Research News

Interaction of mountains and jet stream play major role in winter


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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