Another Wintertime Cloud Seeding Case Study with Strong Evidence of Seeding Effects

In the 1990s numerous winter orographic cloud seeding experiments were conducted over Utah’s Wasatch Plateau. Several previously published case studies successfully documented the physical response in clouds and precipitation to ground-based seeding with silver iodide and liquid propane. A previously unpublished case study that shows additional evidence of positive seeding effects is presented here. Careful documentation of seeding plume transport and dispersion coupled with aircraft and groundbased measurements within and outside the seeding plume are used to verify the steps in the conceptual model for orographic cloud seeding. It is shown that seeding produced significant increases in ice crystal concentrations (well above 10 times the natural background at aircraft level) and strong indications of increased precipitation at the surface (> 3 times the precipitation rate measured outside the seeding plume). Compared to other Utah results, the most unique aspect of this case study was the apparent detection of the seeding effect in the data from the project Ka-band radar. The radar seeding signature documented the areal extent and duration ofthe seeding effect in a way that was not previously possible using the more intermittent aircraft measurements and the lower spatial resolution precipitation data. The physical evidence of seeding effects documented in this and several other Utah/NOAA experiments supports the development of a larger scale randomized experiment to satisfy the call for proof of cloud seeding effectiveness in a recent National Academies of Science report.