The Impact of Glaciogenic Seeding on Snowfall from Shallow Orographic Clouds over the Medicine Bow Mountains in Wyoming
Keywords:Wyoming Cloud Radar, AgI seeding, Medicine Bow Mountains
The 2013 AgI Seeding Cloud Impact Investigation (ASCII-13) campaign was conducted over the Medicine Bow range in Wyoming to examine the impact of ground-based glaciogenic seeding on winter orographic clouds and precipitation. The campaign was supported by a network of ground-based instruments, included a microwave radiometer, two profiling Ka-band radars, rawinsonde launches, and a disdrometer. The University of Wyoming King Air with profiling Wyoming Cloud Radar (WCR) conducted eight successful flights in ASCII-13. WCR profiles from these flights are combined with those from seven other flights, which followed the same geographically-fixed pattern in 2008-09.
A case study shows that a clear seeding signature is not apparent in individual WCR reflectivity transects downwind of the silver iodide (AgI) generators, and that natural trends in precipitation over short timescales can easily overwhelm any seeding impact. Therefore the ASCII experimental design included control regions, both upwind of the AgI generators and to the side of the estimated AgI nuclei plumes. Frequency-by-altitude displays show slightly higher reflectivity values during seeding near the ground, at least when compared to the control regions. The near-surface precipitation rate trend from the untreated to the treated period in the target region, compared to that in the control region, varies significantly from flight to flight, according to the WCR reflectivity data within the boundary layer. Most of this variation relates to non-homogenous natural trends across the mountain range, and/or to sample unrepresentativeness, esp. for the control region. While WCR reflectivity profiles from flights over the Medicine Bow range do not show a significant increase in near-surface precipitation during seeding (in comparison with upstream trends), similar data from nine separate flights in 2012 over an adjacent range, the Sierra Madre, do.