By James E. Kloeppel
Mountains play a loftier role in the earth's weather than once thought.
In fact, the interaction between mountain ranges and the jet stream may
be the primary factor in determining where severe winter storms drop their
loads of snow and ice, a UI researcher says.
"Storm tracks -- the most probable trajectories of intense storms --
are of major interest to climatologists and weather forecasters," said
Mankin Mak, a professor of atmospheric sciences. "A better understanding
of what causes storm tracks could lead to more accurate weather prediction
and a better understanding of why the earth's atmosphere behaves the way
it does."
There are two primary regions of winter storm activity in the northern hemisphere,
Mak said. One is over the Atlantic Ocean off the east coast of North America;
the other is over the Pacific Ocean off the east coast of Asia. The associated
storm tracks have been linked to mountains: the Rocky Mountains in North
America and the Tibetan Plateau in Asia.
Unlike hurricanes that are spawned over oceans during summer, winter cyclones
preferentially originate downstream of the mountains.
"The jet stream is much stronger during the winter due to the greater
temperature differential between polar and equatorial regions, so you have
a much stronger flow over the mountains," Mak said. "This flow
could account for the existence of storm tracks."
To investigate the role that mountains play in the dynamics of winter storm
tracks, Mak and colleague Woo-Jin Lee of the Korean Meteorological Administration
in Seoul, South Korea, constructed a three-layer hemispheric model. Using
climatological data gathered during the severe winter storms of 1982-83,
the researchers examined whether the mechanical effects of the mountains
interacting with the jet stream were sufficient to produce the storm tracks.
The paths simulated by the model captured many essential features of the
observed storm tracks. Mak and Lee reported their findings in a recent issue
of the Journal of the Atmospheric Sciences.
While mountain ranges may determine the general location of a storm track,
other factors must be considered in order to pinpoint the trajectory and
to predict yearly variations, Mak said. Such factors include moist dynamics,
land-sea thermal contrasts and changes in sea-surface temperature.
"By examining how storm tracks change under a variety of conditions,
we may better forecast the intensity and movement of a storm," Mak
said. "For example, if the storm track over the Midwest is shifted
farther north next winter, then we can expect colder weather with more severe
storms and much more snow."
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