AgI cloud seeding effects as seen in WRF simulations

A silver iodide (AgI) point source module has been implemented into the Thompson microphysics scheme of the Weather Research and Forecast (WRF) model to investigate glaciogenic cloud seeding effects by simulating two-dimensional idealized moist flow over a bell-shaped mountain and four three-dimensional real seeding cases over the southern Idaho during the 2010 to 2011 winter season.

By varying the meteorological conditions, cloud properties, seeding methods and seeding rates, the sensitivity of seeding effects have been explored. The results of both 2D and 3D simulations verified that this AgI module is able to reasonably simulate the physical processes of seeding and showed that: 1) Deposition is the dominant nucleation mode of AgI from simulated aircraft seeding while immersion freezing is the most active mode for ground seeding. Deposition and condensation freezing are also important for ground seeding. Contact freezing is the weakest nucleation mode for both ground and airborne seeding. 2) Through diffusion and riming processes, AgI nucleated ice crystals deplete vapor and liquid water resulting in more ice-phase precipitation on the ground for all seeding cases compared to control cases. The majority of precipitation enhancement comes from vapor depletion. 3) The seeding effect is inversely related to the natural precipitation efficiency, but is positively related to seeding rates. 4) Airborne seeding is generally more efficient than ground seeding in terms of targeting, but its efficiency depends on meteorological conditions.

The sensitivity of seeding effect to vertical resolution is also investigated.