WEATHER
FORECASTERS MAY LOOK SKY-HIGH FOR ANSWERS These
days, weather forecasters are lucky if they can accurately predict the weather
a week into the future. But a new study, funded in part by NASA, says shifting
wind patterns in the stratosphere during the winter may help forecasters predict
weather on the surface two months ahead of time, because they have an affect on
where storms track in the northern hemisphere. Changes
in the stratosphere, the atmospheric layer from six to 30 miles up, usually take
a week or more to work their way down to where they affect weather, giving forecasters
some lead-time. Once the changes affect the weather, they tend to last as long
as two months. "The
study points weather forecasters towards a new source of information that hasn't
been used before in weather prediction," said Mark Baldwin, Senior Research
Scientist at the Northwest Research Associates (NWRA), and lead author in the
study. The article appears in the October 19th issue of Science. According
to the study, the stratosphere plays an important role in how large-scale waves,
which originate near Earth's surface, feed back to affect weather patterns in
the Northern hemisphere. In
the winter, when stratospheric winds most often blow from the west, these waves
tend to slow the winds in the stratosphere. This process starts with the higher
stratospheric winds and over about a week's time, can work its way down to winds
of the lower stratosphere, just above the levels of commercial air traffic. Though
the exact processes have yet to be fully understood, it has been observed that
shifting wind patterns in the stratosphere precede changes in the Arctic Oscillation,
a large-scale see-saw of atmospheric mass between the polar regions and mid-latitudes.
The Arctic Oscillation, also called the North Atlantic Oscillation, is most pronounced
over the Atlantic, and affects the strength of the winds through mid-latitudes,
storm tracks, as well as extreme cold events in North America and Eurasia. When
the winds are weak in the stratosphere, the arctic oscillation is weak in the
60 days that follow, and that moves the paths of storms further south in the northern
hemisphere. When the winds are strong in the stratosphere, the arctic oscillation
is stronger, and the paths of storms are usually more northward. "We
can see it coming," said Baldwin. "It takes over a week to get to the
surface. Once they reach the surface, once we are in one of these weather regimes,
it lasts an average of two months." Though
the effects have been clearly observed, the exact interplay between the stratosphere
and how surface weather patterns change is not fully understood. "It
is an initial step," said Dr. Timothy Dunkerton, Senior Research Scientist
at NWRA, and co-author of the paper. "Our understanding of the role of the
stratosphere in weather and climate could be compared to our knowledge of El Niño
20 years ago." Baldwin
added that there are two likely scenarios for practically applying this information
to forecasts. One is to use a sophisticated forecasting computer model that includes
the stratosphere. Current weather models usually do not include data from that
high in the atmosphere. Still, this kind of modeling is a very involved process
that requires large resources, and at present is not very practical. The
other option is to apply statistical measurements that use observational data
to tell the likelihood that certain weather conditions will occur following shifts
in stratospheric winds. The
study was jointly funded by NASA, the National Oceanic and Atmospheric Administration
and the National Science Foundation. Back
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