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1. Introduction
In arid regions, there has long been an interest in the enhancement of precipitation through cloud seeding (Haupt et al. 2019). A method commonly used in the western United States and elsewhere is the glaciogenic seeding of orographic clouds in the cold season (http://www.nawmc.org/). These mountains are targeted because the region is relatively arid and water supply is largely derived from the high-elevation snowpack. Interest in cloud seeding to enhance precipitation has increased as the Colorado River basin is experiencing a multidecade water shortage (Udall and Overpeck 2017), and as the mountain snowpack is declining in a globally warming climate (Mote et al. 2018).
The efficacy of glaciogenic cloud seeding has repeatedly been called into question in authoritative reports (e.g., National Research Council 2003). The challenge has been that, even though there is much evidence for the presence of supercooled liquid water (SLW) in orographic clouds (Rauber and Grant 1987; Heymsfield and Miloshevich 1993; Kusunoki et al. 2005; Sassen et al. 1990; Lohmann et al. 2016), quantitative precipitation impact assessment is extremely difficult, because precipitation measurements are uncertain and no two storms are the same (Rauber et al. 2019).
The most promising progress in recent years has come through physical process studies, examining properties of natural orographic clouds and microphysical changes resulting from the injection of seeding material, usually silver iodide (AgI), combining observations with process-resolved numerical simulations (Geerts and Rauber 2022). First, two recent field experiments in the interior western United States have provided strong evidence that under the right conditions, glaciogenic cloud seeding from the ground (Pokharel et al. 2017, 2018, and references therein) and from an aircraft (French et al. 2018) enhance ice crystal production and surface precipitation. These campaigns are the 2011/12 AgI Seeding Cloud Impact Investigation (ASCII; Geerts et al. 2013; Pokharel and Geerts 2016) and the 2017 Seeded and Natural Orographic Wintertime Precipitation: The Idaho Experiment (SNOWIE; Tessendorf et al. 2019), respectively. Observational evidence has not established the range of seedable orographic clouds: for instance, the observational evidence mentioned above focused on relatively shallow clouds. In essence, adequate SLW needs to be present, and the seeding material needs to be dispersed such that it blends with the SLW at sufficiently low temperatures.
Second, several numerical schemes have...