The Extra-Area Effect of Orographic Cloud Seeding: Observational Evidence of Precipitation Enhancement Downwind of the Target Mountain

This study uses scanning X-band Doppler on Wheels (DOW) radar data to examine whether ground-based glaciogenic seeding influences orographic precipitation, inadvertently, over the foothills of a mountain similar to 50 km downwind of the target mountain. The data were collected during seven storms during the 2012 AgI Seeding Cloud Impact Investigation (ASCII-12) campaign in Wyoming. The DOW was located on the Sierra Madre (the target range), with excellent low-level coverage toward the Medicine Bow (the downwind range). To examine the seeding impact, two study areas are designated, both over the foothills of the downwind range: one is directly downwind of the remote silver iodide (AgI) generators (target area), and the other is offset sideways (control area). Comparisons are made between radar reflectivity measurements from a treated period and those from an untreated period. The total treated (untreated) period over seven storms is 14.3 h (21.2 h). Independent measurements of ice nuclei concentrations indicate that ground-released AgI nuclei can disperse across two mountain ranges over a distance of similar to 80 km. Analyses of DOW transects, DOW echo-height maps, and Doppler velocities from an airborne profiling radar suggest three different mechanisms for the vertical mixing of AgI nuclei: in all cases boundary layer mixing is active, and in some cases convection, or a lee hydraulic jump, or both are present. In all cases the radar reflectivity is higher during seeding in the target region when compared with the trend over the same period in the control region. Note that the results are not definitive proof of a downwind seeding impact since natural variability of precipitation is large and the sample size examined is small.