"We realized that we had stumbled upon a special kind of new measurement, with an ability to reveal a part of the atmospheric circulation at an unprecedented degree of horizontal detail," Steve Miller of Colorado State University, US, told environmentalresearchweb. "The DNB observations reveal a complex array of gravity waves in the upper atmosphere that have never before been observed globally at this spatial detail. All of this information comes as a windfall of the DNB sensor’s extreme sensitivity to visible and near-infrared light."

When assessing the performance of the new DNB sensor, Miller and colleagues noticed clouds in images taken on nights without moonlight. They realized that the instrument could detect night glow emissions, which in this case were reflecting off the clouds. Night glow, also known as airglow, is due to emissions of light in the upper amosphere from atomic oxygen, sodium and hydroxyl radicals. The emissions are strongest at about 85–95 km, around the mesopause.

"We also began to notice general brightening over widespread areas and surmised that this was the direct upward emission of the night glow itself, in areas where its intensity was strong enough to see directly," said Miller. "Zooming in to the fine scale, we noticed wave structures in these direct emissions that did not match up to anything seen in the infrared imagery that is used to detect clouds at night."

The DNB is part of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the US NOAA/NASA Sumoi National Polar-orbiting Partnership environmental satellite launched in October 2011. The sensor, which detects wavelengths of 505–890 nm, sits 834 km above the Earth so that it takes a "snap-shot of the current atmospheric state as it races by at about 7 km/s". Although the sensor is optimized to image the nocturnal surface and lower atmosphere at extremely low levels of light, the team discovered it can pick up night-glow emissions on dark nights. So when gravity waves near the mesopause disturb these emissions, the DNB can image them with a horizontal resolution of around 0.74 km.

Gravity waves are disturbances to the density of the atmosphere that gravity and buoyancy act to restore; they’re the main form of energy exchange between the lower and upper atmosphere. It can take around an hour for gravity waves formed in the lower atmosphere to propagate up to the mesopause. Once there, the waves alter the local temperature and density, modulating the intensity of nightglow emissions and creating rippling patterns of visible light. The waves may appear as alternating bright and dark bands or as complex patterns.

"The sensor was not optimized for night glow observations, and there are a number of caveats to working with the data, but we will try to make the most of the information available," said Miller. "There is considerable room for ongoing discovery as the observations are so very new. There are many wave structures that we have difficulty attributing or explaining. We anticipate that we have not observed all the possible forms of gravity waves."

The team imaged gravity waves resulting from a number of phenomena, including mountains, hurricanes, thunderstorms, tropical cyclones, the jet streams of intensifying cold fronts, mesospheric bores and, in the first known spaceborne measurement of its kind, a volcano. Chile’s Calbuco volcano erupted in April 2015, sending a plume of ash into the stratosphere and making a concentric-ring gravity wave pattern in the nightglow above.

"The volcanic eruption example points to other seismic-related airglow signals that we have yet to observe from the DNB," said Miller. "Given the implications of such a signal – e.g. a large earthquake, which may produce an observable 'bright sky’ airglow response and/or gravity wave train coupled to a tsunami front – we are okay with waiting."

Because they drive circulation patterns at high altitudes, atmospheric gravity waves also tie back to the weather and climate that we experience near the surface, Miller says. "It is clear that in order to improve predictions of climate and climate change we must understand the fully coupled, three-dimensional atmosphere," he added. "Right now, we have a relatively good handle on the lower atmosphere via measurements taken from surface-based sensors and from satellites, but the information becomes far more sparse as one goes up to higher altitudes. [In] the upper atmosphere, where these gravity waves are seen by the DNB, the information is extremely sparse indeed." So the DNB observations could help researchers understand the structural details of gravity waves produced globally by a variety of mechanisms.

"As a satellite-based sensor, the DNB provides global coverage and hence an ability to capture wave activity over remote regions and above clouds that would obscure surface-based viewing," said Miller. "Even among the existing and legacy satellite observations capable of observing night glow, these DNB measurements are unique. Most conventional observations examine airglow by scanning the limb of the Earth…this provides a strong signal but comes at the expense of poor horizontal spatial resolution, so they cannot observe the detailed wave structures atop thunderstorms, for example."

The IMAP (Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere) VISI (Visible-light and Infrared spectrum Imager) sensor on board the International Space Station is the only other kit that can provide horizontal details of upper atmosphere gravity waves. But its resolution is 10 km, an order of magnitude coarser than the DNB’s.

Since the DNB is slated to fly on NOAA’s Joint Polar Satellite System (JPSS) satellites well into the next decade, it could prove useful to researchers analyzing decadal-scale trends in wave activity as they attempt to improve model parameterizations, Miller says. And by combining DNB observations with measurements of gravity waves in the middle atmosphere from the Advanced Infrared Sounder on NASA’s Aqua satellite or the Cross-track Infrared Sounder (CrIS) on the same JPSS satellites, researchers could study the 3-D structure of the waves.

The researchers would like to develop quantitative ways to extract as much information as possible from the "challenging" DNB imagery. They also hope to collaborate with the IMAP/VISI research team and groups operating surface-based night glow sensors to compare information content at different spatial resolutions. Multi-sensor studies could also help understand the vertical structure of the waves throughout the middle/upper atmosphere. Simulating observed wave structures via numerical modeling could also be of benefit. And adding the technology to a geostationary satellite would allow monitoring of waves as they move, providing information on their propagation.

Miller and colleagues reported their work in PNAS.

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