Putting travel risk on ice

NCAR scientists are expanding their dragnet for icing-related hazards in the air and on the road

by Bob Henson

The nation’s transportation system is assaulted each winter by an evildoer that long predates the terrorists of today’s headlines. Slippery and stealthy, this force sends cars careening off roads and pulls planes down from the sky It’s ice—one of the most troublesome and hard-to-predict weather hazards on the planet.

After studying aviation icing for over a decade, NCAR is taking a look at how winter weather affects highway travel. (Photo © The Denver Post.)

Thousands of men and women face off with this glassy enemy each year. They spray deicing fluid on aircraft, steer planes around risky areas, and blanket roadways with tons of salt and sand. Their efforts pay off, but too often their attack is an uncoordinated one, based largely on rules of thumb rather than state-of-the-art detection and decision support.

NCAR’s Research Applications Program stepped into this fray more than a decade ago. At the behest of the Federal Aviation Administration, RAP worked with partners in industry and academia to develop systems for detecting icing on the runway and in the sky. Some of these products have made their way to the Internet, and a new generation of tools is taking shape. NCAR has also set its sights on the road, where icing threatens millions of drivers. A fine-grained system that promises to revolutionize how roads are treated for snow and ice is being tested this winter in the Midwest.

Flight tools take off

“The FAA has been very good to us,” says Marcia Politovich. She and her NCAR colleagues, in turn, have been very good for the nation’s aviation infrastructure. The project scientist, who leads the in-flight icing team of the FAA Aviation Weather Research Program, has been involved since the program began in 1990. Once the domain of subjective decisions made by human forecasters, in-flight icing diagnosis is now produced through a human-machine mix.

Current Icing Potential, a set of maps available on the Internet (see “On the Web”), shows where two types of icing might be expected across the country at multiple flight levels. The product updates hourly, drawing on surface observations, satellite and radar data, pilot reports, and the Rapid Update Cycle numerical model. CIP gained FAA approval in 2001 as a tool for dispatchers to make fly/no-fly decisions and for flight planning and route changes.

CIP complements forecast and intensity information available from AIRMETs, the traditional National Weather Service alerts issued at six-hour intervals. It’s especially helpful for commuter planes and other propeller-driven aircraft, says Politovich. These planes cruise at lower, ice-prone altitudes, and some of them lack the mechanisms commonly found on jets that prevent ice buildup by heating the front edges of wings. A companion tool, Forecast Icing Potential (FIP), projects potential icing up to 12 hours ahead. It goes up for FAA approval in 2003.

Both CIP and FIP can be found on an experimental Web site created through another RAP-based, FAA-funded project, the Aviation Digital Data Service (see “On the Web,” p. 8). ADDS pools information from a variety of sources to give pilots at-a-glance guidance on icing and other hazards. The old system, based on AIRMET text, requires multiple pages of computer printout or speaking by phone with an FAA specialist, explains codeveloper Greg Thompson. “To know where the icing, turbulence, or convection is expected that day, pilots have to read or listen to a series of cryptic airport identifiers and connect the dots to construct an imaginary polygon.” With ADDS, pilots can check out the CIP and FIP outlooks and view maps built from AIRMETs with the outline of predicted hazardous areas clearly marked, or they can view the AIRMETs in text form. Users can also plot a vertical cross section of conditions along a proposed route.

Politovich and her colleagues have embarked on a new, even more challenging step: trying to depict not just the existence of icing but its intensity. Only AIRMETs now provide that information, but “different planes respond to icing differently,” says Politovich, and an aircraft-normalized view of icing can’t be easily gleaned from the AIRMETs reports. “It’s still a big problem.”

Other NCAR scientists are exploring ways to capture icing data from other sources:

• By next year, the center’s S-Pol radar will include a second wavelength for transmitting and receiving. This should allow for greatly improved detection of liquid water, including the supercooled variety that leads to icing.

• Guifu Zhang is working on a technique to infer regions of icing in vertical temperature and moisture profiles obtained from ground-based radiometers.

• Satellite-based algorithms are being developed by Merritt Deeter and Julie Haggerty that relate ice crystal and droplet sizes within a cloud to measurements obtained at the cloud top.

• A new version of the Rapid Update Cycle model, introduced last spring, includes an improved microphysics package from Roy Rasmussen, Thompson, and Kevin Manning (see “On the Web”), with another upgrade now in the works. Online users can see what type of precipitation to expect (snow, rain, freezing rain, sleet, or a mix) at intervals from 1 to 12 hours ahead.

Reducing the tab for deicing

When winter weather can’t be avoided, deicing fluid—a brew of propylene or ethylene glycol, water, and thickeners—can be a lifesaver. Airlines spend millions of dollars each year spraying planes before takeoff. A system unveiled in 1995 to help determine when and how to deice planes is now entering a new phase.

Project leader Rasmussen has been involved from the start, when the Weather Support to Deicing Decision Making system was deployed at Denver’s former airport, Stapleton International.

It was subsequently demonstrated at Chicago’s O’Hare International Airport and New York’s LaGuardia Airport, where it produced an estimated $1 million per year in savings at the two sites. After being transferred to a private vendor, WSDDM was picked up for deployment at the three major New York airports from 1999 into 2002. Despite the system’s proven value, its price tag of $200,000 per airport seemed too much after the September 11 attacks. “Every purchase is being scrutinized with a fine-toothed comb after 9/11,” notes Rasmussen. “It’s had a devastating effect on aviation.”

This snow-making device was developed by NCAR scientists Alan Hills, Roy Rasmussen, Charles Knight, and (pictured at left) Scott Landolt over the last six years in order to test aircraft deicing fluids in a controlled setting. The machine is housed in an NCAR cloud physics lab, with a clone at APS Aviation in Canada. Its raw material is purified water frozen into ice cores, each about 8 centimeters (3 inches) wide by 120 cm (48 in) long. Using a computer-controlled process, the machine shaves the cores to produce snowfall at virtually any desired rate. Although many ski resorts have giant snowmakers, this one is different in that “the snowflakes it generates have a density, size, and velocity close to that of natural snow,” says Hills, who has led many of the machine’s refinements after initial work by Rasmussen and Knight. The closest analog to the NCAR device may be in Hollywood, where similar ice-shaving technology helps create many of the snowfalls seen on camera. (Photo at left by Carlye Calvin; instrument photo by Alan Hills.)

The city of Denver is slated to pick up the tab for a new version of WSDDM being tested this winter at Denver International Airport. Thanks to the city sponsorship, a wide range of airport users will be able to access the system, and costs will be lowered by using internal airport data networks rather than the dedicated phone lines in place at the New York airports. Future versions of the system will be Web-based, reducing costs even further, says Rasmussen.

In Denver’s version of WSDDM, real-time observations are being updated once a minute, and the start and stop times of snowfall are being predicted an hour in advance. NCAR’s Mei Xu and Andrew Crook are devising techniques to assimilate radar data into the Penn State/NCAR Mesoscale Model (MM5) in order to extend WSDDM’s snowfall forecasts out to 12 hours.

Simply providing a real-time estimate of snowfall as it falls is a nontrivial exercise. In the mid-1990s, Rasmussen and colleagues discovered that icing at ground level is more closely related to the amount of liquid water in snow than to the visibility criteria long used to gauge snowfall intensity (light/moderate/ heavy). Based on this work, the United States and Canada now take the time of day, temperature, and other factors into account when reporting snowfall intensity. A new sensor (see sidebar) may further change the landscape of snow measurement.

A “cold room” at NCAR’s Foothills Laboratory is a convenient year-round venue for testing snowfall and its impact on deicing. Project scientist Alan Hills and associate scientists Scott Landolt and Matt Tryhane use the lab—and, when weather permits, an outdoor site just south of Boulder—to see how snowfall and temperature affect the performance of deicing fluids for aircraft. The data help the FAA to certify specific brands of fluid with confidence.

The next frontier: highways

Next to the sleek setups at airports, the process that deploys trucks along roadways to tackle snow and ice seems positively low-tech. With the right information at hand, though, far more could be accomplished. Such is the philosophy driving the Winter Road Maintenance Decision Support System (MDSS).

“The idea is to help agencies that maintain roads to better gauge where and when to use deicing techniques,” says NCAR’s William Mahoney, who heads up the project. This means providing much more than a generic outlook such as “snow likely tonight,” Mahoney points out. “We need forecasts that are more specific, more timely, and tailored for decision makers who are not meteorologists.”

To develop a prototype, NCAR teamed with four other labs associated with the U. S. Army, NOAA, and the Massachusetts Institute of Technology. The end result pulls together existing road and weather data to create an easy-to-decipher picture of current conditions. The system uses numerical modeling to project hour-by-hour roadway conditions up to two days in advance, with an update furnished every three hours.

“Users can pick a route, look at conditions, see what would happen if they didn’t take any action, and ask the system for a recommended treatment,” says Mahoney.

William Mahoney, Roy Rasmussen, and Marcia Politovich are among the leaders of icing-related studies in NCAR’s Research Applications Program. (Photo by Carlye Calvin.)

From February into April, MDSS will be put through its paces by three state-run maintenance groups serving highways across central Iowa. Each plowing route’s predominant characteristics, such as pavement type, will be specified in advance. With such detail in hand, the system can assess how temperature and precipitation will affect the road surface. Ultimately, users will be able to ask the system to track features as specific as a single bridge paved in concrete along an asphalt road.

“A lot of the folks with experience in highway maintenance are starting to retire,” says Mahoney. “The new guy may not know that when you hit a certain bridge you should do something different. Those are things you learn from experience. The MDSS is designed to help make that process more uniform. It’s a blend of science—weather and road-condition prediction—and art, the state of practice for winter icing and deicing. ”

Only a few drivers will be affected by this winter’s tests, but millions more could benefit in the next few years. About half of the nation’s state transportation departments have already signed on as stakeholders, along with some 25 private-sector companies.

“We’re cutting our teeth in this area,” says Mahoney, who’s been crisscrossing the country pulling together support for the program. “Nothing like this has been done before. It’s pretty snazzy. ”

Measuring snowfall as it sizzles

A sandwich with a silicone filling and two slices of aluminum could be the next big thing for measuring rain and snow. NCAR and the University of Nevada’s Desert Research Institute have patented a hot-plate sensor that gauges precipitation in a novel way. The hamburger-sized device does its job with the help of electrical heaters that warm both of the aluminum plates to a constant temperature. The system works like a Wheatstone bridge, a concept familiar to physics majors. As precipitation falls on the top plate, the plate loses heat at a rate proportional to the amount of water in the snow or rain. By measuring how much power goes into the plate, scientists can infer how hard it’s raining or snowing. Cooling from the wind is factored out by comparing heat loss from the top plate to that of the bottom plate. Right now the system can make accurate readings up to liquid equivalents of 10 millimeters (0.4 inch) per hour, which translates to about 10 cm (4 in) of wet snow per hour. That’s adequate for all but the most furious winter storms. However, the sensor’s developers want to increase the power going into the sensor so it can handle rainfall rates up to 150 mm per hr (6 in/hr). Another challenge is measuring snow and ice pellets that can bounce off the plate. Like a waffle iron, a new version of the sensor includes spikes as high as 1 cm to help trap the frozen pellets. Each of the sensors now costs about $15,000 to make, but costs should go down once a vendor is chosen within the next year. Roy Rasmussen expects the device to someday become the centerpiece of the WSDDM system (see main article).


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India steps up its medium-range prediction

Observationalists meet to map out HIAPER sensors

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Unidata’s new director: Mohan Ramamurthy

President’s Corner: Weather, Climate, and the Evolving U.S. Climate Change Science Program

Web Watch

UCAR Community Calendar

Governance Update