Observations of Natural Cloud Structure and Snow Growth Processes in Orographic Clouds

To advance the knowledge of glaciogenic cloud seeding capabilities on wintertime orographic clouds, we need to understand the structure and microphysical processes in natural clouds. In this study, we use data from airborne and ground-based remote sensing and in-situ instruments deployed during the Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) field campaign to study synoptic moisture fluxes, cloud characteristics and their temporal and spatial evolution. Based on the vertical cloud structure and the environmental conditions, storms can be classified as northwest flow events, inland-penetrating atmospheric rivers, shallow orographic, or convective storms. Northwest flow events had either a single cloud layer, or decoupled cloud layers where the upper cloud did not naturally seed the lower cloud. Alternatively, the atmospheric river events were typically comprised of two cloud layers, where the upper cloud naturally seeded the lower cloud layer (i.e., seeder-feeder mechanism). This study investigates how synoptic weather patterns and moisture pathways, identified by HYSPLIT back trajectories, affect atmospheric profiles of temperature and moisture, and how they relate to the orographic cloud microphysics. Back trajectories suggest that the Burney Gap, north of the Sierra Nevada’s in California, is an important pathway for tropical Pacific moisture to reach southwestern Idaho. Near zonal flow and ascent over the Cascade Mountains of Oregon was the second most common moisture pathway. However, Burney Gap flow events tended to have warmer temperatures and wind speeds at 700-hPa and higher average precipitation rates.