When masses of air flow over massive mountains, invisible waves often roil high into the stratosphere, affecting weather and mixing the chemicals that contribute to ozone depletion. The waves also create turbulence that can be a danger to high-altitude research missions by NASA's lightweight ER-2 aircraft, as well as shuttle flights upon reentry.
In Friday's issue of the journal Science, researchers report for the first time a technique that allows them to see temperature signatures from these invisible mountain waves. The method, involving high-resolution, satellite-based measurement of adjacent pockets of the atmosphere, is expected to aid in spotting turbulence and, one day, improve weather forecasts.
Hanging 10, in 3-D
Much like water waves in the ocean, airborne mountain waves rush, break and tumble. But in the air, which is virtually unconfined compared to a body of water, waves propagate in three dimensions, said Stephen Eckermann of the Naval Research Laboratory in Washington, DC.
"Imagine a rock protruding out of water," explained Eckermann, lead researcher on the study. "You'll get a wave or ripple pattern as water passes over and around the rock. Air over a mountain acts very much the same way."
But a typical wave approaching the beach is pretty much limited to two-dimensional movement. Even a ship's waves, which move outward in many directions, travel only along the surface -- at least for the most part. A submarine, on the other hand, sends out three-dimensional waves that are more closely resemble the airborne mountain waves.
Picture the ripples radiating outward from a stone thrown into a pond, Eckermann suggests, and then attempt to fathom distorting all this motion into a cone that also extends upward from the pond. Well, okay, so it's not so simple. And that, Eckermann says, is why it has taken until now to measure the high-altitude mountain waves.
"To model it three-dimensionally is a complicated problem," he said, adding that it's "an important problem" because of the effects on weather, turbulence and ozone depletion.
Seeing the wind from space
The key to modeling waves in the stratosphere -- which extends upward from about 7 miles to around 30 miles -- was to view them from above. Because the waves move quickly -- changing structure significantly over periods as short as 5 minutes -- a method of quick measurement was needed.
So in 1994, a shuttle temporarily deployed a satellite named CRISTA 1, which produced infrared images of the atmosphere. It measured minute temperature differences between relatively small, adjacent patches of air. Eckermann explained how this, ultimately, revealed the waves:
"The wave is basically moving air particles from side-to-side and up-and-down as the wave propagates through. As it moves air up, the air cools, and as it moves it down, it warms." The wave, moving through, leaves a signature of change, a "temperature wave." In a region of the atmosphere where temperatures range from -80 to -20 degrees Celsius, the satellite picked out temperature differences as little as 2 to 10 degrees.
Eckermann and his colleague, Peter Preusse of the University of Wuppertal in Germany, found that mountain waves come in a variety of forms, and not all of the waves make it to the stratosphere.
"You have very long, slow waves and very short, fast waves, depending on the topography of the mountain underneath," Eckermann said. While some of the faster waves can reach the stratosphere in an hour or less, others lumber up there over several days.
Knowing how the waves mix chemicals in the atmosphere could provide clues to ozone creation and depletion, Eckermann says. Regionally, the new model can help predict how the waves, typically 100 to 200 kilometers across, might affect weather.
Over the Andes in South America, for example, waves routinely reach the stratosphere, having a significant effect on weather along the way. But in western North America, the jet stream inhibits their travels aloft, and rarely do the waves make it to the outer reaches of our atmosphere. In the tropics, where there is less land and little topography, standard convection and thunderstorm activity have a far greater impact on weather.
Because mountain waves can change so quickly, and because computer models used to forecast weather always have a finite capacity for data, most models use estimates of the phenomena based on long-term characteristics. Feeding in better data, based on real-time measurements, ought to improve forecasting.
"This is why we think space-based measurement of mountain waves will be important," Eckermann said.
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