When a stable atmosphere is disturbed vertically, gravity tries to restore equilibrium, producing a wave, similar to what happens when a rock is dropped in water. So-called gravity waves play a number of important roles in the middle and upper atmosphere, where the wave amplitudes become increasingly large with decreasing air density. These roles include transferring energy and momentum from one atmospheric region to another, perturbing the densities of ions and neutral molecules, and inducing instability.
However, scientists still struggle to quantify how gravity waves are distributed and structured and how they change over time. They operate on such a large range of scales—from several kilometers to thousands of kilometers—that any one observational technique cannot completely capture them.
Liu et al. use a whole-atmosphere general circulation model that for the first time, resolves gravity waves on the mesoscale—down to the tens of kilometers level—from Earth’s surface to the lower thermosphere to try to better understand these waves’ structure, function, and impacts. This model, developed from the National Center for Atmospheric Research’s Whole Atmosphere Community Climate Model, simulates a number of gravity wave features, including their spectra, intensity, and distribution both horizontally and vertically. The authors also used the model to assess the resulting impacts on larger-scale Earth conditions such as temperatures, tides, and easterly and westerly zonal winds.
The resulting simulations suggest that the modeling generally produced realistic results. The spatial structure and magnitudes of gravity waves generally echoed those derived from temperature observations in the Sounding of the Atmosphere using Broadband Emission Radiometry satellite mission, the authors found.
Vertical winds, a proxy for gravity wave activity, grew stronger and more widespread at higher levels in the simulations, suggesting that the same holds true for gravity waves. Moreover, the model simulation also yielded daily and semidaily tides that agreed with observational data, an improvement from previous models. However, the model still could not resolve many mesoscale waves on the smallest scales (less than 100 kilometers) very well, which the authors suggest means their model can only partially capture the mesoscale waves in the middle and upper atmosphere. (Geophysical Research Letters, doi:10.1002/2014GL062468, 2014)
—Puneet Kollipara, Freelance Writer
Citation: Kollipara, P. (2015), Global atmospheric model simulates fine details of gravity waves, Eos, 96, doi:10.1029/2015EO029115. Published on 6 May 2015.