The Impact of Ground-Based Glaciogenic Seeding on a Shallow Stratiform Cloud over the Sierra Madre in Wyoming: A Multi-Sensor Study of the 3 March 2012 Case

Abstract

A case study is presented of the impact of ground-based glaciogenic seeding on a shallow, lightly precipitating orographic storm with abundant supercooled cloud droplets, but few ice particles. The storm was observed on 3 March 2012 as part of the AgI (silver iodide) Seeding Cloud Impact Investigating (ASCII) experiment in Wyoming. The cloud base temperature was about -9°C, and cloud tops were at about -16°C. The high concentration of small droplets and low ice particle concentration lead to natural snow growth, mainly by vapor diffusion. The question addressed here is whether the injection of ice nucleating particles (AgI) enhanced snow growth and snowfall. The treated (seeded) period is compared with the preceding untreated (noseeded) period, and natural trends (observed in an adjacent control region) are removed. The main target site, located on a mountain pass at an elevation above cloud base, was impacted by AgI seeding, according to a trace chemistry analysis of freshly fallen snow. Data from three radar systems were used: the Wyoming Cloud Radar, two Ka-band profiling Micro-Rain Radars, and a X-band scanning Doppler-on-Wheels (DOW) radar. Composite data from these radar systems and from gauges in the target area indicate an increase in low-level reflectivity and precipitation rate during seeding. This finding generally agrees with other published ASCII case studies. The increase in reflectivity during seeding in the target area appears to be due mainly to an increase in particle size (aggregation), not number concentration, as suggested by DOW differential reflectivity and by disdrometer and Cloud Particle Imager measurements on the ground