Weather Radar Echo Terms

Page 1 27 M A R C H / A P R I L 2 0 0 6 W hen thunderstorm sea- son begins this year, pi- lots will start hearing some very important changes in the way Air Traffic Control (ATC) describes radar weather echoes to pilots. Beginning in late spring 2006, pilots will hear ATC use four terms, “light,” “moderate,” “heavy,” and “extreme” to describe weather radar echoes. Each term represents a precipitation intensity level paired with a dBZ range (Figure 1) to help pilots interpret the severity of the flight con- ditions present. (A dBZ is a measure of radar reflectivity in the form of a log- arithmic power ratio [in decibels or dB] with respect to radar reflectivity factor “Z.”)

The four terms will be used uni- versally in the National Airspace Sys- tem (NAS) by approach controllers and Air Route Traffic Control Center (ARTCC) and Automated Flight Ser- vice Station (AFSS) specialists. The decision to stan- dardize the termi- nology was easy to make because the ARTCC facilities and many of the terminal approach control facilities now have digital radar display sys- tems with proces- sors that can better determine the intensity (dBZ) of radar weather echoes and display that infor- mation to the controller.

Most of us are familiar with The Weather Channel and local news and weather broadcasts that use the Example of Terminal Digital Radar Weather Display (photo courtesy of AOPA) Introducing Revised ATC Terms for Describing Radar Weather Echoes to Pilots by Christine Soucy and Michael Lenz Figure 1 ATC Weather dBZ Radar Terms Reflectivity Levels LIGHT 40 to 50 dBZ EXTREME >50 dBZ Page 2 28 F A A A v i a t i o n N e w s Doppler NEXRAD (next genera- tion radar) WSR 88D weather radar. Some of you may even use those broadcasts to sup- plement 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 de- signed to detect and display weather, but ATC radar sys- tems are designed to detect and display aircraft. Because the NEXRAD color coding and 16 individualized precipitation levels can provide excess clut- ter and possibly compromise the ability of controllers to safely perform their duties, dif- ferent systems for depicting weather radar echoes needed to be developed for the ATC environment.

In air route traffic control centers, NEXRAD data is fed through the Weather And Radar Processor (WARP) that organizes the 16 NEXRAD lev- els into four reflectivity (dBZ) categories. (See Figure 1). Re- flectivity returns of less than 30 dBZ are classified as “LIGHT” and are filtered out of the center con- trollers’ 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, as you can see in Figure 2, the ARTCC’s WARP system does not display dBZ levels below 30, therefore center con- trollers will not be able to report areas of “light” weather radar echoes. WARP/NEXRAD is a vast im- provement over the Air Route Surveil- lance Radar (ARSR) display of weather radar echoes that center controllers used exclusively prior to the implementation of the NEXRAD type weather radars.

The ARSR dis- plays the echoes to the controller by indicating “moderate” intensities with a slash mark “/” and more intense areas with the letter “H” (see Figure 3 for an example of an ARSR and WARP display). In the approach control world, nei- ther NEXRAD nor WARP is available. Instead, radar weather echoes are dis- played 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 digi- tized 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 (see Figure 1) to the controller. Terminal fa- cilities can and do display “light” (less than 30 dBZ) areas of precipitation. 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 Figure 2 ATC Weather Radar Terms dBZ used in ARTCC Reflectivity Levels MODERATE 30 to 40 dBZ HEAVY >40 to 50 dBZ EXTREME >50 dBZ ARTCC Displaying both WARP/NEXRAD (color) and ARSSR (/////’s and HHHH’s) depicting moderate through extreme precipitation Figure 3 Page 3 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 un- known” because their system does not indicate what dBZ level of reflectiv- ity is present. In the world of ATC, weather radar echoes are all referred to as “precipita- tion” even though, technically, it is possible the echo could be associated with birds, volcanic ash, etc., or pre- cipitation that is not reaching the Earth’s surface (virga). Controllers will tell pilots the location of significant areas of “precipitation” when it ap- pears that it may affect the aircraft’s flight path.

They will also provide as- sistance in the form of course devia- tions when requested by the pilot. Rainfall rates (i.e., inches/hour) as they relate to intensity (dBZ) have not been correlated with the ATC displays. Therefore, the terms (light/moderate/ heavy/extreme) cannot be equated/correlated to rates of rainfall per se, at this time. Generally however, the more intense the echo, the more likely there is to be greater intensities of precipitation; and when conditions are favorable for convective activity, turbulence and other weather hazards should be expected. As the intensity of precipitation increases, so too, does the likelihood of more severe weather conditions.

Pilots should also remember that turbulence can be present in areas where ATC does not display precipitation at all. Therefore, pilots should always exercise care when transiting areas of known or suspected convective activity. Pilots of light general aviation air- craft should even approach areas of “light” precipitation with caution. A rapidly growing thunderstorm can in- crease at a rate of 6,000 feet per minute! Think of the time-lapse pho- tographs and weather radar loops showing building thunderstorms. “Light” precipitation could grow to “moderate” and “heavy” levels within a very short period of time, given the right conditions.

The following tips are offered to assist pilots in navigating stormy skies safely. • Request course deviations early. Don’t wait until the last moment. • Ask for information updates as needed. The ARTCC WARP/NEXRAD updates every one to six minutes. Terminal (ASR based) systems show near “real time” echoes. • Make sure the controller under- stands what services you want. • Maintain situational awareness concerning your position and the weather areas you wish to avoid. • Include the information that you are on a heading assigned/ap- proved by ATC for weather avoidance, when you report onto the next controller’s fre- quency. • Verify what additional services ATC is providing to you.

Is it what you need? PIPE UP WITH A PIREP! Pilot reports (PIREP) of flight con- ditions are an invaluable source of in- formation for other pilots and con- trollers as well. PIREPs should include reports of turbulence, icing, cloud tops and bases, intensity of rain, presence of hail, sleet, etc. A PIREP is often the only source of information regarding actual flight conditions a pilot may en- counter. Do your part for flight safety and pipe up with a PIREP! PIREP REPORTING FORMAT For anyone who has never sub- mitted a PIREP, the Aeronautical Infor- mation Manual (AIM) explains how to submit one, the uses of a PIREP, and the format a pilot should use in report- ing information.

AIM paragraph 7-1- 21, Pilot Weather Reports, is the refer- ence. Table 7-1-6, PIREP Element Code Chart, explains the reporting for- mat with the elements explained. Christine Soucy is with FAA’s Of- fice of Accident Investigation, Accident Coordination Branch and Michael Lenz is a Program Analyst in Flight Standards’ General Aviation and Com- mercial Division. 29 M A R C H / A P R I L 2 0 0 6 Precipitation echo depiction similar to non-digital ATC display - Intensity is unknown 5