Like waves that ripple across a pond’s surface from a tossed pebble, gravity waves ripple toward space from disturbances in the Earth’s lower atmosphere.
Gravity waves are created when air masses are moved up or down, by disturbances such as a thunderstorm, or when wind is forced up and over a mountain range. But in the lower atmosphere, their impacts usually remain local.
By the time they reach the upper atmosphere, however, the waves have built up in amplitude and extent. There, they can rule over atmospheric processes on a much greater scale, sometimes threatening reliability of Earth-based communication systems.
Now scientists have found a way, for the first time, to observe the propagation of gravity waves toward space.
What’s Seen is Fascinating
Here’s the trick, according to a team of researchers led by National Center for Atmospheric Research Senior Scientist Hanli Liu:
Push the NCAR-based Whole Atmosphere Community Climate Model to a resolution that is fine enough to pick up gravity waves at their source, when they’re still relatively small.
Liu, who studies the upper atmosphere at NCAR’s High Altitude Observatory, said:
“We’ve never seen a global picture of gravity waves in the upper atmosphere before, either from observations or simulations, even though we have suspected their importance up there. This is the first time we have been able to capture these waves with a computer model of the whole atmosphere.”
The model’s standard version yeilds only a blurry look at phenomena that take place on scales less than 1,250 miles across, and it’s blind to anything smaller than 200 km.
The higher-resolution model has much crisper vision all the way down to 125 miles. The intense computing power of the NCA -Wyoming Supercomputing Center’s Yellowstone system made the higher-resolution runs possible.
Liu and his colleagues report the finer-scaled model’s abilities by showing how gravity waves such as those created by a tropical cyclone east of Australia grew as they traveled upwards. The model shows that what starts out as a localized phenomenon extends across the entire Pacific Region at 100 km above Earth’s surface.
“For the middle and lower atmosphere, if you miss the gravity wave, you’re not missing too much,” Liu said. “But it’s a different story in the upper atmosphere.”
Disturbances in the upper atmosphere can endanger satellites, distort GPS readings, and shut down radio transmissions.
They are often thought of as originating from the Sun, where solar storms can kick off geomagnetic storms around Earth. But the ionosphere, the upper reaches of the atmosphere affected by this kind of space weather, is also influenced by disturbances originating on Earth.
“When gravity waves propagate to the bottom side of the ionosphere, they can kick off instabilities,” Liu said. “If you want to have a better understanding of space weather—the ionosphere—you need this kind of modeling capability.”
Liu, H.-L., J. M. McInerney, S. Santos, P. H. Lauritzen, M. A. Taylor, and N. M. Pedatella (2014)
Gravity waves simulated by high-resolution Whole Atmosphere Community Climate Model
Geophys. Res. Lett., 41, 9106–9112, doi:10.1002/2014GL062468.
Illustration: A model simulation illustrates how gravity waves kicked off by a cyclone east of Australia build as they travel toward space. The simulation was created using a high-resolution version of the Whole Atmosphere Community Climate Model (WACCM). Clockwise from upper left corner: Vertical winds at 11, 30, 87, and 100 km above Earth’s surface. Credit: Hanli Liu, NCAR/em>