HAPpy Hour Seminar: Orographic Effects on Cloud Seeding Feasibility in Idaho's Lemhi River Basin

Abstract: Wintertime precipitation is vital to water resources in mountainous river basins in the western United States. The use of silver iodide (AgI) as a cloud seeding agent has the potential to enhance wintertime precipitation. In this study, the Lemhi River Basin in Idaho was the focus of our analysis to determine the feasibility of enhancing wintertime precipitation using both ground-based and airborne dispersion methods.
The WRF-WxMod (Xue et al. 2013) model was used to simulate and analyze a case study for both ground based and airborne seeding. Various meteorological conditions with different wind directions and atmospheric stabilities were explored. For ground-based seeding, wind direction and Froude number analyses were performed to assess flow blocking and the effective dispersion of AgI over the targeted mountain barriers. For airborne seeding, different flight track lengths, altitudes, and directions were investigated to determine the effectiveness of AgI dispersion compared to ground-based methods.

The results show that some regions in the study area experienced substantial flow blocking, impacting seeding potential—particularly for ground-based methods. The impact of orographic forcing on airborne seeding is twofold. The strong updrafts and downdrafts induced by the mountains can cause ineffective and dry sedimentation of the seeding agent, while they can also enhance its mixing and seeding effect. Simulation results and recommendations on how to most effectively target the basin with ground based and airborne cloud seeding will be presented.