A Multi-sensor Study of the Impact of Ground-based Glaciogenic Seeding on Clouds and Precipitation over Mountains in Wyoming in Relation to Ambient and Cloud Conditions

The AgI Seeding Cloud Impact Investigation (ASCII) campaign was conducted over the mountain ranges in southern Wyoming to examine the impact of ground-based glaciogenic seeding on winter orographic clouds and precipitation. The field experiment was conducted over the Sierra Madre and Medicine Bow mountains in early 2012 and 2013, respectively, in the context of the Wyoming Weather Modification Pilot Project. The campaign was supported by a network of ground-based instruments, including a microwave radiometer, two profiling Ka-band Micro-Rain Radars (MRRs), a Doppler on Wheels (DOW) X-band radar, rawinsonde launches, and a disdrometer. The University of Wyoming King Air with profiling Wyoming Cloud Radar (WCR) conducted 17 successful flights (9 over the Sierra Madre and 8 over the Medicine Bow mountians) in ASCII. Data from 7 flights over the Medicine Bow in 2008-09 are included in the composite analysis. This analysis also includes 9 cases with DOW data and 5 cases with paired MRR data over the Sierra Madre, not all of them overlapping with the 9 WCR cases over that range. In most cases, two hours without seeding (NOSEED) were followed by two hours of seeding (SEED). The WCR reflectivity data from all 24 cases were analyzed to examine how glaciogenic seeding affects snow growth in orographic clouds. The double difference (SEED – NOSEED in the target region, compared to the same in the control region) suggests an increase in precipitation, esp. over the Sierra Madre, and especially in stratiform precipitation cases. But there is much uncertainty related to sampling representativeness and to differences in natural trends between control and target regions. Thus the double difference varies significantly from case to case. Therefore it is difficult to examine the sensitivity of seeding impact to atmospheric conditions based on the available cases. In situ data at the ground (disdrometer) and at flight level (2D-probes) indicate an increase in concentrations of small particles during seeding.