Gravity waves generated near Earth’s surface can propagate to very high altitudes in the atmosphere due to the pull of gravity, somewhat like waves on the ocean. These gravity waves transport energy to, and reach very large amplitudes in, the upper atmosphere. The large amplitudes lead to instabilities and turbulence that have important influences on the buffer of air around our planet. Fritts et al. have demonstrated a new way to view the effects of these gravity waves.
The authors employed images of noctilucent (“night shining”) clouds observed from the ground to reveal details of gravity wave effects. The noctilucent clouds, which are composed of tiny ice crystals and found at altitudes near 82 kilometers, can be seen in summer at high latitudes during twilight. The effects of gravity waves are apparent in the undulations of these clouds, and in the ways that those change over time.
The authors captured images of noctilucent clouds from Germany, looking 300-600 kilometers to the north over Norway on two nights in 2009. These were the highest resolution images of those clouds yet obtained, both in space and time. Thus the authors could see details of the clouds that were less than 100 meters across and could track their evolutions in great detail over many minutes. The first set of images revealed deep Kelvin-Helmholtz billows in the noctilucent clouds that were induced by gravity waves and changed quickly in time. The second set of images revealed billows that were shallower and evolved more slowly.
The analysis demonstrates that imaging noctilucent clouds can capture a range of motions, including those at very small scales. Further investigation of the subtleties of noctilucent cloud behavior could reveal more about how gravity waves affect the atmosphere as a whole. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2014JD021833, 2014)
—Shannon Palus, Freelance Writer
Citation: Palus, S. (2015), Clouds visible at twilight reveal mysteries of gravity waves, Eos, 96, doi:10.1029/2015EO023485. Published on 11 February 2015.