The Impact of Ground-based Glaciogenic Seeding on Orographic Clouds and Precipitation over a Mountain: A Detailed Case Study

We examine the impact of ground-based glaciogenic seeding on a shallow, lightly precipitating orographic cloud, observed on 22 February 2012, as part of the AgI Seeding Cloud Impact Investigation (ASCII) experiment in Wyoming. In this case, the temperature in the cloudy turbulent boundary layer in the target region was -5 to -10 oC, the target orographic clouds contained abundant amount of supercooled liquid water, and the storm was remarkably steady during the measurement period, consisting of a 2-hour untreated period, followed by a 2-hourtreated period. Even though the cloud base temperature was rather warm for AgI seeding, the turbulent boundary layer depth was rather deep , thus AgI nuclei likely reached air temperatures cold enough for ice initiation. Three silver iodide (AgI) generators were used, located on the windward slopes of the target mountain. The target mountain pass site was impacted by seeding, according to a trace element analysis of the falling snow. Data from three radar systems were used in the analysis of the impact of seeding on precipitation: the airborne W-band (3 mm wavelength) profiling Wyoming Cloud Radar (WCR), two Ka-band (1.2 cm) profiling Micro-Rain Radars (MRR), and a X-band (3 cm) scanning Doppler-on-Wheels (DOW) radar. The WCR was aboard a research aircraft flying geographically fixed tracks, the DOW and one MRR were located at the target mountain pass, and another MRR was upstream of the AgI generators. Composite data from the three radar systems, each with their own target and upwind control regions, are consistent, at least in sign, suggesting that the observed changes in reflectivity are due to AgI seeding. In-situ measurements at the mountain pass site shows that the concentration of both smaller and larger ice crystals was higher downwind of the AgI generators during seeding. The in-situ data are consistent with the radar data, providing additional evidence of positive seeding impact.