Experiments with pulsed seeding by AgI and liquid propane in slightly supercooled winter orographic clouds over Utah's Wasatch Plateau

A series of orographic cloud seeding experiments were conducted on Utah's Wasatch Plateau during the 1994-95 and 1995-96 winters. Their purpose was to permit physical assessment of the effects of both silver iodide and liquid propane seeding, particularly at only slightly supercooled temperatures. Seeding materials were released in 1-hour and half-hour pulses from a location well up the plateau's windward slope. The terrain often channeled the seeding plumes to an observation site, or target, located at a canyon head on the Plateau top's windward edge. Snow particles were detected at the target with a vane-mounted 2D-C optical array probe whose strobing speed was governed by a heated anemometer. AgI nuclei were detected there by an NCAR ice nucleus counter to confirm the presence and successful targeting of seeding materials. The experiments were carried out in conditions both favorable and unfavorable for expected effectiveness of the seeding agents. There was no evidence for the effectiveness of AgI plumes injected into clouds at a release site temperature warmer than about -3 degrees C and target temperature of about -5 degrees C. Increases in ice particle concentrations and precipitation rates were observable for AgI seeding with target air temperatures colder than -6 degrees C. Liquid propane was released into air at temperatures of -0.4 to -3.4 degrees C, resulting in about 10 ice particles L super(-) super(1) at the target with only one release nozzle and about 20 L super(-) super(1) for two nozzles. The difference in ice particle concentrations formed by adding the second nozzle was statistically detectable at a 6 percent level or better, depending on the measurement type. While a temperature dependence of the liquid propane was not observable over such a small temperature range, the experiments have shown that liquid propane is an effective seeding agent for slightly supercooled clouds at temperatures where AgI is ineffective. Seeding plume detection appeared to be limited during periods of abundant natural snowfall. The larger natural crystals appeared to compete for the available supercooled liquid water and may have removed many of the seeded embryos by aggregation.