Can tell if returns are light, moderate, heavy,
  or severe
• Earth curves away from beam so you see below
  clouds close to the radar site and above clouds
  far away
• Farther away, you get less resolution since beam
  spreads out

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Typical Reflectivity Image
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Doppler Radar

• Doppler radar can measure winds towards and away
  from the radar. But how?
• Because of the same reason why a train whistle
  sounds louder as it approaches you than as it
  moves away
• Doppler radar measures the change in frequency of
  targets (actually measures change in phase which
  is related to freq.)

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Typical Velocity Image
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Velocity Images

• Color scale is again important.
• Winds toward the radar are usually negative
  winds away from radar usually positive
• There are limitations in the maximum wind speeds
  you can detect.
• In event of very strong winds, can get velocity
  or range folding (where it goes off one extreme
  end of the scale and onto the other)

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NEXRAD WSR-88D

• This is what the National Weather Service calls
  their radars.
• NEXRAD stands for NEXt generation RADar
• WSR-88D stands for Weather Survillance Radar 1988
  Doppler
• For short, you can use one or the other to
  describe the current NWS radar network

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Phased Array Radar

• This is the next step in Radar development
• Originally developed for use by the Navy
• Currently being tested at the National Severe
  Storms Laboratory

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How a Phased Array Works

• Unlike conventional radar, phased arrays
• Use multiple beams
• Use multiple frequencies
• Controlled electronically
• Advantages
• Up to 6 times faster scanning atmosphere (3-D
  view)
• Can track multiple targets (several storms at
  once)
• Potentially increase tornado warning lead times
  from 12-22 minutes

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Good web sites to visit

• For more info on radars
• University of Illinois site or Intellicast site
• For current radar imagery
• OK-FIRST Data -- Great data for the southern
  plains. Use username "synoptic", and password
  "scale" to access the data. You will need to
  download the software first.
• See radar links here http//weather.ou.edu/wx/
• A good pay site is www.weathertap.com
  (5.95/month).
• For a little more detail about Phased Arrays
• http//www.noaanews.noaa.gov/stories/s850.htm

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For Next Time

• Read Chapter 10 of the Weather Book (Sky
  Watching)
• Were going to go over some basics again with
  cloud types next week
• After this, were in the home stretch
• Numerical Models
• Winter Storms (may switch with Severe Wx)
• Severe Weather
• Job Opportunities