Sometimes it’s what you can’t see that can hurt you, and that is certainly the case with airborne weather radar. Pilots who use radar should be keenly aware of the implications of precipitation attenuation, which may manifest itself as a “radar shadow” on the display. This is just one of several scenarios where aircraft weather radar can lure the unaware into serious trouble. The good news is—in the U.S. at least—there is something pilots can do about this, provided NEXRAD data is available.
Of course, any information advocating the use of NEXRAD imagery must be prefaced with the caveat that it is never to be used tactically as a replacement for airborne weather radar. But it can augment radar nicely, and if employed properly can significantly enhance safety.
Total precipitation attenuation manifests itself as an area of black on the radar display. There are four attenuation scenarios that pilots must thoroughly understand. First, while on the ground with the tilt up, the pilot does not have the luxury of painting ground returns to look for shadows. But any convective cell that has a crescent shape (curves opposite the range-rings coupled with a steep gradient on the backside), is a classic shape that represents a cell so intense it is attenuating all the radar energy and only a portion of it is displayed. This is a very intense cell.
Airborne, pilots can paint ground returns, so anytime total precipitation attenuation is suspected, attempt to paint ground. An absence of ground returns where they should otherwise be is confirmation of attenuation; this is valid for all airborne scenarios.
In a stratus environment, precipitation attenuation typically manifests itself as precipitation returns out to a certain distance on the display, then appears black beyond (assuming low-altitude operation with a short range selected). Attenuation is confirmed if the outer perimeter of the weather remains in the same relative position that the aircraft travels. In essence, the radar is saying that radar energy can travel only part way through the precipitation and still make the return trip to the antenna.
The embedded-cell scenario is the most complicated to recognize because it is not intuitive; it manifests itself as a combination of the stratus and convective indicators described above. The majority of what is displayed will likely be stratus returns and, like the status-precipitation scenario, those returns may end at some distance short of the outer edge/perimeter of the display, with a subtle but critically important difference.
The outer boundary of returns may be characterized by a series of dips and bulges. An untrained pilot—who just wants to get out of the turbulence and heavy rain—may think the quickest way out of the precipitation is to fly towards the black, the dip on the outer perimeter. Not so fast!
In an embedded-cell scenario, that dip may be caused by increased precipitation intensity—possibly an embedded cell preceding the dip. The bulges, on the other hand, are areas where the precipitation intensity is less; radar energy is able to travel further and still make the trip back.
While every pilot who uses weather radar should be highly proficient identifying precipitation attenuation in all of those scenarios, on-board NEXRAD may offer a huge advantage provided it is employed wisely (and its limitations are understood) as it may reveal important details. This ranges from individual cell shape/gradient details to the depth of weather, including revealing when numerous lines/bands of weather lay in succession behind one another.
When viewing cells at longer ranges, aircraft weather radar is susceptible to another limitation known as the “dispersal effect.” This results in very intense weather, when viewed at longer distances, appearing to be understated—it looks less intense than is actually the case. Approaching a line of weather is a common scenario; the closest cells will appear most intense. But in the distance, say a hundred miles or so, the line appears to weaken. As the aircraft turns to parallel the line to head towards those apparently weak cells, they continually become more intense as distance to them decreases. Are they really growing in intensity? Maybe. But most likely what is causing this illusion is the dispersal effect. NEXRAD can reveal the truth about whether that line is actually weakening in the distance or if they are simply being displayed weaker than fact on radar due to the dispersal effect.
Certain areas of the country are susceptible to high-based convective weather (e.g. DEN, LAS, PHX, ABQ, etc.). In these environments, it is entirely possible for the bases of these cells to be 10,000 feet AGL, or higher. For aircraft on the ground getting ready to depart, this may pose a serious threat as it is possible that a high-based thunderstorm may reside above the scan of the beam, even with the radar antenna-tilt at full up (typically +15˚). Here, the rain shaft is characterized by VIRGA—a rain shaft that in any other environment would extend to the ground. But because the underlying air is so dry in these environments, it evaporates the rain-shaft before it reaches the ground, thus eluding radar. However, the cold air, which is a byproduct of the evaporation, continues crashing downward. If NEXRAD is available, always use it to augment the radar in locating cells in these environments.
It is always a good idea—when it is safe to do so—to operate the radar during taxi to the departure runway. The primary justification for doing so is that the very hazards pilots try to avoid may emanate from any cell anywhere around the airfield. Additionally, should an emergency return be required for any reason, knowing ahead of time where the hazardous weather is located may significantly simplify planning. But because the scan of aircraft radar is limited typically to 120˚ (60˚ either side of centerline), there’s usually a wedge of airspace somewhere around the airport that cannot be scanned; NEXRAD may provide information about the mystery sector.
Most pilots who use inflight NEXRAD are familiar with the advantage it provides evaluating weather beyond practical range of the on-board radar. This is helpful to gauge trend information (intensity, direction of travel, etc.) of distant weather. But it also provides an opportunity prior to top of descent to evaluate the area between the destination and the alternate—what if any convective weather resides there. While the common divert plan assumes straight-line travel to the alternate, NEXRAD can provide critical insight—early—if intervening weather makes that straight-line divert impractical.
While the NEXRAD system possesses many limitations that users should be aware of, two of them are worth reviewing. First, there are areas of the country (mostly out west) where large geographic gaps in NEXRAD coverage exist. In these areas, as well as in most of the rest of the world, NEXRAD will be of no help at all. Second, there are areas within the U.S., again, mostly out west, where large geographic areas are covered by a single NEXRAD site. While latency exists in every NEXRAD image, it will be exacerbated in these locations.
While black on a radar display may represent an area with little or no precipitation, there are several scenarios where serious hazards may be cloaked in black. Used properly, NEXRAD can augment airborne weather radar to reveal important details about these areas.
Radar Training International’s Erik Eliel conducts convective weather avoidance seminars covering use of airborne weather radar, human factors, and integrating NEXRAD technology into the overall weather strategy. He is a pilot for a major U.S. airline.