Two approaches are guiding the general evaluation of the WWMPP: a) a randomized experiment that builds a data set of seeded and control (unseeded) cases, and b) exploratory studies to investigate a wide variety of ideas on detecting seeding effects, including physical studies to document the precipitation formation events hypothesized to be important to snowfall production in orographic storms. The main emphasis of our evaluation is on the statistical analyses required to provide the evidence regarding cloud seeding efficacy. However, efforts to measure and model some of the physical effects of seeding are also supported within the WWMPP as well as through collaborations with other researchers, while remaining focused on a randomized statistical experiment. The overall goal of the WWMPP is to provide the best advice possible to the WWDC regarding the potential for meaningful augmentation of snowpack through cloud seeding.
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Design of the Randomized Seeding Experiment
The experimental design includes the following elements:
- Target areas near the crests of the Medicine Bows (near the Brooklyn Lake SNOTEL) and Sierra Madres (near the Old Battle SNOTEL)
- A cross–over design in which one range is randomly chosen for seeding and the other becomes the control, resulting in paired cases
- A seeding period of four hours with a "buffer" period of four hours to clear the areas of seeding material
- High–resolution precipitation gauge data for determining the seeding response
- Two control gauge sites (unaffected by seeding) in each range to help describe the natural variability in precipitation between target areas and between snowfall events
- Seeding conditions occurring in each range simultaneously (i.e., cold enough temperatures for the seeding material to work, wind flow of the right direction for the seeding generators to affect snowfall in the target areas, and the presence of supercooled liquid water (SLW))
- Enough cases to provide statistical significance of the results
The map of the Medicine Bows-Sierra Madres in southern Wyoming shows the locations of instruments and equipment used in the experiment. These include 16 ground–based seeding generators (8 in each range), 23 precipitation gauges at 8 sites (with redundancy at each site and some experimental gauges), 12 weather stations (at each gauge site and four at generator sites), two microwave radiometers for detecting SLW, a radiosonde unit ("weather balloon" for measuring temperature and winds at cloud heights), and SNOTEL sites (used for data checking and climatological studies).
Evaluation Activities in the Wind River Range
Of the three ranges of interest to the WWMPP – Wind River, Medicine Bow, and Sierra Madre – the Winds are particularly challenging to evaluate. The range encompasses a large area that includes significant expanses of designated wilderness and tribal lands, and it is therefore difficult to collect ground-based observations. The data network in and around the Winds is very sparse (compared to the size of the area), and special observations are needed to assess and verify physical processes in precipitating systems.
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