Cloud-resolving Large Eddy Simulations of the Impact of Ground-based Glaciogenic Seeding on Shallow Convective Orographic Clouds and Precipitation

The efficacy of cloud seeding in enhancing snowfall remains controversial, mainly because of the challenge of detecting the seeding signal in a highly variable, noisy field, i.e. precipitation. The 2012-13 AgI Seeding Cloud Impact Investigation (ASCII) made some progress in trying to isolate the seeding impact on particle size distribution, snow mass, and snowfall, both using case studies and composite studies (Geerts et al., 2010, 2013; Miao and Geerts 2013; Pokharel et al. 2014a, b; Pokharel and Geerts 2014; Jing et al. 2014). While these studies add to an increasing body of evidence that glaciogenic seeding of orographic clouds “works” under some conditions, they all mention considerable uncertainty about observationally isolating the seeding effect. To address this challenge, we use the Weather Research and Forecast model (WRF) based cloud-resolving larger eddy simulations (LES) with horizontal grid spacings of 300 m and 100 m in a domain covering the entire mountain range, and a glaciogenic seeding parameterization coupled with the Thompson microphysics scheme to simulate ground-based AgI dispersion and its impact on orographic clouds and precipitation. Chu et al. (2014) use this strategy for a stratiform winter storm and find a positive impact of seeding on hydrometeor mixing ratio and snowfall downwind of AgI generators, consistent with observations, but this trend of overwhelmed by natural mountain-scale storm changes, so it can only be seen in observations through vertical profile analysis and comparisons with a control region. This study examines a storm with shallow convection embedded in stratiform precipitation, on 13 February 2012 over the Sierra Madre in Wyoming. The 100-m LES reproduces the storm structure well, including convective cells. The two key questions we aim to answer are: (a) Do convective motions disperse the ground-based AgI nuclei and the ice nucleation events over a greater depth than for more laminar, stratified flow over the mountain? (b) Does the presence of convection move the seeding-induced snowfall enhancement further downwind, across the mountain crest, as is suggested by observations (Jing et al. 2014)?