The impact of ground-based glaciogenic seeding on clouds and precipitation over mountains: A case study of a shallow orographic cloud with large supercooled droplets

Abstract

This paper examines the impact of ground-based glaciogenic seeding on a shallow, lightly precipitating orographic cloud with rather large (~35 μm) supercooled droplets. The storm was observed on 22 February 2012 as part of the AgI (silver iodide) Seeding Cloud Impact Investigation experiment in Wyoming. The cloud base (top) temperature was about -5°C (-12°C). Vertical velocity data from an airborne Doppler W-band (3 mm) profiling Wyoming Cloud Radar (WCR) indicate broad ascent due to the strong wind (20 m s-1) impinging on the terrain and small pockets of intense updrafts. The large droplets, low droplet and ice particle concentrations, and strong updrafts lead to natural snow growth mainly by accretion (riming). The treated (seeded) period is compared with the preceding untreated period. The main target site, located on a mountain pass, was impacted by AgI seeding, according to a trace chemistry analysis of the falling snow. Data from three radar systems were used in the analysis of the impact of seeding on snow growth: the WCR, two Ka-band (1.2 cm) profiling Micro Rain Radars, and an X-band (3 cm) scanning polarization Doppler-on-Wheels radar. This case is complicated somewhat by a natural increase in cloud liquid water and in snow growth by riming, starting halfway during the seeding period, and continuing after seeding ended. Composite data from the centimeter-wave radar systems indicate an increase in low-level reflectivity during seeding, even after accounting for the natural trend observed in the upwind control region. A precipitation particle probe at the main target site shows an increase in concentration of both small and large hydrometeors.