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Mysterious "Rain on Snow" Events Tracked in Arctic

Graeme Stemp-Morlock in Waterloo, Ontario
for National Geographic News
March 4, 2008
 
A few warm, springlike days might sound appealing if you live in the frigid Arctic Circle.

But a rise in temperature can spell doom for native peoples and the caribou, musk-oxen, and reindeer that they depend on in Earth's northernmost regions.

That's because a mysterious phenomenon known as "rain on snow," when sudden warm air turns northern snows to rain or slush, can cause animals to starve.

Rather than melting the snow, rain seeps through the snowpack and pools on top of the frozen soil.

When the extreme cold returns, the water freezes into an impermeable shell that prevents animals from grazing.

In October 2003 on Banks Island in Canada's Northwest Territories, a rain-on-snow event caused the deaths of more than a quarter of the musk-ox population—20,000 animals.

Some native people reported the unusual sight of musk-oxen walking onto floating sea ice in search of food, drifting to watery graves.

"When the ice layer formed, the musk-ox on the northern end of the island couldn't break through the ice to get the food," said Tom Grenfell, an atmospheric research scientist at the University of Washington who tracks rain-on-snow events.

"They starved to death or got hypothermia because they have to eat something to stay warm."

Arctic Enigma

While the results of rain-on-snow events are clear, many of the details about why and how the phenomena form remain an enigma.

Jaakko Putkonen, a research professor in the University of Washington's Department of Earth and Space Sciences, is one of a handful of scientists studying the incidents.

"When I [first] tried to get more information, there was almost nothing on rain-on-snow events," he said.

"They are very elusive, so we don't know how often they occur, whether they have changed over time, or their spatial distribution."

Stories told by local people suggest that these events occur in Russia, Sweden, Finland, and Canada, and affect approximately four million Arctic inhabitants.

A small percentage are indigenous people. Canada has more than 50,000 Inuit, one of the largest concentrations of native people in the far north. These native people hunt musk-oxen under a quota system, eating their meat and making clothes and bedding from their wool.

(Related: "Bigger Than Yellowstone, Canada Park to Protect Cultures, Creatures" [October 20, 2006].)

Rain-on-snow events are so difficult to detect that a weather station on the southern end of Banks Island picked up on the 2003 spate of warm weather, but didn't record any precipitation.

Even more surprising, the scientists working at the weather station had no inkling that a massive musk-ox die-off was going on that year—until Canadian Wildlife Services officials went to count the herd in the spring.

Satellite Insights

Though current weather monitors still miss some information, older models were even more poorly designed.

Precipitation was collected with devices that melted snow, so it was never clear what was originally rain, what was snow, or in what amounts and ratios the precipitation fell.

So Grenfell, who studies the characteristics of ice and snow using satellite data, began to collaborate with Putkonen in 2005 to develop methods better suited to observing the events than static weather stations.

In Grenfell's previous work, he had used information from passive microwave detectors to find out more about snow and ice in the Arctic.

"Anything above absolute zero [-459.67 degrees Fahrenheit or -273.15 degrees Celsius] gives off microwave radiation. You and I just sitting here emit microwaves," Grenfell said.

"So the special sensing microwave-imager satellite just picks information up like radio waves, although they aren't being broadcast—they are given off by the medium itself."

He and Putkonen decided that the same satellites could work for tracking rain-on-snow events.

"With a rain-on-snow event, the snowpack's signature [changes]," Grenfell said. "It changes the snow's grain size, which microwaves can detect. Also microwaves can determine layers, such as an ice layer in a snowpack."

The researchers used the microwave-satellite data to create a "cookbook algorithm to understand when and how much water is in the snowpack," Putkonen said.

From that they were able to scientifically confirm the 2003 rain-on-snow event, evidence for which had previously been only anecdotal.

Promising Research

Joey Comiso, a senior scientist at NASA Goddard Space Flight Center's Cryospheric Sciences Branch, said that Grenfell and Putkonen's research is very promising.

"The detection of these incidences would be very useful for monitoring the ecology of the region," Comiso said.

"With so many animals dying in one year, and the population not being that large, such a study would have a tremendous impact."

But "real verification that the study works will require actual measurements from the ground to make sure you have a layer of ice underneath."

Warning System

Putkonen and Grenfell said they plan to look for more rain-on-snow events in the meteorological record—provided they can get funding.

"The next step is to take to all the data that exists to find out how often rain-on-snow events occur, where they happen, and are there special places [where they take place]," Putkonen said.

"We can look back into time because the satellite data has been archived for 25 years, so we can find out if these changed with time and climate, and could they change or drift with future warmer conditions."

In fact, Putkonen's previous climate modeling work suggests that in the next hundred years there could be a 40 percent increase in the area affected by rain-on-snow events.

Based on these models, Grenfell "strongly suspects" the trend of climate change will make rain-on-snow events more common in the Arctic.

Also in the works is a system to monitor weather conditions in the Arctic for rain-on-snow events so the native people can be warned.

Such a system could give people who depend on local animals a chance to observe and feed the animal population so it doesn't suffer a major die-off like in 2003.

"This is one of those fairly rare occasions where there is a very interesting scientific problem to understand natural properties that we know very little about," Putkonen said, "but [which] have very high societal value."

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