Dynamical and Microphysical Drivers of Activation and Snow Growth from Ground-Based Seeding in Complex Terrain. Part I: Numerical Model Analysis

This study used a model-based analysis with the Weather Research and Forecasting Model with the weather modification module (WRF-WxMod) to examine how thermodynamic, microphysical, and dynamical conditions influenced ground-based silver iodide (AgI) cloud seeding during intensive observation period 7 (IOP07) of the Seeded and Natural Orographic Wintertime Clouds—the Idaho Experiment (SNOWIE). This case featured a shallow cloud layer (2.3–3.3 km MSL) with cloud temperatures ranging between –7.58 and –14.38C and widespread available liquid water content, indicating a favorable microphysical environment for cloud seeding. Terrain-driven updrafts and downdrafts produced fixed wave patterns that strongly influenced the vertical transport of AgI. Although only 0.02% of released AgI was activated—and most (93.9%) remained inactivated within valley layers below 2.1 km MSL—the model showed that AgI was activated within 5–10 min after release, with snowfall from seeding developing 50–60 min later, roughly 30 km downwind from the ground generators. AgI pathways followed midlevel southwesterly (SW)–west-southwesterly (WSW) winds, revealing distinct activation patterns based on generator location: High-elevation generators near cloud base efficiently injected AgI into the supercooled liquid water (SLW) cloud and supported rapid downwind snow growth, while low-elevation generators were hindered by persistent downdrafts that trapped AgI in valleys until farther downwind, where upslope flow enabled eventual activation. These results demonstrate how generator elevation and local dynamical structures critically modulate seeding efficiency in complex terrain. SIGNIFICANCE STATEMENT: Ground-based cloud seeding is used to enhance snowfall in winter storms, yet its ef fectiveness in complex terrain remains uncertain because the boundary layer can vary widely. This study uses a numerical weather model to examine how thermodynamic, microphysical, and dynamical conditions influenced the transport and activation of silver iodide (AgI) released from ground-based generators during a winter storm in Idaho. Results show that only a small fraction of released AgI became activated and that terrain-driven airflow strongly controlled whether AgI reached supercooled liquid water within clouds. Generators located near cloud base were more effective than those at lower elevations, where downdrafts limited vertical transport. These findings demonstrate that generator elevation and local dynamical structures critically influence ground-based cloud seeding efficiency.