Boulder scientists have for the first time shown conclusively that cloud seeding can enhance snowfall when the conditions are right.
In a study published this week in Proceedings of the National Academy of Sciences, researchers at Boulder’s National Center for Atmospheric Research and the University of Colorado Boulder report they successfully deployed a combination of snow gauges and radars to measure the effect of cloud seeding on snowfall.
“This is a revelation,” Sarah Tessendorf, an NCAR scientist and co-author of a new paper about the research, said in a statement. “We can definitely say that cloud seeding enhances snowfall under the right conditions.”
“The news here is that we can measure it over an entire (water) catchment”area,” the study’s lead author, Katja Friedrich, said in an interview.
“So far, people have shown in individual gauges that we can see an increase, which they can link to cloud seeding,” said Friedrich, an associate professor at CU Boulder’s Department of Atmospheric and Oceanic Sciences.
“But the problem is always to separate the natural precipitation from the seeding. In our case, there was no natural precipitation, so if we didn’t seed on that day, we didn’t see any precipitation. So the news here is that over an entire catchment we can quantify how much water we can produce from one cloud seeding event.”
The study draws on analysis of observations recorded in a cloud seeding experiment in Idaho during late winter 2017.
“This campaign was actually in collaboration with the Idaho Power Company,” Friedrich said. “And their goal is to enhance, to actually put more water into their reservoirs, which they can use for hydropower. So the goal is always to enhance the snowpack for whatever reason, whether it’s for hydropower or whether it is generating more water for consumption.”
The project, dubbed SNOWIE (Seeded and Natural Orographic Wintertime Clouds), was co-funded by Idaho Power and the National Science Foundation.
Researchers deployed airborne and ground-based radars, high-resolution snow gauges and computer modeling to quantify the impact of injecting silver iodide into moisture-laded clouds over the Payette Basin north of Boise. The seeding aircraft released silver iodide along a flight path that resulted in a zigzag pattern of seeding effects in the clouds.
According to a news release, that approach enabled researchers to watch the entire process and compare side-by-side the seeded and unseeded areas, tracking the seeding plume from the time it was injected into the clouds until it produced snow that reached the ground.
On at least three instances, scientists found that the seeding measurably boosted the snowfall across significant regions. A cloud seeding flight on Jan. 19, 2017, for example, produced snow for about 67 minutes, depositing a tenth of a millimeter of snow above the negligible amount that was falling naturally on about 900 square miles.
The three cases highlighted in the study produced a combined total of 571 acre-feet of water, or roughly the equivalent content of about 285 Olympic-sized swimming pools.
But other cloud seeding attempts were not as easily detected and may not have been successful. Friedrich and Tessendorf said their team is continuing to analyze data from 18 other attempts during the project, hoping to determine under what conditions the seeding effect can be detected and trigger a precipitation increase.
Early results produced from the study provide encouragement that some precision may finally be coming to a practice that for decades has been clouded by uncertainty.
“There are studies out there that claim they can increase the snowpack by 0 to 50%. This is a random number,” Friedrich said, who noted that since the ’60s and ’70s, cloud seeding had been “oversold” in light of a deficit of data.
“And the difficulty is that it is very difficult to separate from what is naturally occurring in a cloud, through precipitation, and what is the precipitation from cloud seeding. … And I think this is what we are suffering from now, that people thought they could solve all kinds of problems with cloud seeding, with very little scientific evidence.”
The NCAR-CU Boulder study, which included partners at the University of Wyoming and University of Illinois at Urbana-Champaign, is seen as a significant step toward remedying that situation.
“We are still analyzing the data. This paper is based on three cases,” Friedrich said. “In total we have 24 cases, so there is so much more data to be analyzed.
“Again, the end goal is not to look at three individual cases to see how much water we can produce, but rather, looking at an entire water year, looking over an entire winter over these 24 cloud seeding events, to see how much snow can be produced.”
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