When the atmosphere is disturbed, gravity waves ripple through the air, similar to ripples on the surface of a lake. Unlike ripples, however, gravity waves can propagate to very high altitudes, where they interact and dissipate. Gravity waves can have very large effects on the “weather” at these altitudes, even changing the direction of the wind. Scientists have struggled to accurately model these waves, however, which adds to the challenge of predicting weather on Earth’s surface. To improve forecasts, modeling will need to explore the details of how gravity waves propagate, interact, and dissipate in the upper atmosphere.
In a new study, Fritts et al. modeled how gravity waves travel through the mesosphere—a layer of the atmosphere between 50 and 90 kilometers above Earth—and thermosphere, the layer extending from 90 kilometers to much higher altitudes. Gravity waves reach much larger amplitudes in the cold, thin air of this region than they do closer to Earth: as much as 10 times larger or more. The large amplitudes cause them to break, like ocean waves on a beach. This generates strong turbulence and deposits large amounts of energy and momentum, which strongly influence the local environment.
The team focused on mesospheric inversion layers (MILs)—layers of the atmosphere that can be hundreds or thousands of kilometers wide—in which temperatures rise, rather than fall, with increasing altitude. These inversions can arise when gravity waves and larger planetary waves mix layers of air, which influences the propagation of new gravity waves entering these layers.
The team simulated what happens when gravity waves encounter an MIL. They found that when large-amplitude waves encounter MILs, atmospheric instability and turbulence develop at lower altitudes than normal, an important general finding with regard to weather close to Earth’s surface. Smaller waves tended to pass through the inversions without losing fidelity or were merely partially reflected by it. Yet transmission also depended on the MIL itself: Waves could go farther through weak inversions, for example, whereas strong, deeper mesospheric inversions tended to pose a bigger obstacle.
The findings will help researchers more accurately model how energy travels through the whole atmosphere and will contribute to advances for predicting weather. Yet there is still a long way to go, the authors caution. One pressing challenge, for example, is to understand how gravity waves are likely to drive atmospheric dynamics as Earth’s climate changes. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1002/2017JD027440, 2018)
—Emily Underwood, Freelance Writer