Case Studies of Microphysical Responses to Valley-Released Operational AgI Seeding of the Wasatch Plateau, Utah

Arlin B Super

Abstract


An experimental field program was conducted on the Wasatch Plateau of central Utah in early 1991. Objectives included monitoring of the transport and dispersion of valley-released silver iodide and associated microphysical effects within the seeded clouds. Silver iodide ice nuclei were monitored by acoustical ice nucleus counters in a truck, an aircraft and at a Plateau-top observatory. An aircraft-mounted ice particle imaging probe was used to search for ice particle concentration differences between the seeded cloud and crosswind, nonseeded cloud. Other instruments monitored wind, cloud liquid water content, temperature and other parameters of interest. Six experiments are described in detail, representing all aircraft sampling periods during which valley-released AgI (silver iodide) was transported to both Plateau-top and aircraft altitudes. Embedded convection was present during five of the experiments which probably enhanced vertical AgI transport. Clouds bases were generally a few hundred meters below the Plateau top with one exception which had bases above the Plateau. The valley-released AgI consistently had a pronounced vertical gradient above the Plateau. Concentrations of AgI ice nuclei, effective at -20°C, were typically at least an order of magnitude less at lowest aircraft altitudes than found on top of the Plateau. Silver iodide was seldom detected as high as 1 km above the Plateau top. No evidence was found for enhanced ice particle concentrations at aircraft altitudes during the three experiments with sampling zone temperatures of -9°C and above. However, suggestions of increased ice particle concentrations were found when sampling zone temperatures were colder. Silver iodide ice nuclei estimates, effective at prevailing supercooled liquid cloud temperatures, were lower than believed desirable for effective seeding during the three experiments with warmer cloud at aircraft sampling altitudes. The results presented are believed to indicate that current operational cloud seeding in Utah produces insignificant increases in ice particle concentrations and snowfall rates during warmer storm episodes. The main problem appears to be that concentrations of effective ice nuclei are too low for the typical mildly supercooled liquid water clouds reached by the AgI. Seeding appeared to markedly increase the ice particle concentration in colder supercooled clouds, and precipitation observations suggested associated limited snowfall increases during some cases. A number of recommendations are made for increasing ice particles concentrations in the warmer supercooled liquid clouds.

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