Implications of Early 1991 Observations of Supercooled Liquid Water, Precipitation and Silver Iodide on Utah's Wasatch Plateau

Abstract

The Utah/NOAA Atmospheric Modification Program conducted an observational program in early 1991, with additional support from the Bureau of Reclamation. A summary is presented of observations obtained on the Wasatch Plateau of central Utah which includes SLW (supercooled liquid water), precipitation, Agl (silver iodide ice nuclei), air and dewpoint temperature, and wind velocity. With the exception of Agl ice nuclei, measurements were made on 20 days with storm conditions. Silver iodide was monitored during part or all of a subset of 12 days when valley Agl generators were being operated. It is shown that abundant SLW existed during many hours, and a large fraction of these hours did not have precipitation observed on top the Plateau. The SLW flux over the Plateau-top’s windward edge exceeded the average precipitation on top the Plateau. These findings suggest significant seeding potential may exist. Acoustical ice nucleus counter observations were adjusted to temperatures typical of Agl plume tops. Aircraft measurements showed the plume tops were usually less than 1 km above the Plateau. The adjusted ice nucleus observations suggest effective Agl ice nuclei concentrations were too low for productive seeding much of the time when SLW was present. The main problem appears to be the warm temperatures of the SLW during most storm periods. Effective concentrations of Agl ice nuclei are not expected at such temperatures with the generators and release rates used in the Utah operational seeding program. However, these estimates were based on a 1981 generator calibration in a cloud simulation laboratory which may not be totally representative of winter orographic clouds. Direct observations are needed of ice particle concentrations caused by seeding orographic clouds for the range of conditions typical of winter storms. The challenge is to develop means of routinely targeting the SLW zone with adequate concentrations of seeding-caused ice crystals which can start the precipitation formation processes in naturally inefficient clouds. A number of approaches are suggested which could make the Utah operational seeding program more effective.