Mountain Wind Flows Make Waves With Scientists

Like water waves breaking on a beach, atmospheric waves breaking high above mountains can pack a powerful punch.

On Dec. 9, 1992, their turbulence was so rough at 31,000 feet it tore an engine pod and part of a wing off a DC-8 cargo plane, which happily landed safely. Now analysis of data from a lidar (laser "radar") that was probing the atmosphere near the accident site near Boulder, Colo., gives new insight into this dangerous phenomenon.

Mountain-wave turbulence is a kind of internal friction. It is part of the system by which energy flows through the atmosphere and keeps the weather machine running. Understanding that process is basic to understanding how the atmosphere works. The new findings should also help forecasters sharpen their predictions to warn pilots when such dangerous turbulence is developing, according to F. Martin Ralph of the Environmental Technology Laboratory of the National Oceanic and Atmospheric Administration in Boulder.

Mountain-wave turbulence may have caused a plane crash in Colorado Springs in 1991. Alerted to this danger, the National Transportation Safety Board has asked scientists to probe the wind flow over the mountains. A separate study, going on currently, includes new measurements with the lidar. It should help modelers improve the accuracy of forecasting programs.

Dr. Ralph, his colleague Paul J. Neiman, and David Levinson at the University of Colorado in Boulder laid out the details of their new findings related to the Boulder accident in a recent issue of the journal Geophysical Research Letters. In a telephone interview, Ralph pointed out that this is part of ongoing research that is shedding light on why aircraft observations of mountain-wave behavior don't jibe with computer simulations.

Meteorologists have long known about mountain waves and where they fit into the general scheme of atmospheric circulation. They just don't know the details.

In a broad sense, you can think of the atmosphere as a kind of engine. It's powered by energy from the warm tropics and from Earth's surface in middle latitudes. It's cooled by losing heat in high latitudes and at high altitudes. If you turned off the energy supply, there's enough wind energy in the system to keep things going for about a week. Eventually, like a car without fuel, friction would bring everything to a halt.

With a car, much of the friction occurs where the rubber meets the road. Likewise, much of the atmosphere's friction occurs in the so-called boundary layer, where winds blow across the planet's surface. But a car also has some internal friction in its axles, gears, and other working parts. The atmosphere has internal friction too, and some of that involves the mountain waves.

Over the Rocky Mountains, for example, the terrain sets the generally west-east air flow to undulating up and down. This wave motion can be felt up into the stratosphere. At times, the waves grow so large they break like large waves at sea. The resulting turbulence draws energy out of the general wind flow, contributing to what meteorologists call mountain-wave drag.

Computer-based models of the atmosphere account for this. But they have been projecting an effect several times larger than research aircraft find. The modelers tell their programs to calm things down a bit in simulations, but it's been a minor hang-up in computer-based forecasting. Lower readings from the aircraft occur, Ralph says, because it takes them several hours to find out what's happening, while the waves are changing faster than that. Lidar, on the other hand, gives an almost instant picture.

Ralph says he expects that the detailed knowledge this research will give will let scientists make computer models more realistic. This should help improve computer-based weather forecasting in general and make flying over mountainous regions safer. That's an impressive return on an investment whose original aim was to get to know the atmosphere a little better.