Abstract:
To examine the feasibility of cloud seeding to enhance precipitation and runoff in Wyoming, the State of Wyoming commissioned the Wyoming Weather Modification Pilot Project (WWMPP, 2007-14). The focus of this project was a randomized statistical experiment (RSE) examining the impact of ground-based silver iodide (AgI) seeding on snowfall in the Medicine Bow and Sierra Madre mountains. The Wind River Range (WRR) was seeded as well, by a network of 10 AgI generators, whenever the meteorological conditions were suitable, but no RSE was conducted. A final WWMPP report (Rasmussen 2014) and a host of published observational and numerical modelling studies, supported by the UW Water Research Program and the National Science Foundation, indicate that glaciogenic seeding of orographic clouds in the Medicine Bow and Sierra Madre can increase precipitation, although the amounts and optimal seeding conditions remain rather uncertain.
Less is known about the impact of cloud seeding over the WRR on snowpack and streamflow. The execution of another properly designed RSE would cost millions of dollars and require many years of data collection. Instead, we propose to numerically simulate the impact of cloud seeding during the past decade in the WRR, using a state-of-the-science seeding module developed by the National Center for Atmospheric Research (NCAR) (Xue et al. 2013a, b). This module is integrated into the Weather Research and Forecasting (WRF), a widely used atmospheric model, and a detailed land surface scheme (Noah Multi-Physics). This approach will enable us to realistically assess the impacts of seeding on snowpack in the WRR and streamflow in the Green River in less time, for less money. This project, a collaboration with Drs. Rasmussen and Xue at NCAR, will build on previous state-funded modelling work completed for the WRR.
In Year 1, we propose to conduct two separate high-resolution WRF simulations for all 175 seeding operations that were conducted over the WRR during the past nine cold seasons, once with seeding and once without (control). This will allow us to quantify the impact of seeding on snowfall and snowpack evolution. In fact an ensemble will be run for each case, with each simulation slightly different in terms of model boundary conditions, aerosol concentrations and model physics, to also provide a measure of uncertainty. In Year 2, WRF-Hydro simulations will be conducted for the same period (Oct 2007 – Sept 2016) and the same two scenarios (seeding and control) to quantify the impact of cloud seeding on the streamflow in the Green River, down to Flaming Gorge Reservoir. In Year 3, possible discrepancies between observed and simulated basin-side precipitation will be addressed. In particular, we will explore whether or not gridded precipitation products, such as PRISM, underestimate snowfall in higher elevations of the WRR, as has been suggested by several recent multi-year WRF simulations (Liu et al. 2016; Wang et al. 2016). These discrepancies may be due to lack of measurements, especially at higher elevations. Thus this proposal aims to provide informed guidance about optimal locations for new measurements in the WRR and elsewhere in Wyoming.
As detailed in a letter of support from the Wyoming Water Development Office (see Appendix 3), “the results of this study will help facilitate collaborative funding solutions regarding the continuation of operational cloud seeding activities in the WRR.”
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