UW Scientists Contribute to Cloud

The University of Wyoming played a key role in a nearly decade-long study by the state that concluded cloud seeding has some effect, with a 5-15 percent positive impact on snow accumulation shown after the weather modification technique is used.

However, UW researchers were cautious in their comments on the findings, which were presented to the Wyoming Water Development Commission at that board’s meeting in Cheyenne Dec. 10.

While the overall draft executive summary report concluded that cloud seeding is a strategy for long-term water management in Wyoming, Terry Deshler, a retired UW professor of atmospheric science, stresses that not all winter snowstorms are conducive to using cloud seeding. During cloud seeding, silver iodide is released into the clouds through generators. In the case of the state’s $13 million study, generators were strategically placed upwind of the ridges of the Medicine Bow and Sierra Madre mountains in southern Wyoming. The silver iodide facilitates ice crystal formation in super-cooled water clouds.

“Not all winter precipitation is seedable,” Deshler told board members during his portion of the presentation on the draft study results of the state’s nine-year Weather Modification Pilot Program (WMPP). “Estimates of seeding impact cannot be applied to the entire winter snowpack.”

During his climatology report on cloud seeding, Deshler told the commission that seeding of orographic wintertime clouds is appropriate only under certain meteorological conditions. On average, atmospheric conditions -- temperatures colder than minus 8 degrees Celsius (17 degrees Fahrenheit), wind moving in the right direction and the presence of super-cooled liquid water in the atmosphere -- met the seeding criteria less than one-third of the time during the winter, Deshler says. In addition, such conditions were accompanied by precipitation approximately half of the time that atmospheric conditions met the seeding criteria.

“Considering only conditions when precipitation was occurring during seedable conditions indicates, on average, 30 percent of the wintertime snowpack over the Medicine Bow and Sierra Madre ranges for the years 2000-2008 would have been seeded under the conditions specified for the RSE (randomized statistical experiment),” says Deshler, a member of the cloud-seeding project’s Technical Advisory Committee.

“NCAR (National Center for Atmospheric Research) is suggesting snowpack increases of 5 to 15 percent for seedable storms. But, what percentage of winter storms are actually seedable?” says Barry Lawrence, project manager of the Wyoming Water Development Office. “Terry Deshler has kept us credible.”

Deshler’s work shows that the 5-15 percent increase due to seeding translates to a 1.5-5 percent increase in snowpack.

This experiment by the state of Wyoming is the first statistically valid experiment that has been run in the U.S. on cloud seeding since the 1960s to the 1970s, Deshler says.

Due to water shortages and drought in some states and in countries around the world, cloud seeding is seen as a potential way to increase water supplies for communities and to irrigate crops. Cloud seeding typically is paid for by water resource managers, power companies (hydropower) and agricultural interests.

In 2005, the Wyoming State Legislature approved a five-year weather modification study administered by the Wyoming Water Development Office. The WMPP began in 2006-07. Randomized cloud seeding began during 2008-09, with the seeding period running from Nov. 15 through April 15 each year. The original expectation was to run 65-70 four-hour experiments during the seeding period each year. However, based on conditions ripe for cloud seeding, experiments each season numbered in the 25-30 range.

Since the project started, the Legislature has twice funded two-year extensions to the original five-year appropriation because the number of cloud-seeding experiments was not sufficient to reach statistically significant conclusions for the pilot project.

The project, which also included some cloud-seeding experiments over the Wind River Mountains, concluded in April of this year. Since then, scientists and statisticians with NCAR -- working independently from the state and the contracted cloud-seeding operator -- tested the compiled data before presenting the results last week.

“Our final conclusion is that there is a 5 to 15 percent positive seeding impact in Medicine Bow and the Sierra Madre for seedable cases based on the RSE criteria and for which sufficient ground-based silver iodide seeding was achieved,” Roy Rasmussen, a senior scientist with NCAR, says referencing the written report. “This is accumulated from statistical, modeling and physical studies.”

The high-resolution modeling studies conducted by NCAR -- that simulated half of the total number of seeding cases -- showed positive seeding effects between 10 and 15 percent.

However, Bart Geerts, another UW professor of atmospheric science, pointed to the statistical study only, which revealed a mere 3 percent increase in precipitation with a 28 percent probability that the snowpack result occurred by chance.

“This 28 percent is much larger than the 5 percent they had set as the threshold for a statistically significant seeding effect,” Geerts says of the state’s statistical study results. “This again underscores the difficulty in demonstrating seeding efficacy by means of randomized seeding experiments.”

Geerts piggybacked on the state’s project. Geerts’ project, titled "Detecting the Signature of Glaciogenic Cloud Seeding in Orographic Snowstorms in Wyoming Using the Wyoming Cloud Radar,” was funded by the Wyoming Water Resources Research Institute at UW through the university's Office of Water Programs (OWP). Using radar and lidar instrumentation from UW’s King Air research aircraft, Geerts gathered data for his own cloud-seeding research.

During 2010, Geerts’ study estimated up to a 25 percent increase in precipitation for seven lightly precipitating storms, a small sample set. Geerts garnered additional funding from the National Science Foundation for two more years of aircraft study measurements.

That NSF-funded project focused on the processes in clouds over mountains and on ways in which these processes were altered by seeding. The preponderance of evidence in that study suggests that seeding can increase the snow concentration and overall snowfall over Wyoming’s mountains. That study also showed that suitable seeding conditions remain poorly understood and poorly predicted.  

Still, regarding the pilot project report, Geerts credits NCAR with making tremendous progress with the numerical simulation of cloud seeding.

“While the details of the seeding and cloud processes in these simulations require further scrutiny, this approach is very promising because models allow perfect isolation of the seeding effect, something that eludes us in the real world,” Geerts says.

The state’s draft study results also showed there was no impact on snowfall in areas surrounding the cloud-seeding target sites and negligible environmental impact, with silver traces found in water sources at concentrations in parts per trillion and concentrations in the soil measuring in the parts per billion range.

“It was important to bring the report home and present the results assembled,” Lawrence says. “Certainly, we can build on successes here to go forward in an operational mode.”

A final report is expected to be presented to the Wyoming Water Development Commission in March, Lawrence adds. A draft executive summary of that report was presented this month because researchers wanted to present their findings to the American Meteorological Society in January, when that group hosts its annual meeting in Phoenix, Ariz.