The WWDC funded a feasibility study to assess the potential for cloud seeding in the Bighorn Mountains in north-central Wyoming. RAL leads this program with collaboration from WMI and HEC. This project began in June 2015 and highlights from a few tasks completed in FY16 are provided below. A draft report summarizing the results of this study was submitted to the WWDC in August 2016.
A climatological study of the project area was conducted to determine the characteristics of wintertime precipitation in the Bighorn Mountains and estimate how frequently environmental conditions would be amenable to seeding. The climatology analysis indicated that the typical wind regimes in the Bighorns are westerly to northwesterly, with few easterly (upslope) events on the eastern slopes. A spatial mapping analysis revealed that liquid water content (LWC) most frequently develops on the western and northeastern slopes of the mountains with the most frequent seeding opportunities on the western slopes. A microwave radiometer and high-resolution snow gauges were deployed to the northern end of the Bighorns during the winter of 2015–2016 in order to collect measurements of liquid water path and snowfall, to aid in this investigation.
Based only on temperature and LWC criteria, ground seeding had equal or more frequent opportunities than airborne seeding during the November–April wintertime period. When considering additional criteria required for ground-based seeding (wind direction and stability for transporting ground-released AgI into the targeted clouds), ground-seeding opportunities dropped to nearly zero in the eastern and southern regions, and were substantially reduced in the western region. In fact, airborne seeding potential in the western region is greater than ground seeding potential in the western region, especially when considering the additional criteria necessary to implement ground-based seeding (Figure).
Three test cases were simulated with the NCAR cloud seeding model to evaluate the impact of six groups of proposed ground generators and several potential aircraft tracks (Figure). The test cases were selected to represent various meteorological scenarios encountered in the Bighorn Mountains, but not every scenario may have been represented by this limited sample.
Based upon the model simulations of the three test cases, in general, ground seeding had very limited spatial impact on the region, and Groups A and B rarely impacted the target area. Group C had positive simulated seeding effects in some meteorological conditions, especially when the winds had a north-northeasterly component. Groups D and F yielded the best results for ground-based seeding, yet the impact area was still rather small and confined to very narrow plumes (Figure 7). In the cases tested, Group E had quite minimal overall simulated impacts (Figure 7).
Airborne seeding tended to yield the most widespread and biggest simulated seeding effects in these cases (Figure 7). The benefit of airborne seeding is that it can be performed wherever the most SLW is present. This can include situations where elevated SLW extends quite far upwind (to the west) of the mountains, as occurred in some of these cases. In fact, airborne seeding is the only way to impact elevated SLW layers over the Bighorn River Basin because the air is too stable in this valley to use ground generators to reach those higher altitudes. Yet, it should be noted that not all of the simulated seeding effects from seeding further upwind impact the higher elevations of the Bighorn Mountains; rather they broadly impact the Bighorn Basin in general. Based on the modeled climatology, easterly upslope events include SLW, but occur infrequently. Therefore, due to the lower frequency of occurrence, siting ground generators on the eastern slope of the Bighorns is not advised, but it should be noted that airborne seeding is versatile enough that it can also be used to target easterly upslope events.
