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Mountain waves and aviation safety

Lenticular clouds at sunset over the Front Range.

Lenticular, or lee wave, clouds indicate the presence of mountain wave activity. Lenticular clouds such as these are often seen along the Front Range of the Rocky Mountains. (Image ©UCAR.)

On December 20, 2008, a Boeing 737 with Continental Airlines encountered a crosswind gust during takeoff at Denver International Airport (DIA), causing it to veer off the runway. A post-crash fire ensued, along with 37 injuries and substantial aircraft damage.

Simulations from NCAR's Research Applications Laboratory indicate that a mountain lee wave amplified over DIA within minutes of the accident. It marks the first time that research models have simulated these observed wind gusts at the airport.

Mountain waves, a type of atmospheric gravity wave, are generated when air blowing across a mountain range is forced up and over the top. On the lee side, gravity pulls the air down and forms waves. Patches of turbulence associated with large amplitude waves can pose a hazard to aviation.

In support of a National Transportation Safety Board (NTSB) accident investigation, the NCAR team ran a high-resolution, numerical model (Clark-Hall) to simulate weather conditions at the time of the accident. Results from the simulation indicate mountain wave activity with a well-defined lee wave increasing significantly in amplitude shortly before the accident, creating strong gusts propagating across the runway of the accident.

“Instead of a downslope windstorm, we saw a mountain lee wave amplifying in a stable layer aloft above the airport,” says NCAR scientist Teddie Keller.

The team’s research could lead to changes in the weather information given to pilots, as analysis of wind data from the Low Level Windshear Alert System (LLWAS) revealed substantial variability in gustiness across the airport, indicating that data from more than a single sensor should be used for airport operations. The NTSB report recommends that the Federal Aviation Administration require air traffic controllers to provide pilots with additional LLWAS information that better captures localized crosswinds and gusts.