Even if you have a datalink weather capability in the cockpit, don’t give up on your air traffic controllers. Many other technologies are at work—albeit unseen, but critical to get the pilot back on the ground safely.
This is especially true when flying through a high-impact terminal area during the warm season when convective SIGMETs are sprinkled nearly everywhere. Your satellite-based weather receiver becomes less effective in the busy terminal area due to its latency—especially if an update or two is missed. The good news is that some approach controllers are armed with near real-time weather data from two additional sources to include the Airport Surveillance Radar (ASR) and Terminal Doppler Weather Radar (TDWR). Even though the controller can’t see your view outside of the cockpit, they can offer near real-time guidance around the most significant cells. This is especially critical when microbursts have reared their ugly head in and around the terminal area.
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During the warm season when convective SIGMETs outnumber all other advisories, many pilots can’t seem to get enough ground-based weather radar images from the WSR-88D NEXRAD Doppler radars. The NEXRAD image often plays a pivotal role helping pilots locate a route that will minimize their exposure to dangerous deep, moist convection. Through the magic of a datalink weather receiver, NEXRAD also travels lightly with the pilot.
The spatial scales of en route operations are nearly limitless with a lot of elbow room to get around convective weather. If you don’t like the visual you see ahead or your datalink weather display doesn’t look inviting along your current route, then take a 20-mile excursion around the cells you are painting on your multifunction display (MFD) or your tablet. While these deviations may eat into your fuel reserves, the entire process is fairly painless.
As the flight nears its end, however, the options begin to dwindle quickly. In fact, many aircraft can cover almost 10 nm between datalink weather updates, thus limiting its intrinsic value. Plus, the airspace is more congested, giving you less options to deviate. Air traffic controllers (ATCs), on the other hand, can see weather updates on a minute-by-minute basis providing the necessary monitoring in and around the busy terminal airspace when confronted with significant convective weather. And, of course, diverting to another airport is always an option when the fuel reserve is low and your destination is restricted by a line or area of nasty convection.
Many approach control facilities have ASRs like the ASR-9 or ASR-11 that are physically located on the field. Both the ASR-9 and ASR-11 are dual fan-beam Doppler radars that not only target your aircraft but have a weather channel that can detect hydrometeors such as rain, snow, and hail. Both feature a dedicated weather reflectivity processor that outputs “six-level” maps of precipitation reflectivity. A Weather Systems Processor (WSP) has been added to many of the ASR-9s to measure Doppler wind velocity and thereby support detection of low-level wind shear events. The WSP also improves the quality of the ASR-9’s precipitation reflectivity measurements by eliminating ground clutter that may occur during anomalous propagation weather conditions. This is something you may see when there’s a surface-based temperature inversion that tends to duct the radar beam into the ground creating false returns.
The Airport Surveillance Radar/Model 11 (ASR-11), shown at the Charlotte Douglas International Airport (KCLT) in North Carolina, is a Doppler radar that not only tracks air traffic but also weather. [Courtesy: Scott Dennstaedt]
The WSP provides full resolution reflectivity and velocity (Doppler) imagery out to the ASR-9’s range of 60 nm. These images are updated every antenna sweep (4.6-4.8 seconds) out to 15 nm where the wind shear detection algorithms operate, and every minute at greater ranges. Not too shabby. Wind shear alerts due to gust fronts, outflow boundaries, and microbursts are automatically detected and tracked providing the approach control facility with a warning of hazardous wind shear that in turn is passed along to the pilot. While WSP adds additional capability to the ASR-9, the TDWR is a completely separate system. It is similar in many respects to NEXRAD, but in many situations it is of better quality especially as it relates to wind. There are 45 TDWRs in operation across the U.S., and most are located at high-impact airports or near airports vulnerable to wind shear conditions from thunderstorms and frontal passages. One of the first things you’ll notice about TDWRs is that they are located off the airport about 7 miles from the center of its runway complex. This is because forward-pointing radars have a cone of silence called a “zenith cone”—not a good thing to have a blind spot right over the airport you are trying to survey and protect. Instead, the TDWR is located in a place that has good visibility to the approach and departure corridors and the runway environment.
The angular (azimuthal) resolution of the TDWR is nearly twice that of the NEXRAD Doppler radars. Each radial in the TDWR has a beam width of 0.55 degrees as compared to the average beam width for NEXRAD of 0.95 degrees. The higher resolution comes with a penalty, however. With a 5-centimeter wavelength, TDWR is more subject to attenuation or signal loss, and that can lead to more misleading data than its 10-cm wavelength counterpart, namely, NEXRAD. Although signal attenuation for the TDWR is slightly greater than for the WSR-88D radar in precipitation, TDWR velocity data (winds) are largely unaffected. And that’s good news because it’s the wind that we really want to track.
Not only does the TDWR help ATC identify dangerous wind shear potential, it also assists the forecasters at the local weather offices. For example, Charlotte Douglas International Airport (KCLT) is located over 75 miles north-northeast from the closest NEXRAD radar located near the Greenville-Spartanburg Airport (KGSP) in Greer, South Carolina. The Charlotte region is one of the largest metropolitan areas in the country that isn’t covered by a nearby NEXRAD radar, endangering millions of people including the flying public. As of press time, there is legislation introduced into Congress to fund a new NEXRAD radar installation in the Charlotte region.
A ribbon display mounted near the air traffic controller’s sector suite or in the tower cab provides any recent microbursts alerts. [Adobe Stock]
One of the greatest advantages of the Charlotte TDWR (TCLT) is its location under KGSP’s NEXRAD radar coverage area. Due to the curvature of the Earth’s surface, the lowest standard atmosphere view from the KGSP radar over center city Charlotte is 7,245 feet agl. In comparison, the lowest scan from the TCLT radar is only 315 feet. Both the finer resolution and closer proximity of the TDWR to the Carolina Piedmont makes significant improvements to the Doppler radar data used by meteorologists issuing severe thunderstorm and tornado warnings. Essentially, if you want to see a tornadic vortex signature on radar, the lower the scan the better. Since TWDR and the Low-Level Wind Shear Alert System (LLWAS) were first installed, wind shear accident fatalities in the U.S. have dropped from several hundred over a period of a decade to zero since 1995. Given that convective wind shear happens on such a small spatial scale, it’s hard to predict or detect until it is too late. But these technologies have proven to be a useful tool to essentially eliminate these tragic accidents.
This drop in fatalities has also been attributed to pilot training. Here’s the problem. Pilots flying light, medium, or heavy aircraft are not going to be bitten by a supercell thunderstorm. That’s because those storms look ugly, and pilots are more likely than not to avoid them at all cost. However, a benign-looking cell such as a high-based rain shower is far more inviting. The high base gives the pilot the impression that it’s safe to fly under and lures them into a false sense of security. Often, there’s no visible rain shaft present until it’s too late. For example, Delta Flight 191, an L-1011 approaching the Dallas-Fort Worth International Airport (KDFW), was taken down by a microburst in 1985 from a benign-appearing cell, not a supercell thunderstorm. Drs. Ted Fujita and Fernado Caracena stated a long time ago that most microbursts occur from benign-looking cells such as these. You may recognize the name Fujita. He was the research meteorologist and professor at the University of Chicago that actually provided a definition for the microburst. Furthermore, how tornado intensity is quantified today is named after him, namely, the Fujita scale.
Here’s a quote from an article written by Captain William Melvin titled “Wind Shear Revisited” in the November 1994 issue of Air Line Pilot magazine:
“Many pilots have been trained to avoid large supercell-type thunderstorms in the belief that this will prevent encounters with microbursts. Yet no evidence exists that any of the known microburst encounters have occurred in supercell storms. Dr. Ted Fujita and Dr. Fernando Caracena recognized authorities in this field have repeatedly emphasized that microbursts are frequently generated from benign-appearing cells. Many ‘experts’ who disagree with Drs. Fujita and Caracena have emphasized the supercell storms with warnings of dangers of gust fronts. These so-called experts are leading pilots down the primrose path for microburst encounters.”
The training that pilots receive today along with airborne and ground detection systems continue to be a true godsend. Most pilots are used to looking at the NEXRAD surveillance scan of the radar that measures the base reflectivity. However, it’s the Doppler scan of the TDWR that is the crown jewel when it comes to detecting a wet or dry microburst. This is referred to as the base velocity data. A microburst is a rush of concentrated air flowing nearly straight down (although there is usually a rotation of this air as it descends) out of the cell that may strike the surface and spread outward like pouring pancake batter on a griddle. The TDWR velocity data does not show the velocity of the downward motion of the air itself.
It simply quantifies the magnitude of the velocity component of the wind directly toward or away from the radar site once the air from the microburst or downburst is moving horizontally.
As the microburst begins to make its outward movement, those hydrometeors (i.e., raindrops) and other particles to include dust, pollen, sand, and insects intersect the radar beam and are detected creating a dual-node signature. Green colors are motion toward the radar site, whereas red colors are motion away from the radar site. It is this classic dual-node signature that can be quickly detected and tracked by the TDWR and alerts generated. Hydrometeors that move tangentially to the radar beam show up as a zero velocity although the actual magnitude of the wind in that direction may be extreme.
Once detected by the TDWR, microburst alerts are provided to controllers through a ribbon display with an audible. For example, an alert such as “17A MBA 30K- 1MF 150 11” states, “Runway 17 arrival, microburst alert, 30 knot loss, 1 mile final, threshold wind 150 at 11 knots.” In plain language, the controller will tell the crew that on approach to Runway 17, there is a microburst alert on the approach corridor to the runway and to expect a 30 knot loss of airspeed at approximately 1 mile out on final approach. With that information, the crew is forewarned and should be prepared to apply any wind shear/microburst escape procedures should it decide to continue the approach.
During the warm season, many GA pilots will tell you that they are dependent on their datalink weather in the cockpit. For most flights, that’s usually good enough. But unexpected weather is quite common, so having those extra eyes on the sky through terminal radar is greatly appreciated.
This feature first appeared in the April Issue 957 of the FLYING print edition.
