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December 2004 - January 2005

Prospecting for ice

By guiding a NASA plane into clouds that harbor cold water drops, RAL researchers hope to improve icing forecasts for the aviation industry.

Winter’s here and the time is right for flying into ice. These are early mornings for RAL’s Ben Bernstein, Frank McDonough, and Cory Wolff. The trio is issuing predawn forecasts from their Foothills Lab offices to help guide a NASA Twin Otter turboprop airplane based in Cleveland into clouds with a high icing potential. Their long-term goal: improve forecasts of icing so pilots can be warned away from treacherous areas.

At times, they travel to Cleveland and ride on the plane, comparing the accuracy of their forecasts with their own observations of ice building up on the wings.

Most of the flying public would rather not head into hazardous weather conditions. But, Cory says, “there’s no better way to verify your forecast than to get on the airplane and see what’s actually going on.”

The hunt for perfect ice

Protecting planes from in-flight icing is a high priority for the aviation community. Ice formation on wings has been a contributing factor in a number of fatal accidents, including the 1994 crash of an American Eagle ATR-72 outside Chicago that killed all 68 people on board. A 1997 icing-related crash outside Detroit resulted in the deaths of the 29 people on board.

Ice on the wing of the NASA Twin Otter. (Photo courtesy NASA Glenn Research Center.)

But detecting treacherous regions in clouds that can produce ice on aircraft wings remains a work in progress.

Icing occurs when supercooled water drops adhere to an aircraft wing and freeze (supercooled drops are liquid even though the temperature is below 32°F, or 0°C). When ice builds up on the wings of an aircraft, it can simultaneously slow velocity and decrease lift, potentially sending a plane into a catastrophic dive.

The RAL team has collaborated with the NASA Glenn Research Center in Cleveland every winter since 1997. The project helps both agencies: RAL gets instant feedback on its forecasts and obtains valuable information about the clouds from a suite of on-board instruments, while NASA tests the performance of its aircraft under actual icing conditions.

The challenge for the forecasters is to locate what one might call the “Goldilocks” region, where conditions are just right. A few degrees too warm, and the water drops won’t freeze on the wings of an aircraft; a few degrees too cold, and the water drops will turn into ice crystals or snow instead.

The narrowness of that range came into focus on March 20, 2000, when United Airlines had to cancel numerous flights because of severe in-flight icing conditions at Denver International Airport. The icing conditions surprised meteorologists, who had expected a heavy snowfall instead. Looking over data after that event, the RAL team found the cloud tops that day were warmer and lower than predicted. This meant that supercooled water drops forming in the lower atmosphere were not being cleared out by snow that would have formed if the clouds had been deeper.

“There’s no absolute situation, no one type of weather, no anything that tells you for certain there’s icing,” Ben explains. “You just kind of have to mix up this big soup of data and decide whether or not there’s going to be ice.”

Over the years, the team has found that one perfect setup for ice is a large, relatively uniform area of deep stratus or stratocumulus clouds whose tops are a few degrees below freezing (-12 to -5° C or 11 to 23°F). But subtle factors can dramatically affect the severity of icing. Depending on wind direction, for example, a 20-to-50-mile-wide (32-80 kilometer) band outside Cleveland often produces icing because winds in that narrow area blow across both Lake Huron and Lake Erie, creating a lake-effect icing cloud. This phenomenon is like a lake-effect snowstorm, but it produces supercooled liquid water instead of snow.

To make the forecasts, the team members look at satellite and radar data, and they weigh surface observations along with pilot reports on ice. As Frank puts it, “We use pretty much anything we can get our hands on that can give us clues as to what’s going on in the atmosphere.”

Although forecasting icing conditions may be as much art as science, the RAL team is getting good at it. The team can guide the Twin Otter into icing conditions almost every time. When the collaboration started, the success rate was only about 70%.

New tools for pilots

Thanks to the insights gleaned from the NASA collaboration, RAL launched a new forecasting tool for airlines in 2002. The Current Icing Potential (CIP) provides pilots with an online display of high-precision maps that identifies areas of potential aircraft icing produced by cloud drops, freezing rain, and drizzle. It draws on surface observations, numerical models, satellite and radar data, lightning observations, and pilot reports. A companion product that is based solely on numerical model output, the Forecast Icing Potential (FIP), provides outlooks of icing conditions up to 12 hours in advance.

The ice forecasters (left to right): Frank McDonough, Ben Bernstein, and Cory Wolff.

CIP and FIP have an 85% success rate in detecting icing conditions, Ben says. But the tools often warn of icing conditions when none exist, and the team wants to reduce the number of false positives. Another goal is to provide more detailed forecasts that will, in addition to alerting pilots about the location of icing, also rate the severity of the icing hazard.

The team also is putting a greater focus on a particular type of supercooled drops, larger than 50 micrometers, which can be especially dangerous. Small supercooled drops usually freeze when they hit the front of an airplane wing, and many planes are equipped with deicing devices designed to melt them. Large supercooled drops, however, flow back over the wing before freezing, or strike the wing rear of the deicing equipment, creating crusty ridges of ice that can be destabilizing.

The Federal Aviation Administration is expected to issue a rule that could require aircraft manufacturers to create devices to protect wings from large-droplet icing. But designing such devices may be difficult until more is known about the behavior of large-droplet icing.

In two years, RAL will begin gathering even more data on icing conditions. That’s because NASA will begin deploying an S-3 Viking jet with the range to cruise over the entire continental United States instead of being restricted to a radius around Cleveland. This will allow the team to examine regions such as the Pacific Northwest, where icing conditions are believed to vary considerably from the Great Lakes.

“It will open up some exciting research possibilities,” Ben says.

map

The CIP identifies areas of potential icing. This graphic, from the CIP Web site, shows icing at 15,000 feet.

•David Hosansky

On the Web

Current Icing Potential


Also in this issue...

The 2004 Outstanding Accomplishment Awards

Recollections from a pioneering woman scientist

IMAGe comes into focus

Native American visitors

Turning off the juice

Delphi questions

Happy Holidays!


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