Comparison of Silver Iodide Outputs from Two Different Generators and Solutions Measured by Acoustic Ice Nucleas Counters

Abstract

Field testing in late September 2009 permitted comparisons of the output of a WMI remote-controlled seeding generator burning a modern solution with an older style AgI generator and solution previously calibrated in the Colorado State University (CSU) Cloud Simulation Laboratory.  That facility is no longer available for seeding generator calibrations. Acoustical Ice Nucleus Counters (AINCs), usually operated at -20°C, were used to monitor ice nucleus concentrations from passage of AgI lines released upwind by mobile generators towed approximately perpendicular to the prevailing wind direction. Considerable variability existed for total ice nuclei per AgI plume passage as could be anticipated given variations in atmospheric conditions. However, examination of all tests with useable data revealed no major difference between the outputs of the two generator types using different solutions. The WMI generator uses a newer solution, expected to produce ice nuclei which operate primarily by condensation-freezing in winter orographic clouds. It had yields in the -15 to -20°C range similar to the Montana State University Skyfire generator producing a relatively pure AgI aerosol likely to operate by contact nucleation. However, AINCs with standard configurations as used in this investigation cannot differentiate between ice nucleation processes. The observations also documented that a newly-manufactured AINC compares favorably with previously tested units.  Recommendations are made for future testing expanded to warmer temperatures than practical with the standard configurations of the AINCs available for this study.