The role of moisture pathways on natural snowfall production during SNOWIE

Glaciogenic cloud seeding has proven to be a viable method for increasing wintertime snowpack in years with below-average snowfall, providing valuable water for hydropower and human consumption. To optimize the role of glaciogenic cloud seeding in producing snowfall, it is imperative to understand the cloud structure, ice generation, and kinematic and thermodynamic properties of the natural cloud systems targeted for seeding. Recent studies focused on extreme wintertime precipitation in the Intermountain West of United States have shown that airflow follows distinct pathways from the Pacific Ocean through gaps in terrain. However, the role of these pathways on atmospheric conditions and snowfall patterns remains largely uncharacterized. In this study, we utilize a Lagrangian methodology to determine backwards trajectories initiated at 3 km MSL above Packer John Mountain in the southwestern Central Mountains of Idaho. We use in-situ and remote sensing observations from 24 precipitation events during SNOWIE (Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment) spanning January – March 2017. Backwards trajectories are categorized into three regimes based on their paths: (1) Sierra blocked – where airflow enters the Central Valley of California, is redirected northward by the Sierra Nevada, and spills over into the Snake River Plain through the Burney Gap in the northeastern corner of the Central Valley; (2) Southwesterly – where air flows overtop the Sierra blocked flow, or mountains in northern California and Oregon, with mean wind direction between 202.5 – 247.5°; (3) Zonal – where airflow is similar to southwesterly, but the mean wind direction is between 247.5 – 292.5°. Here we show that Sierra blocked flow was responsible for producing around two-thirds of the natural snowfall in less than half the overall time observed by a ground-based scanning radar located on Packer John Mountain. In general, we find that pathways are strongly tied to temperature, moisture content, and structure of horizontal moisture flux, where strong, low-level (2-4 km MSL) moisture flux appears to be the driving factor in echo top heights and natural snowfall production. These results suggest that airflow within certain pathways are more effective in generating natural snowfall while others are less effective and are potential targets for cloud seeding. Thus, cloud seeding operations may vary based on pathway frequency in a given winter.