Observational Study on the Impact of Silver Iodide on Cloud Formation and Evolution in Orographic Winter Storms

Given the challenge to measure aerosol-cloud-precipitation interactions in a continuously evolving cloud system, and uncertainties regarding ice nucleating properties of natural aerosol, we examine the response of controlled emissions of well-characterized ice nucleating particles into a well-characterized cloud of mostly supercooled liquid water. Here a pyrotechnically-dispersed AgI-salt complex commonly used for cloud seeding intended to enhance precipitation is used. While the physical process is well understood by which silver iodide (AgI) introduced into a supercooled cloud produces ice crystals, the impacts of cloud seeding on cloud formation and evolution in orographic winter storms and the amount and spatial distribution of snow is still difficult to quantify. Many microphysical, thermodynamic, and kinematic factors can influence the cloud formation and the amount of snow that reaches the ground. We use observations from two ground-based dual-polarization scanning X-band Doppler radars to analyze the temporal and spatial evolution of seeded and natural clouds during three events. The environment in which they form and evolve will be described through data from soundings and microwave radiometers. The data were collected during the 2017 Seeded and Natural Orographic Wintertime clouds—the Idaho Experiment. Microphysical and dynamical processes within these seeded clouds were analyzed using vertical cross sections of radar observations at high temporal resolution combined with high-resolution volume scans as the seeded clouds evolve and snow fell out over the higher terrain. The analysis of dual-polarization parameters over 90 minutes shows the growth of ice crystals from moderately rimed particles to larger crystals that fell to the ground. Dual-polarization signatures of surrounding clouds, that were not seeded with AgI, will be compared to signatures found in seeded clouds.