Several wintertime weather modification research projects in the 1990’s showed the usefulness of mesoscale modeling for evaluating seeding plume transport and dispersion, for revealing the characteristics of gravity waves, and for showing the evolution of airflow in and around mountainous regions where cloud seeding was being conducted. Modeling, combined with quality verification data sets, provides a means of determining how effectively cloud seeding can be conducted in a wide variety of meteorological situations. Modeling historical and current situations allows analysis beyond what can typically be evaluated with detailed and expensive measurement systems.

A mesoscale modeling effort within Nevada’s Weather Damage Modification Program (WDMP), a cooperative research effort with the U.S. Bureau of Reclamation, focused on simulating ground seeding plume transport and dispersion in both well documented historical cases and in storms encountered during the winter of 2003-04. The project used the Mesoscale Model 5 (MM5) for predicting the evolution of storms over the Sierra Nevada, and the DRI Lagrangian Random Particle Dispersion (LAP) model for simulating plume transport and dispersion. Both models have been tested extensively in this region with good results. The LAP model uses MM5-predicted meteorology as input, keeps track of millions of individual hypothetical particles from multiple emission sources, and employs realistic sub-grid scale turbulence.

This paper presents the modeling results from a ground and airborne cloud seeding case in the Walker River Basin for a period from 1-3 February 2004. The inner domain of MM5 was centered on a region that encompassed all ground seeding generators, the aircraft seeding track and the downwind instrument and snow sampling sites. MM5 output variables are compared to precipitation data at several locations, radiometer vertically integrated cloud water and vapor, and aircraft data. LAP seeding plume simulations are used to evaluate the effectiveness of ground seeding sites for this case, and a new elevated line source simulation is used for the first time to show the predicted evolution of the aircraft seeding plume.