UW Researcher Leads Study of First Quantifiable Observation of Cloud Seeding

Cloud-seeding graphic via Science Matters

From the University of Wyoming:

A University of Wyoming researcher contributed to a paper that demonstrated, for the first time, direct observation of cloud seeding — from the growth of the ice crystals through the processes that occur in the clouds to the eventual fallout of the ice crystals that become snow — and how the impacts could be quantified.

The research, dubbed SNOWIE (Seeded and Natural Orographic Wintertime Clouds — the Idaho Experiment), took place Jan. 7-March 17, 2017, within and near the Payette Basin, located approximately 50 miles north of Boise, Idaho. The research was in concert with Boise-based Idaho Power Co., which provides a good share of its electrical power through hydroelectric dams.

“No one has ever had a full comprehensive set of observations of what really happens after you seed the cloud,” says Jeff French, an assistant professor in UW’s Department of Atmospheric Science. “There have only been hypotheses. There has never been a set of observations from one campaign that shows all the steps that occur in cloud seeding.”

French credits modern technology, citing the use of ground-based radar, radar on UW’s King Air research aircraft and multiple passes of the mountain range near Boise with making the detailed cloud-seeding observations happen. Despite numerous experiments spanning several decades, no direct observation of this process existed before SNOWIE, he says.

French is the lead author of a paper, titled “Precipitation Formation from Orographic Cloud Seeding,” which appears in the Jan. 22 (today’s) issue of the Proceedings of the National Academy of Sciences (PNAS), one of the world’s most prestigious multidisciplinary scientific journals, with coverage spanning the biological, physical and social sciences.

Other contributors to the paper were from the University of Colorado-Boulder, University of Illinois at Urbana-Champaign, the National Center for Atmospheric Research (NCAR) and Idaho Power Co.

“SNOWIE was a great collaborative effort, and it shows the value of private, public and academic partnerships,” says NCAR scientist Sarah Tessendorf, a co-author of the paper.

Tessendorf notes that SNOWIE grew out of research that Idaho Power Co. had conducted with NCAR to improve its cloud-seeding program. This included the development of high-resolution computer modeling approaches to simulate cloud seeding, enabling researchers to better evaluate its impacts.

“This research shows that modern tools can be applied to long-standing scientific problems,” says Nick Anderson, program director in the National Science Foundation (NSF)’s Division of Atmospheric and Geospace Sciences, which funded the study. “We now have direct observations that seeding of certain clouds follows the pathway first theorized in the mid-20th century.”

Cloud seeding is a process by which silver iodide is released into the clouds, either from the air or via ground-based generators. In the case of the SNOWIE Project, the silver iodide was released by a second aircraft funded through Idaho Power Co., while the UW King Air took measurements to understand the impact of the silver iodide, French says.

In all, the UW King Air made 24 research flights or intense observation periods (IOPs) lasting 4-6 hours each during SNOWIE. Of those IOPs, cloud seeding occurred during 21 of the flights. During the last three flights, Idaho Power had to suspend cloud seeding because there was so much snow in the mountains already, French says.

While a good deal of research took place aboard the King Air, much of it also occurred on the ground. Numerical modeling of precipitation measurements was conducted using the supercomputer, nicknamed Cheyenne, at the NCAR-Wyoming Supercomputing Center. The numerical models simulated clouds and snow precipitation — created in natural storms and with cloud seeding — over the Payette Basin in Boise. The numerical models also allow researchers to study future storm events where measurements have not been obtained in the field.

“Midterm, it will help with those simulations we are running on Cheyenne,” French says. “Long term, we can answer questions with this data and simulations on how effective cloud seeding is in orographic clouds, and what conditions one can expect cloud seeding to be effective.”

Throughout the western U.S. and other semiarid mountainous regions across the globe, water supplies are fed primarily through snowpack melt. Growing populations place higher demand on water, while warmer winters and earlier spring reduce water supplies. Water managers see cloud seeding as a potential way to increase winter snowfall.

“Ultimately, water managers and state and federal agencies can make the decision whether cloud seeding is a viable option for them in terms of adding additional water to their supplies in addition to snowpack in the mountains,” French says.

From Science Magazine (Angus Chen):

…despite decades of cloud seeding operations, proof that the technique works outside miniaturized clouds created in the lab has been elusive. One reason: Instruments of decades past couldn’t measure water droplet size in clouds in real time. Without knowing how a cloud evolves after seeding, scientists were unsure whether the silver iodide was doing anything at all. Another: The chaotic nature of weather makes controlled, natural experiments almost impossible. “Once you seed, you’re contaminating the cloud. You can’t repeat the experiment because you’ll never have the same atmospheric conditions again,” says Katja Friedrich, an atmospheric scientist at the University of Colorado in Boulder.

But newer instruments convinced Friedrich and her colleagues that the time was ripe for another approach—and the National Science Foundation and Idaho Power provided the funding. The team took its experiment to the mountains of southwestern Idaho, where it waited until supercooled clouds appeared in the sky. At temperatures of 0°C to –15°C, they are cold enough to freeze, but are at low odds of doing so.

When the right clouds came along, the team sprang into action. It launched one aircraft that made laps between two ground-based radars, dropping canisters that spread silver iodide into the clouds. The same plane also flew through the cloud while streaming silver iodide from its wings. Another plane loaded with cloud measuring equipment paced a perpendicular path to take readings.

At first, there was nothing. “The radar can only see [water] particles that are big enough, and these clouds had tiny droplets not detectable by radar,” Friedrich says. “Suddenly, we saw lines appear. It was really astonishing.” The zig-zagged lines matched the flight path of the first plane. Within these lines, the cloud’s water particles were getting bigger as they hit the silver iodide and froze. After a couple of hours, the snowflakes had grown from a few microns in diameter to 8 millimeters in diameter—heavy enough to fall to the ground, Friedrich and her colleagues report today in the Proceedings of the National Academy of Sciences. “We were super, super excited. Nobody had seen that before,” she says.

The experiments have also been met with enthusiasm from cloud seeding companies. “Those of us working on cloud physics for a long time have felt that [cloud seeding] was working,” says Bruce Boe, a meteorologist at cloud-seeding company Weather Modification in Fargo, North Dakota. “This verification and incontrovertible evidence this is occurring is really, really nice for us.”

Still up for study is whether the approach is economical. “Does it make enough snow to make an impact on a water budget?” Friedrich wonders. “We still have to answer those fundamental questions.”

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