NCAR, NOAA, and university scientists search for clues to aircraft icing at and near Denver's Stapleton airport by Robert Henson UCAR Outreach In this year's phase of a continuing study, NCAR, government, and university scientists engaged in a two-month field program to investigate what causes supercooled water droplets and ice crystals to form in clouds. Besides NCAR, participants in the Winter Icing and Storms Project (WISP) 1994 included Colorado State University, the universities of Illinois and Wisconsin, the University of Nevada's Desert Research Institute, Yale University, and the National Oceanic and Atmospheric Administration's Environmental Technology and Forecast Systems laboratories. The research was sponsored by the National Science Foundation (NSF) through an interagency agreement in response to requirements and funding by the Federal Aviation Administration's Aviation Weather Development Program. United Airlines ground operations at Stapleton International Airport in Denver, Colorado, also cooperated with studies designed to improve forecasting for ground de-icing efforts. Tiny water droplets, suspended in air below 32¡F, freeze almost instantly when they encounter a hard surface, such as an airplane, causing ice to build up on its body and wings. To improve prediction of aircraft icing, meteorologists need to better understand important factors leading to its cause--supercooled water droplets. Led by operations codirectors Marcia Politovich and Roy Rasmussen, both of NCAR, approximately 25 scientists examined in-cloud ice as part of the multiyear study based at NCAR. They used four radars, two airplanes, and an array of surface and airborne sensors to sample clouds along Colorado's Front Range from 25 January to 25 March. Now in its fourth winter field program, WISP has advanced understanding of how regions of supercooled water develop and evolve. Ironically, the worst icing conditions for aircraft can occur in a shallow cloud deck that may not contain ice itself, or produce any rain or snow on the ground, but is full of supercooled water. Since ice crystals aloft can reduce the icing threat to an airplane by depleting supercooled cloud drops, this year's study focused on the origins of in-cloud ice. A number of innovative instruments sampled clouds from aboard the NSF/University of Wyoming King Air and the NSF/NCAR four- engine Electra research aircraft. Air samples taken from just outside ice-bearing clouds were rushed to a cloud chamber at Colorado State University, where scientists tried to recreate the ice-production process that the air would have encountered within the cloud. Researchers are working to connect these small-scale observations to large-scale weather, so that future forecasts can predict how much a given cloud or storm might threaten aircraft. This could eventually result in more direct routing, lower fuel costs, and safer flights for the public. The WISP participants are working with operational aviation weather forecasters at the National Weather Service's National Aviation Weather Advisory Unit at Kansas City, Missouri, to transfer the icing forecast technology being developed. As a result of this cooperative effort, the techniques developed through this winter's research will be available for operational use next winter in support of aviation.