The overarching goal of SNOWIE (Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment) is to understand the natural dynamical and microphysical processes by which precipitation forms and evolves within orographic winter storms and to determine the physical processes by which cloud seeding with silver iodide (AgI), either from ground generators or aircraft, impacts the amount and spatial distribution of snow falling across a river basin. The core scientific objectives build on results of recent investigations of orographic clouds by the NSF-funded 2012-13 AgI Seeding Cloud Impact Investigation (ASCII) field program in Wyoming (Geerts et al. 2013; Pokharel et al. 2015).
SNOWIE will be conducted in the Payette Mountains in Idaho and partners with Idaho Power Company (IPC) who maintains an operational seeding program in the region. SNOWIE uses similar observational and modeling tools as ASCII. However, unlike ASCII, which used only ground-based seeding, SNOWIE focuses a significant effort investigating cases of airborne seeding. This new focus allows detailed examination of the cloud microphysical response to AgI seeding in a region above the planetary boundary layer (PBL) that is accessible by the cloud-physics research aircraft and in turn provides a data set to evaluate and improve the numerical model’s ability to capture key details of ice and precipitation development in the studied clouds. SNOWIE differentiates itself from ASCII in other significant ways: (1) the seeding aircraft will, on a few flights, target a relatively simple stratus cloud away from the mountains, in order to repeat with state-of-the-art instruments the original experiment by Schaeffer (1946) and Vonnegut (1947) to allow for direct, unambiguous microphysical change-of-events verification; (2) natural and seeded storm structures will be analyzed in more detail, with higher temporal resolution, and over a larger domain, especially by means of one airborne and two scanning Doppler radars, in order to better isolate seeding signatures from natural cloud evolution; (3) aerosol size distributions and concentrations will be characterized using ground-based measurements in the airmass impinging on the target mountain; and (4) the modeling component of SNOWIE will also use historical data to evaluate seeding effectiveness. ASCII has shown to some degree how high-resolution cloud and aerosol resolving numerical simulations can be validated with observations and be used for seeding evaluation—but SNOWIE provides this and the ability to run the model retrospectively with and without seeding to quantify the fraction of seedable storms and the impact on the seasonal snowpack.
Our core scientific objectives are to: (1) Evaluate the role of mesoscale and microscale dynamics and of the underlying terrain in the formation, growth, and fallout of natural ice crystals in winter storms through observations; (2) Investigate how the natural snow growth process is altered as a result of airborne AgI seeding through both observations and model simulations, and (3) Evaluate the effects of ground seeding on snowfall amount and distribution. In addressing the last two objectives, we will evaluate and improve the newly-developed cloud seeding module in the Thompson cloud microphysical scheme currently used in the Weather Research and Forecasting (WRF) model (Xue et al. 2013 a,b). Select cases will be simulated using a bin resolving microphysics scheme to further investigate the microphysical evolution of the natural and seeded clouds.