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Most of us are familiar with The Weather Channel and local news and weather broadcasts
that use the Doppler NEXRAD (next generation radar) WSR 88D weather radar. Some of you
may even use those broadcasts to supplement your flight planning and overall weather
awareness. However, there are significant differences with how weather information
is displayed on a controller's radarscope and the local news weather broadcast depictions.
NEXRAD is designed to detect and display weather, but ATC radar systems are designed
to detect and display aircraft. Because the NEXRAD color coding and 16 individualized
precipitation levels can provide excess clutter and possibly compromise the ability
of controllers to safely perform their duties, different systems for depicting weather
radar echoes needed to be developed for the ATC environment.
Air Route Traffic Control Centers
In air route traffic control centers, NEXRAD data is fed through the Weather And Radar
Processor (WARP) that organizes the 16 NEXRAD levels into four reflectivity (dBZ)
categories. Reflectivity returns of less than 30 dBZ are classified as "LIGHT"
and are filtered out of the center controller's display. The remaining three
categories correlate to bands of dBZ values to assist pilots in evaluating the
severity of flight conditions that might be associated with those precipitation
returns. Therefore, the wide range of color coding available to NEXRAD is not available
to the controller, and the ARTCC's WARP system does not display dBZ levels below 30.
Therefore, center controllers will not be able to report areas of "light"
weather radar echoes.
WARP/NEXRAD is a vast improvement over the Air Route Surveillance Radar (ARSR)
display of weather radar echoes that center controllers used exclusively prior to
the implementation of the NEXRAD type weather radars. The ARSR displays the echoes
to the controller by indicating "moderate" intensities with a slash mark
"/" and more intense areas with the letter "H."
Approach Control and Terminal Facilities
In the approach control world, neither NEXRAD nor WARP is available. Instead, radar
weather echoes are displayed by the Airport Surveillance Radar (ASR) systems using
Common Automated Radar Terminal System (Common ARTS) or Standard Terminal Automation
Replacement System (STARS) digital processors. The digitized ASR 9 and 11 systems (and
some ASR 8 systems that have been digitized) paired with a weather processor, display
the four weather radar echo intensity categories to the controller.
Terminal facilities can and do display "light"(less than 30 dBZ) areas
of precipitation.
...But Keep in Mind...
Of course, there are no absolutes. In the universe of terminal radars, the NAS
still has a few non-digital ASR systems. While these systems do a good job of
displaying weather radar echoes, they lack processors that can discern the intensity
of the echoes. These facilities will not be able to use the terms,
"light," "moderate," "heavy," or "extreme."
Controllers who work from these displays will be able to tell pilots the position
of weather radar echoes but will state, "intensity unknown" because their
system does not indicate what dBZ level of reflectivity is present.
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