While spring may be right around the corner, Colorado scientists are in the midst of a major study on winter storms. During the Winter Icing and Storms Project 1994 (WISP-94), a two-month field program lasting until 25 March, about 25 researchers from NCAR's Research Applications Program, the National Oceanic and Atmospheric Administration (NOAA), and several universities are searching for the secrets that cause supercooled water droplets and ice crystals to form in clouds. This is the fourth winter since 1990 of WISP research, sponsored through an interagency agreement with NSF and the Federal Aviation Administration (FAA).
"Although it seems like we haven't had a lot of big winter storms this year, we've actually been very busy," says Marcia Politovich. She and Roy Rasmussen, both of RAP, are the two operations directors for WISP-94. "We're set up to study just about any kind of cloud--everything from deep storms to mountain-wave clouds."
Before meteorologists can better predict aircraft icing, they need to better understand its cause: supercooled water droplets. These tiny droplets, suspended in air below 32¡F (0¡C), freeze almost instantly when they encounter a hard surface such as an airplane, causing ice to accumulate. Ironically, the worst icing conditions for aircraft can occur in a cloud deck that may not contain ice itself or produce any rain or snow at the ground, but that is full of supercooled water.
The WISP team has identified shallow stratus decks as the source of the largest supercooled droplets, those with diameters from 50 to several hundred microns (about 0.002 to 0.01 inches). "The droplets tend to form near the tops of the clouds," says Marcia, "but we've seen some surprises." A number of innovative new instruments are sampling clouds from aboard the University of Wyoming's King Air and NCAR's Electra research aircraft. A U-band radar on the King Air, operated jointly by the universities of Wyoming and Massachusetts, will help resolve the fine-scale structure near cloud tops that may be responsible for generating the large droplets. Both aircraft flew to Kansas and Nebraska last week for "very successful" probes of extensive stratus decks there, and early this week the King Air studied intense snow and ice storms in Oklahoma.
Ice crystals aloft can reduce the icing threat to an airplane by scouring out supercooled cloud droplets. This year's study is focusing on the origins of in- cloud ice. Air samples taken from just outside ice-bearing clouds are being rushed to a cloud chamber at Colorado State University, where scientists are recreating the ice-production process that the air would have encountered within the cloud. This technique, coupled with improved processing methods for ice-nucleus filter samples, should yield a better understanding of nucleation mechanisms at work in winter storms.
Another project, led by Greg Kok (RAP/Atmospheric Chemistry Division), Ron Smith (Yale University) and Bob Rauber and Harry Ochs (University of Illinois), seeks to compare the ratios of deuterium isotopes in ice crystals and water vapor in order to estimate the altitude at which the ice or water vapor originated. Microscale observations such as these will be connected to large- scale weather features later, so that future forecasts can predict how a given cloud or storm might evolve.
Though icing conditions can be very localized, traditional icing forecasts for aviation are so general--often covering entire states--that they can be of little help in fine-tuning flight plans. The WISP team is working with the FAA, NOAA's Forecast Systems Laboratory, and the National Aviation Weather Advisory Board on a new aircraft-icing forecast system scheduled to be demonstrated next year. Three mesoscale forecast models are being used with various computer algorithms that give real-time, in-flight icing forecasts; the goal is to find which algorithm-and-model pair works best.
Closer to ground level, the United Airlines ground operations unit at Denver's Stapleton International Airport is involved in a WISP project designed to improve airport de-icing efforts. "We're using some techniques for tracking snow bands by radar, in combination with snow gauges at the airport, to see how snowfall rates can be predicted from radar returns," says Marcia. "This year we're developing the relationship between snowfall and radar returns--next year we hope to do real-time forecasts." Better snowfall predictions will help airlines decide when and how often to de-ice planes.
Also taking part in WISP are the University of Wisconsin, the University of Nevada's Desert Research Institute, NCAR's Mesoscale and Microscale Meteorology Division and Atmospheric Technology Division, and NOAA's Environmental Technology Laboratory. --BH
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