Rain-on-Snow Kills Mammals

Public release date: 18-Mar-2008
University of Washington

Rain falling on snow sounds like a relatively harmless weather event,
but when it happens in the far north it can mean lingering death for
reindeer, musk oxen and other animals that normally graze on the
Arctic tundra.

That was the case in October 2003 on Canada’s Banks Island, at the
edge of the Beaufort Sea inside the Arctic Circle. Rain fell for
several days on top of a 6-inch snow cover, and the rain seeped
through the snow to the soil surface. The temperature then plunged
and the water became a thick layer of ice that lasted the winter and
prevented browsing animals from reaching their food supply of lichens
and mosses at the soil’s surface. Some 20,000 musk oxen starved to
death.

“Starvation happened over a period of many months and no one knew
until they went up to do the population count the next spring,” said
Thomas Grenfell, a University of Washington research professor of
atmospheric sciences who has studied the Banks Island event.

Grenfell and Jaakko Putkonen, a UW research associate professor of
Earth and space sciences, have found evidence of the 2003
rain-on-snow occurrence in passive satellite microwave imagery, which
they believe could provide a signature to help detect such events
anywhere. They detail their work in a paper to be published March 25
in Water Resources Research, a journal of the American Geophysical
Union.

Their methods could provide native people, whose livelihood depends
on hoofed animals such as musk oxen, reindeer and caribou with a
realistic chance of getting food to the herds to prevent mass
starvation.

“We are talking about Banks Island, but this applies to the whole
Arctic – Alaska, northern Canada, Siberia, Scandinavia – wherever
there is permafrost,” Putkonen said.

Grenfell has conducted more than 40 field experiments in polar
regions and has become quite familiar with precipitation
characteristics there. Much of the previous work he did was with
researchers who were interested in the nature of the snowpack, but he
found that the presence of water interfered with interpreting
satellite microwave readings.

But for the new research, the signal from water was key. Grenfell and
Putkonen looked for patterns in satellite microwave data that
correlated with rain-on-snow events. They examined data from 10
different satellite microwave channels, each providing slightly
different information on the condition of the snowpack.

“The subtleties in the microwave levels mean there can be high error
margins on this information, but the Banks Island event stood out
like a sore thumb in the data,” Grenfell said.

The researchers hope to examine other satellite microwave records in
search of evidence of rain-on-snow events during the last 30 years
that are known from anecdotal information.

The 2003 rain-on-snow event affected the northern part of the
43,000-square-mile Banks Island. The musk oxen population of 70,000
was cut by nearly 30 percent, but a caribou herd on the southern part
of the island was unaffected. The closest weather station, about 60
miles from the musk oxen range, didn’t record any rainfall at the
time of the event that resulted in the massive die off, so few people
recognized that the oxen were in distress.

Currently there is no way to know exactly where or how often these
potentially devastating rain-on-snow events occur, the researchers
say, but using satellite data to locate them could make up for a
scarcity of weather stations in the sparsely populated Arctic.

Rain-on-snow events historically have occurred mostly in coastal
areas. However, in earlier research Putkonen found that models
predict that climate change will push winter rainfall much farther
into northern continents and large islands.

While food shortages can trigger a large die off, there also can be
severe consequences from milder events that force animals to exert
more energy to get food. That reduces body weight and limits
reproduction, which in turn can cause long-term damage to herds.

“Because the Arctic stays well below freezing for eight to 10 months
of the year, the ice layer can stay around for months. If a
rain-on-snow event happens in the fall, these animals can go the
whole winter without access to food,” Putkonen said. “The native
people in the north depend on these animals for food and for many
other things.”

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For more information, contact Grenfell at (206) 543-9411, (206)
543-4576 or tcg@atmos.washington.edu; or Putkonen at (206) 543-0689
or putkonen@u.washington.edu.

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