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Summer 1999

Mt. Washington's wild weather sheds light on aircraft icing

by Bob Henson and Anatta

Two of the NOAA radars used in MWISP.

Seated on the radar trailer are, left to right, Robert Kropfli, Carroll Campbell, Roger Reinking (all NOAA); standing, left to right, Duane Hazan, Michelle Ryan, Bruce Bartram (all NOAA), and Marcia Politovich (NCAR). (Photos by Bob Henson.)

Researchers from several institutions spent most of April working at the Northeast's highest, coldest, and windiest peak. The Mt. Washington Icing Sensor Project (MWISP) tested methods for remote sensing and improved prediction of in-flight icing conditions, particularly freezing drizzle and freezing rain, which can down aircraft.

MWISP was the largest field program ever conducted on New Hampshire's Mt. Washington, where some of the first research on icing began in the 1930s. During April, the summit--best known for its high winds--is typically enclosed in clouds over 60% of the time. "It's a wonderful late-winter, early-spring cloud lab," says NCAR's Marcia Politovich (Research Applications Program). She joined Charles Ryerson (U.S. Army Cold Regions Research and Engineering Laboratory, or CRREL) and Kenneth Rancourt (Mount Washington Observatory, or MWO) in leading the field campaign.

During mid-April, the peak saw prolonged snow and cold, with winds gusting above 160 kilometers per hour (100 miles per hour). "The fast-moving clouds provided constantly changing conditions that were a challenge for the radars to track," says Politovich. The researchers found "very dynamic features, even in clouds that were pretty uniform visually." The month ended with ten intensive study days.

The group hopes to be publishing early results from Mt. Washington by this fall. "This is a very rich data set, and we'll be working on it in one form or another for several years," she says.

The goal of MWISP is to improve in-flight icing detection, mainly from remote sensors, and to improve forecasts issued by computer models. Pilot reports can be used for long-term, large-scale comparisons with a model. However, "To get down to smaller scales, like the terminal areas around airports, we need measurements with more detail than just the pilots' 'yes' or 'no' for icing," says Politovich. Better icing-prediction software is expected to become part of the weather research and forecasting model (see Weather Research and Forecasting Model ).

Remote sensors at MWISP included a five-channel, fully polarimetric radiometer at the summit that sensed radiation emitted by clouds. Another set of instruments was located near Bretton Woods, a few miles west of the peak. These included X-, K-, and W-band radars; a lidar; and a second multichannel radiometer. Balloon-borne instruments were launched from various points by a mobile unit from NCAR. Plymouth State and Lyndon State colleges provided weather forecasts (see How Unidata helps small schools tackle big projects ).

The NASA Glenn Research Center flew its Twin Otter aircraft through clouds above the peak to help document the uniformity in time and space of summit-cloaking clouds. Scientists from NCAR, NOAA, CRREL, and Quadrant Engineering analyzed data from remote sensors and compared the results with on-site measurements from the summit and aircraft.

A previous set of WISP experiments took place across northeast Colorado in the early 1990s. However, few tests had been done on supercooled clouds with relatively high liquid-water content, rare across Colorado but more common in the Northeast. Because of this geographical bias, says Politovich, "This is one of the first documentations of the effect of freezing rain on flight over an extended period." According to NCAR scientist Ben Bernstein (Research Applications Program), "Much of the research and development of operational forecast tools on supercooled drops has focused on freezing drizzle and ignored freezing rain." Bernstein has analyzed a case from February 1998 in which the same NASA Twin Otter aircraft now flying in MWISP suffered over 90 minutes of exposure to freezing rain above the Midwest. The result was a major degradation of the plane's performance, including an increase in drag of up to 200%.

The U.S. Army's participation in MWISP through CRREL stems in part from the huge number of Army helicopters and other aircraft vulnerable to weather. "The Army has the largest fleet of airborne vehicles in the military," notes Ryerson. Although airport weather safety has increased in recent years, he adds, "helicopters don't use airports." CRREL has joined NASA and the Federal Aviation Administration (FAA), along with scientists at NCAR and elsewhere, for an ambitious ten-year program to develop an in-flight safety system that addresses icing, wind shear, and thunderstorms. The next collaborative icing study is being planned near Ottawa, Canada, for next winter, with participation from that nation's Atmospheric Environment Service. And "we are thinking of going back [to Mt. Washington] in two years," Politovitch says. "That gives us time to analyze the data, see what different or new instruments to deploy, and do some more homework."

MWISP was funded largely by the FAA and NASA, along with CRREL and MWO. Providing instruments were NCAR; NOAA's Environmental Technology Laboratory; the University of Massachusetts; the Defense Research Establishment at Val Cartier, Canada; ATEK Data Corporation; and Stratton Park Engineering.


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