The impact of ground-based glaciogenic seeding on clouds and precipitation over mountains: A multi-sensor case study of shallow precipitating orographic cumuli

This paper examines reflectivity data from three different radar systems, as well as airborne and ground-based in situ particle imaging data, to study the impact of ground-based glaciogenic seeding on shallow, lightly precipitating orographic cumuli, observed on 13 February 2012, as part of the AgI Seeding Cloud Impact Investigation (ASCII) experiment in Wyoming. Three silver iodide (AgI) generators were used, located on the windward slopes of the target mountain. This case was chosen for several reasons: the AgI generators were near the lifting condensation level, where the temperature was about - 6. °C; cloud droplets were present in the cumulus clouds, which were rooted in the boundary layer; and the airflow, although weak, ascended over the mountain. The target mountain pass site was almost certainly 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 onboard 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, indicate that the observed changes in reflectivity profiles can be explained largely by the natural emergence of shallow cumuli. A comparison with lateral control regions (i.e., over the mountain, but to the side of the AgI plumes) suggests that seeding may have further enhanced snowfall, but the signal is weak.Particle probes at flight level and at the mountain pass site show that the concentration of small ice crystals (<. 1. mm) was significantly larger downwind of the AgI generators during seeding. This too is consistent with the emergence of shallow convection, but a comparison between flight sections downwind of the AgI point sources and those to the side suggests that glaciogenic seeding increased the concentration of ice crystals of all sizes in the shallow convection.