PRODUCTION AND DEPLETION OF SUPERCOOLED LIQUID WATER IN A COLORADO WINTER STORM

During the 1990 Winter Icing and Storms Project(WISP), a shallow cold front passed through northeastern Colorado, followed by a secondary co ld front. A broad high pressure area behind the initial front set up a Denver cyclone circulation within a well-mixed boundary layer, which was capped by a stable, nearly saturated layer of air left in place by the initial cold front. As the secondary cold front passed through th e WISP domain, these layers of air were lifted. The lifted boundary la yer formed only broken cloud, but the lifted moist layer formed a stra tiform cloud that contained high liquid water contents. Cloud characte ristics were measured in situ with a research aircraft, and remotely b y ground-based radars, microwave radiometers, and a lidar ceilometer. Moderate to severe icing conditions were reported by aircraft flying i n the area during the event and also affected the flight of the resear ch aircraft through an increase in drag on the airframe. Liquid water was depleted in portions of the lower stratiform cloud as ice crystals , produced in midlevel clouds embedded in westerly now, fell into the lower cloud, and quickly rimed to form showers of graupel at the groun d. After these midlevel clouds passed over the area, liquid production resumed. Supercooled liquid cloud persisted for 36 h as cloud formed within the surface cold air mass behind the secondary cold front as it entered the Denver area and was lifted over the local terrain. The ev olution of weather events is discussed using a variety of datasets, in cluding radar, surface mesonet, balloon-borne soundings, research airc raft, satellite imagery, microwave radiometers, and standard National Weather Service observations. By combining information from these vari ed sources, processes governing the production and depletion of superc ooled liquid from the synoptic to the microscale are examined. The sto rm is also discussed in terms of its potential for causing moderate to severe aircraft icing. The effect of accreted ice on the research air craft is described, as are implications of the meteorology for detecti on and forecasting inflight icing.