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The joint NASA/NOAA Suomi National Polar-orbiting Partnership (Suomi NPP) satellite carries five sophisticated instruments including the Visible/Infrared Imager Radiometer Suite (VIIRS). VIIRS is an advanced scanning radiometer operating at 22 channels, ranging from the visible through the thermal infrared, and including a Day/Night Band (DNB) that is sensitive to extremely low levels of light. CIRA scientists are involved in several key aspects of the Suomi NPP mission, including calibration and validation of the VIIRS instrument and pioneering new scientific techniques such as using the low-light abilities of the DNB to take a new look at tropical storms using moonlight. Traditional sensors use emitted infrared energy for nighttime observation. Since infrared emission depends on the temperature of the emitting object, it is difficult to distinguish objects of similar temperatures such as low clouds and the ocean surface. The DNB of VIIRS is capable of distinguishing low cloud from surface signatures and can function in low light situations where visible sensors cannot.
In a groundbreaking study published in the Proceedings of the National Academy of Sciences (PNAS) by CIRA research scientist Steve Miller along with colleagues from NOAA, Northrop Grumman, and the U.S. Department of Defense, it was discovered that the DNB of VIIRS is able to detect clouds on dark nights that are illuminated by the very low light from air glow and starlight.
During the day, ultraviolet radiation from the sun bombards Earth's upper atmosphere and breaks apart gaseous molecules and atoms. During the night, these molecules and atoms recombine emitting faint visible light — called airglow — in the process. Most weather satellites aren't sensitive enough to see city lights. Reflected moonlight is much fainter at nearly a million times fainter than sunlight. And air glow/starlight sources are 100-1000 times fainter still.
While working on noise reduction for the DNB data, scientists found that the instrument was sensitive enough to see clouds and other objects in what would appear to the human eye as complete darkness. No one had expected to see clouds on moonless nights. This new capability to sense clouds at night could significantly advance weather and climate observations for meteorologists and researchers, including improving our views of low clouds and features such as sea ice on dark nights.