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Profiling airborne radar data and accompanying Large Eddy Simulation (LES) modelling are used to examine the impact of ground-based glaciogenic seeding on cloud and precipitation in a shallow stratiform orographic winter storm. This storm occurred on 18 February 2009 over a mountain in Wyoming. The numerical simulations use the Weather Research and Forecast model in LES mode 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. A series of non-LES simulations at 900 m resolution, each with different initial/boundary conditions, is validated against sounding, cloud and precipitation data. The LES runs then are driven by the most representative 900 m non-LES simulation. The 100-m LES results compare reasonably well with the vertical plane radar data. The modeled vertical motion field reveals a turbulent boundary layer and gravity waves above this layer, as observed. The storm structure also validates well, but the model storm thins and weakens more rapidly than observed. Radar reflectivity frequency-by-altitude diagrams suggest a positive seeding effect, but time- and space-matched model reflectivity diagrams only confirm this in a relative sense, in comparison with the trend in the control region upwind of seeding generators, not in an absolute sense. A model sensitivity run shows that in this case natural storm weakening dwarfs the seeding effect, which does enhance snow mass and snowfall. Since the kinematic and microphysical structure of the storm is simulated well, we will examine how glaciogenic seeding impacts clouds and precipitation processes within the LES, in Part II of this study.




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Jun 30 2015 (withdrawn)
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