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Doppler Radar Information and Definitions

WSR88d exterior The NEXRAD WSR-88D Radar is one of the most advanced operational weather radar in the world. It gives meteorologists and the general public the ability to see "inside" a storm. Using the NEXRAD radar, forecasters can predict the weather with the aid of data such as reflectivity, velocity, and spectrum width of an atmospheric disturbance. Since this data was unavailable in the past, the WSR88-d NEXRAD system has the capability to save more lives than radars of the past. For full technical details on the NEXRAD Radar system, visit
NOAA's Radar Operation Center.

What is and why does APRSFL use Level II Radar Data?

There are two different sets of data that NEXRAD sites put out. One is named NIDS and one is named Level II data. There are many advantages to Level II produced radar images. Level II, for example, are shown with 128 colors instead of 15 providing more detail in intensity. This allows the ability to delineate certain phenomena (e.g. gust fronts, hook echoes) that are sometimes hard to see using NIDS data. The Level II data is available in 0.5 dBZ increments, while NIDS reflectivity data is only available in 5.0 dBZ increments. This lower precision in the NIDS data causes large errors in any estimates of precipitation and precipitation rate. There are many other differences, but for this brief explaination, it is clear why we use Level II data to render the images you see.

Base Reflectivity, Base Velocity & Spectrum Width

  • Base Reflectivity is one of the basic quantities that Doppler radar measures. Color intensity corresponds to the amount of radiation that is scattered or reflected back to the radar by whatever targets are located in the radar beam at a given location. These targets can be hydrometeors (snow, rain drops, hail, cloud drops or ice particles) or other targets (dust, smoke, birds, airplanes, insects).

  • Base Velocity is the average radial velocity of the targets in the radar beam at a given location. Radial velocity is the relationship between the target's motion and the direction of the radar beam. Positive values (warm colors) denote out-bound velocities that are moving away from the radar. Negative values (cool colors) are in-bound velocities that are moving towards the radar.

  • Base Spectrum Width is a measure of velocity dispersion within the radar sample volume. The primary use of this product is to estimate turbulence associated with mesocyclones and boundaries.

The parabolic reflector inside the radome

Since this is primarily an Amateur Radio site, you have to see this!

Here's some of the specifications for the WSR88D....

The inside of the Kylstron transmitter cabinet

  • Transmitter Type: S-band, klystron tube

  • Amplifier (53 dB gain typical)

  • Frequency: 2700 to 3000 MHz

  • Power: 750 kw peak at klystron output

  • Transmitter to antenna loss: 2 dB typical

  • Average Power: 300 to 1300 watts

  • Antenna Type: center fed paraboloid

  • Antenna Size: 28 feet in diameter

  • Polarization: linear horizontal

  • Gain at 2850 MHz: 45.5 dB (including radome loss)

  • Beamwidth at 2850 MHz: 0.925 deg

  • First sidelobe: -29 dB (others less than -40 dB beyond 10 deg)

  • Radome: fiberglass foam sandwich frequency tuned, 39 foot truncated sphere

  • Radome two way loss: 0.24 dB at 2850 MHz

Photos taken during a tour of a WSR-88d site back in 2005...

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