New insights into hydrometeor response to ground-based glaciogenic seeding of orographic clouds from profiling radar data

Data from an airborne vertically-pointing mm-wave Doppler radar are used to study the cloud microphysical effect of glaciogenic seeding of cold-season orographic clouds. Fixed flight tracks were flown downstream of ground-based silver iodide (AgI) generators in the Medicine Bow Mountains of Wyoming. Composite data from seven flights, each with a no-seeding period followed by a seeding period, indicate that radar reflectivity was higher within the boundary layer (BL) during the seeding periods. Several physical considerations argue in favor of the hypothesis that the increase in near-surface reflectivity is attributed to AgI seeding. The flight level generally remained above the BL and thus above the region of AgI seeding impact, but the profiling radar data can be used to identify sections where the aircraft intersected BL air. In these sections more numerous ice crystals and a lower liquid water content are encountered during seeding. While the increase in near-surface reflectivity and thus snowfall rate are statistically significant, caution is warranted in view of the large natural variability of weather conditions and the small size of the dataset. Therefore a proposal has been submitted to NSF for further, more comprehensive observational and modeling work, to be conducted in the context of ongoing glaciogenic seeding operations in Wyoming.