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Smoother skies

Can weather research help ease the nation’s congested aviation system?

by Bob Henson

aviation weather team

NCAR’s aviation weather team includes (clockwise from top left) Robert Sharman, Marcia Politovich, Bruce Carmichael, John Williams, and Robert Barron. (Photo by Carlye Calvin.)

During the haunted weeks and months after 11 September 2001, it seemed that Americans might never take to the skies as they had before. But today, U.S. airlines are hauling more passengers than ever.

That’s good for air carriers and the overall economy, but it doesn’t always make for a pleasant consumer experience. For one thing, planes are packed these days. According to the U.S. Department of Transportation, flights in 2007 averaged close to 80% full, a new record. And more than 26% of U.S. commercial flights were delayed or cancelled in 2007, the second-worst performance on record behind 2000. About 40% of those delays were weather-related.

Some 80 scientists and technicians at NCAR’s Research Applications Laboratory (RAL) are now working full or part time on projects designed to help keep aircraft safe from weather threats. Funded mainly by the Federal Aviation Administration (FAA), this work extends back more than 15 years in some cases. It’s now being reshaped by the demands of the boom in air travel, with a new focus on using prediction software and observing tools to bring planes into and out of airports both safely and efficiently.

Fewer disasters, more delays

“We’ve always had a safety orientation,” says Bruce Carmichael, director of aviation applications for RAL. In the 1980s, RAL (then known as RAP) got its start by working with the FAA and university partners to show how wind shear could be successfully detected and avoided. At that time, accidents related to wind shear were killing hundreds of U.S. passengers every few years. But only one such crash has occurred in the United States since a warning and detection system based on RAL’s research went into place in the 1990s.

Since 9/11, the nation’s airspace has become safer still. Less than 100 passengers have died in U.S. airline accidents since 2002. Yet the crowding and delays show no signs of abating. Passenger loads, estimated by the FAA at 780 million for 2007, could exceed one billion by 2015 and 1.5 billion by 2025. With funds for infrastructure scarce, most U.S. airports aren’t planning major expansions in the near future. (The increase in flying also has environmental implications, as discussed below.)

Given the recent contrast between airlines’ stellar safety records and their deteriorating punctuality, “the federal emphasis has shifted,” says Robert Barron, RAL deputy director of aviation applications. Today, even in ideal weather, many airports are incapable of moving the number of aircraft scheduled by their resident airlines without some delay. “Throw a little weather on top, and suddenly it becomes a nightmare,” says Barron.

Policymakers are well aware of the trend toward more crowded skies and airports. In 2003, Congress called for a complete revamping of the nation’s aviation system by 2025. That legislation spawned the Next Generation Air Transportation System (NextGen), a public-private partnership to modernize every major aspect of air traffic, including weathermonitoring and prediction. NextGen programs are expected to cost up to $22 billion by 2025, with a similar amount needed to equip aircraft with NextGen-spawned technology.

Weather is a major concern for the Joint Planning and Development Office, the interagency office charged with implementing NextGen. One of nine NextGen working groups is focused on weather; the co-chairs of that group are Mark Andrews (NOAA) and Steven Brown (National Business Aviation Association).

NextGen envisions a largely decentralized system for orchestrating air traffic. Currently, the system emphasizes human controllers, which means extra-large margins of error must be employed to account for the controllers’ inherently limited viewpoints. In the future, as described in a NextGen brochure, “Flight crews will have increased control over their flight trajectories and ground controllers will become traffic flow managers.”

To make this power-sharing approach workable, pilots and controllers must have access to a common weather picture. As Carmichael puts it, “Everybody needs to be using the same sheet of music.”

Weather cubed

The cornerstone of NextGen’s approach to aviation weather is a four-dimensional virtual “cube” that will hold frequently updated 3-D information on past, present, and future weather. As NCAR works to help develop this cube, it will leverage years of experience from building the online Aviation Digital Data Service, which provides the aviation community with easy-to-access weather guidance.

The NextGen cube will provide a single authoritative source for current and forecast information for each of the aviation weather impact areas, including icing, turbulence, ceilings and visibilities, and storms. The Rapid Refresh version of the multi-agency Weather Research and Forecasting model will be a fundamental contributor to each of these areas.

NextGen is prompting consideration of weather-related factors for which guidance is not yet available to users, such as wake vortices. Like a tiny tornado on its side, a wake vortex spins off horizontally from the wingtip of an aircraft in motion. Due to the vortices’ small sizes, current operational observing systems at airports can’t spot them, and


De-icing treatments are common for U.S. aircraft when winter weather strikes. . (Photo by Carlye Calvin.)

they’re normally invisible to the naked eye. To be on the safe side, aircraft are typically separated by as much as six miles (10 kilometers) on takeoff and landing. That’s a far greater distance than is otherwise needed, and perhaps far more than the vortices themselves require.

“There’s a whole lot of tightening up that could be done if we knew where the wake vortices were and where they’re migrating,” says Carmichael.

Ceiling and visibility (C&V) is another area getting increased attention. At San Francisco International Airport, for example, the main runways are only 750 feet (230 meters) apart. A low deck of marine stratus often blankets the airport in the morning, causing one of the two runways to be shut down. In this scenario, many aircraft scheduled to fly to San Francisco may be held at their departure cities until the stratus clears. With a reliable forecast of the burn-off time, these aircraft can take off sooner, approaching the airport just as clearing occurs and airport capacity rebounds.

An NCAR group led by Paul Herzegh has completed its first system designed to monitor and predict C&V. The diagnosis component will become operational this summer, while a forecast prototype is now in the process of being approved by the FAA and may be available by year’s end. These tools will help pilots and controllers better determine where and when low clouds or fog may cause trouble.

Another part of NextGen could transform the C&V problem in years to come. A synthetic display now being developed will use weather data and other information to give pilots a simulated view of the airspace ahead of them, even when instrument flight rule (IFR) conditions block the actual view. Once the new display is ready, says Carmichael, “C&V will cease to be a problem for large planes that are IFR-certified.”

Avoiding bumps, dodging ice

Other meteorological components of NextGen draw on areas studied for years by NCAR and its partners. Turbulence and icing guidance is available on the ADDS Web site, for instance, but more detailed and sophisticated products are on the way.

For example, the Graphical Turbulence Guidance forecast product, already available, is being greatly enhanced to cover all altitudes and types of turbulence. Part of this enhancement is due to improved capability to sense turbulence in both clear and cloudy air. An in situ (in-place) capability to compute atmospheric turbulence using information already available on the aircraft is being deployed extensively on commercial airlines. This will provide reports of turbulence along each flight track. In the near future, significant portions of the fleets of United, Delta, and Southwest Airlines will be downlinking these routine, quantitative reports. In addition, an improved system developed at NCAR for helping pilots spot turbulence in clouds and storms has been tested by United Airlines over the last three summers. It uses data from the National Weather Service’s network of ground-based Doppler radars.

“Pinpointing turbulence in clouds and thunderstorms is a major scientific challenge,” says NCAR’s John Williams. “Our goal is to use these radar measurements to create a three-dimensional mosaic showing turbulence across the country that can help pilots avoid hazardous areas, or at least give them enough warning to turn on the ‘fasten seat belt’ sign.”

de-ing map

Conditions were ripe for icing at 10,000 feet above sea level (about 3,050 meters) across a broad swath of the Ohio Valley and intermountain West on the night of 21–22 February, according to this depiction from the upgraded Flight Path Tool of the Web-based Aviation Digital Data Service ( This NOAA site, created with NCAR expertise, allows pilots and controllers to map a variety of current and projected weather conditions. The color bar at bottom indicates the chance of icing, which occurs when aircraft encounter supercooled water droplets. (Image courtesy ADDS.)

Final testing of the algorithm will take place this summer. If all goes well, the algorithm will contribute to a nationwide system providing turbulence outlooks every 15 minutes for pilots and controllers. These outlooks should help pilots reduce the rerouting around storms that now occurs in the absence of more detailed data.

In all, more than a dozen separate tools for turbulence and other thunderstorm-related hazards have been created by NCAR and partners over the last 20 years. A portion of that knowledge and technology is now being folded into a program called Consolidated Storm Prediction for Aviation (CoSPA). A product of NCAR, NOAA, and the MIT/Lincoln Labs,

CoSPA will lead to frequently updated forecasts, extending out to 12 hours, of both summer and winter storms. The first real-time demonstration of CoSPA, producing 0-to-6-hour forecasts at a resolution of 3.3 kilometers (2.0 miles), will take place this summer. CoSPA products will gradually replace existing guidance over the next few years.

Another tool now being refined is the Forecast Icing Product (FIP), which is available on ADDS but currently restricted to meteorologists and dispatchers. It maps in 3-D the areas most likely over the next three hours to contain supercooled liquid water that could freeze on contact with an airplane. Every hour, the FIP takes model data, calculates cloud depths, determines precipitation type, and uses a fuzzy-logic method to estimate the risk of icing.

Right now the FIP can only provide an index of relative risk (such as a value of 70 in one spot and 30 in another) rather than actual probabilities (such as a 70 percent chance of icing). An NCAR team led by Marcia Politovich is now calibrating the FIP so that it provides truly probabilistic guidance. In the meantime, pilots and controllers are making use of the Current Icing Product, or CIP, which applies the same techniques on current data rather than model projections. Pilots can now access CIP in their cockpits to make course adjustments as needed.

Greener skies ahead?

While a doubling of air travel by 2025 may be good for the bottom line, it raises eyebrows among those concerned about climate change and other environmental ills. “I think the environmental impact of aviation is going to become more important to us over the next ten years. People are getting serious about this,” says RAL’s Carmichael.

In late February, Carmichael and several other NCAR staff attended an international conference held by the FAA Environmental Research Program to examine the status of research involving aviation and climate change. From this conference will emerge a joint international initiative to answer remaining questions about aviation’s impact on climate change and on strategies to mitigate that impact.

Guy Brasseur, the head of NCAR’s Earth & Sun Systems Laboratory, is serving as lead coordinating author for the conference report, cosponsored by NASA and NOAA. “This will be an opportunity for the U.S. scientific community to better understand how aviation will affect our climate, including the impact of contrails and the cirrus clouds they help produce,” says Brasseur. NCAR’s Andy Heymsfield is compiling the analysis on contrails and cirrus, which have drawn increasing attention.

Even though NextGen’s emphasis is on making room for more airplanes and passengers, the program has a green side. One of its nine working groups deals with the

On the Web

Aviation research at NCAR/RAL (from 2007 NCAR Annual Report)

Aviation Digital Data Service


environment, and NextGen includes a number of capabilities that will lead to quieter, less fuel-intensive flights.

One smart technique is “continuous descent approach”—essentially, allowing an aircraft to land at a steady angle of about 3 degrees instead of the more conventional stairstepping approach. Studies at the Georgia Institute of Technology and the Massachusetts Institute of Technology have shown that this reduces noise and saves both time and fuel, since less thrust is required and pre-landing altitudes are higher. The pioneer in this technique is British Airways, which is now using it on more than 95% of its landings at London’s Heathrow and Gatwick airports.

Another factor limiting airport capacity is noise.  Noise restrictions are already in place at a number of airports around the world. NCAR’s Robert Sharman has been investigating the possibility of generating a “noise forecast” that would factor in atmospheric conditions, such as temperature inversions and low-level wind shears, to define flight trajectories that would minimize noise annoyance.

These and other strategies to come will help NextGen shave time and fuel from air travel. Some of the gains will no doubt be offset by the anticipated increase in flights. But regardless of how often people choose to fly, the research now under way will help make a given journey more efficient. With safety and capacity already considered vital, “the environment could become the third leg on the stool,” says NCAR’s Barron.

The trouble with night flights

Taking a red-eye flight could help you pack more into your schedule, but it might well have a bigger warming effect on climate than taking the same flight during daylight hours. Contrails, and the cirrus clouds they help form, help trap upwelling radiation from Earth around the clock. This can be at least partially offset if the contrails and clouds reflect sunlight—but that’s possible only during daylight hours.

In a 2006 study in Nature, a group led by Piers Forster (University of Leeds) estimated that nighttime flights led to 60–80% of the atmospheric forcing from all contrails above the busy southeast England corridor. Yet those nighttime flights were outnumbered three-to-one by daytime flights over the same area. The work was extended in 2007 for Atmospheric Chemistry and Physics. In that journal, Nicola Stuber (University of Reading) and Forster reported that night flights accounted for less than 40% of the global distance traveled by aircraft but 60% of the average global annual forcing.

piers forster

Piers Forster.

Reducing the total forcing from contrails could take a substantial chunk out of aviation’s overall climate impact. The 2007 assessment from the Intergovernmental Panel on Climate Change estimates that persistent linear contrails produce a climate forcing of between 0.006 and 0.015 watts per square meter, compared to 0.025 for the carbon dioxide emitted from aircraft. Other aviation-induced cloudiness may increase the overall warming effect by several times, the IPCC notes.

“I think there is a clear case for flying by day,” says Forster, a coordinating lead author for the IPCC chapter cited above. He notes that switching flight times on a global basis could have a much more immediate impact on climate than reducing greenhouse-gas emissions from aircraft, which would take decades to achieve its full benefit. When we analyze aviation’s impact on climate, he says, “the choice of the time horizon is crucial.”


Contrails are a significant part of aviation’s impact on climate. (Photo by Carlye Calvin.)




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