Estimating the Fraction of Winter Orographic Precipitation Which Meets the Criteria for Seeding During the Wyoming Weather Modification Pilot Project

Annual precipitation increases of 10% or more are often quoted for the impact of winter orographic cloud seeding; however, establishing the basis for such values is problematic for two reasons. First, the impact of glaciogenic seeding on any particular orographic storm has not been firmly established. Second, not all winter precipitation is produced by seedable storms. Addressing the first question motivated the Wyoming state legislature to fund a multi-year, crossover, randomized cloud seeding experiment in southeast Wyoming, to quantify the impact of glaciogenic seeding of wintertime orographic clouds. The crossover design requires two barriers, randomly selected for seeding, for comparison of seeded and non-seeded precipitation under relatively homogeneous atmospheric conditions. Addressing the second question motivated the work here. The seeding criteria, 700 hPa temperature ≤ -8ºC, 700 hPa winds between 210º and 315º, and supercooled liquid water present, were applied to eight winters to determine the percent of winter precipitation, which may fall under the seeding criteria. Since no datasets provide precipitation and all the atmospheric variables required for this study, estimates of these quantities from an eight-year run of the Weather Research and Forecasting model were used. The accuracy of the model was tested against several measurements and the small model biases were removed. On average, ~26% of the time, between 15 November and 15 April, atmospheric conditions were seedable over the barriers in southeast Wyoming. These seedable conditions were accompanied by precipitation ~13-14% of this time, indicating ~30% of the winter precipitation resulted from seedable clouds.