Adam Mann

PNAS September 24, 2019 116 (39) 19218-19221

Figure: Atmospheric gravity waves—seen here over the Indian Ocean via the Moderate Resolution Imaging Spectroradiometer housed in NASA’s Terra satellite—emerge when parcels of air are forced upward. Image credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC.

On September 3, 2018, an unpowered experimental sailplane made history by flying into the stratosphere. After leaving from El Calafate, a town near the Southern Patagonian Ice Field in Argentina, glider pilots Jim Payne and Tim Gardner surfed on enormous airborne waves emanating from the Andes Mountains. They achieved a world record height of 23,203 meters—higher than a Lockheed U-2 spy plane’s cruising altitude. No other unpowered aircraft has ever achieved such elevation. It was a feat made possible by not just human ingenuity but also the incredible strength of a phenomenon known as atmospheric gravity waves.

Unlike the similarly named (and perhaps more famous) cosmic gravitational waves, which are ripples in the fabric of space-time, atmospheric gravity waves are a wholly terrestrial occurrence. They emerge when parcels of air are forced upward, for instance, by a tall mountain range, moving from a dense atmospheric layer to a thinner one. The heavier blobs of air then succumb to the force of gravity and fall back down, resulting in a periodic oscillation that can carry energy and momentum over vast distances. They are the smallest atmospheric waves that researchers study, generally between a few hundred meters and a few hundred kilometers in size. But their cumulative effects can be dramatic.

The actions of atmospheric gravity waves bring energy from the troposphere—the lowest atmospheric layer, which ends at an altitude of about 10 kilometers—out past the edge of space, 500 or more kilometers above Earth’s surface. They play important roles in daily weather as well as long-term climate fluctuations, decelerating powerful jet streams and affecting the circulating polar vortexes that appear over our planet’s poles. They influence atmospheric turbulence, temperature, and chemistry, yet limited computing power continues to prevent their direct inclusion in most atmospheric simulations.

Although researchers believe they have a fairly good handle on the basics of atmospheric gravity waves, a more detailed and realistic look at the waves’ behavior would provide increased accuracy for simulations predicting both local weather and potential adverse effects from climate change. In recent years, new efforts have sprung up to provide better observations and modeling of these important players in our planet’s dynamic atmosphere.

“They seem small,” says atmospheric researcher Joan Alexander of NorthWest Research Associates in Boulder, CO. “But they’re affecting forecasting and predictions on many timescales, and if we don’t include them we get big biases in our models.”

See https://www.pnas.org/content/116/39/19218