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The pathways air travels from the Pacific Ocean to the Intermountain West of the United States are important for understanding how air characteristics change and how this translates to the amount and distribution of snowfall. Recent studies have identified the most common moisture pathways in the Intermountain West, especially for heavy precipitation events. However, the role of moisture pathways on snowfall amount and distribution in specific regions remains unclear. Here, we investigate 24 precipitation events in the Payette Mountains of Idaho during January–March 2017 to understand how local atmospheric conditions are tied to three moisture pathways and how it impacts snowfall amount and distribution. During one pathway, southwesterly, moist, tropical air is directed into the Central Valley of California where the air is blocked by the Sierra Nevada, redirected northward and over lower terrain north of Lake Tahoe into the Snake River Plain of Idaho. Other pathways consist of unblocked flows that approach the coast of California from the southwest and then override the northern Sierra Nevada and southern Cascades, and zonal flows approaching the coast of Oregon overriding the Oregon Cascades. Air masses in the Payette Mountains of Idaho associated with Sierra-blocked flow were observed to be warmer, moister, and windier compared to the other moisture pathways. During Sierra-blocked flow, higher snowfall rates, in terms of mean reflectivity, were observed more uniformly distributed throughout the region compared to the other flows, which observed lower snowfall rates that were predominantly collocated with areas of higher terrain. Of the total estimated snowfall captured in this study, 67% was observed during Sierra-blocked flow.

 

Climate change and population growth have increased demand for water in arid regions. For over half a century, cloud seeding has been evaluated as a technology to increase water supply; statistical approaches have compared seeded to nonseeded events through precipitation gauge analyses. Here, a physically based approach to quantify snowfall from cloud seeding in mountain cloud systems is presented. Areas of precipitation unambiguously attributed to cloud seeding are isolated from natural precipitation (<1 mm h⁻¹). Spatial and temporal evolution of precipitation generated by cloud seeding is then quantified using radar observations and snow gauge measurements. This study uses the approach of combining radar technology and precipitation gauge measurements to quantify the spatial and temporal evolution of snowfall generated from glaciogenic cloud seeding of winter mountain cloud systems and its spatial and temporal evolution. The results represent a critical step toward quantifying cloud seeding impact. For the cases presented, precipitation gauges measured increases between 0.05 and 0.3 mm as precipitation generated by cloud seeding passed over the instruments. The total amount of water generated by cloud seeding ranged from 1.2 × 10⁵m³(100 ac ft) for 20 min of cloud seeding, 2.4 × 10⁵m³(196 ac ft) for 86 min of seeding to 3.4 x 10⁵m³(275 ac ft) for 24 min of cloud seeding.

 

Abstract This paper reviews research conducted over the last six decades to understand and quantify the efficacy of wintertime orographic cloud seeding to increase winter snowpack and water supplies within a mountain basin. The fundamental hypothesis underlying cloud seeding as a method to enhance precipitation from wintertime orographic cloud systems is that a cloud’s natural precipitation efficiency can be enhanced by converting supercooled water to ice upstream and over a mountain range in such a manner that newly created ice particles can grow and fall to the ground as additional snow on a specified target area. The review summarizes the results of physical, statistical, and modeling studies aimed at evaluating this underlying hypothesis, with a focus on results from more recent experiments that take advantage of modern instrumentation and advanced computation capabilities. Recent advances in assessment and operations are also reviewed, and recommendations for future experiments, based on the successes and failures of experiments of the past, are given.