Precipitation Formation in Wintertime Orographic Clouds: Environmental Conditions, Cloud Characteristics

Cloud seeding has been widely used in the Western U.S. to enhance snow production efficiency in wintertime orographic clouds and to enhance snowpack. However, due to the scarcity of long-term, high-resolution cloud and precipitation observations in complex terrain, only a few studies have explored orographic snow production by comparing environmental conditions and cloud characteristics with surface snowfall distribution and quantity. This study analyzes environmental conditions and cloud characteristics to surface snowfall patterns for the 24 snowfall events observed in the 2017 Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). Our investigation aimed to understand 1) whether environmental factors like wind, turbulence, and updraft influence snowfall amounts and distribution, 2) whether cloud properties can serve as indicators of precipitation-forming effectiveness, and 3) whether cloud seeding can compensate for the deficiency of ice nucleating particles (INP). The analysis reveals that low liquid-equivalent snowfall accumulated over the observational domain of 7200 km² (< 0.9 x 10⁷ m³), and snowfall rates < 0.45 mm h^-1 occurred for about half of the events. These low snowfall events were characterized by higher cloud top temperatures (> -20°C), fewer larger droplets, higher liquid water content, and lower ice water content compared to the other events. These low snowfall cases also coincided with the observation of seeding lines and little background natural snowfall, suggesting cloud seeding was mainly responsible for the generated snowfall during these cases. While the amount of Silver Iodide (AgI) released during cloud seeding did not exhibit any significant correlation with snowfall amount and rate, there is a suggestion that snowfall might slightly increase with increasing amounts of AgI for low accumulation cases.