New Satellite Sensor Sees Earth at Night Almost as If It Were Day

No one likes to be “in the dark” about what is going on around them, especially in times of peril. Yet when night overtakes a continent or ocean, scientists and forecasters suddenly lose important satellite imagery in the visible-light range—information that can reveal swirling storms, the choking smoke of wildfires, massive chunks of sea ice that threaten ships, and much more.

A new sensor called the Day Night Band is beginning to fill that void. Part of the Visible Infrared Imaging Radiometer Suite flying on the Suomi-NPP satellite, the instrument is so sensitive that it can measure the glow of a single streetlamp, the deck light of a lone boat in the middle of a pitch-black Atlantic Ocean or a flickering gas flare in the vast North Dakota oil fields. Even on a moonless night, the sensor can discern clouds and snowfields, illuminated by the atmosphere's own faint, nocturnal glow.

In the past three years researchers who work with the sensor have seen fascinating features of Earth's forces, including great waves of energy launched high into the atmosphere by violent thunderstorms. And they have improved forecasters' ability to warn residents about the path of hurricanes, helped firefighters monitor shifting plumes of deadly smoke and directed lost ships away from moving sea-ice flows [see images below].

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The Day Night Band, very useful on its own, also complements infrared sensors, which have trouble identifying low clouds and snow cover that tend to blend into their surroundings at night. What is more, scientists are beginning to merge the Day Night Band data with software to specify how much moonlight is present on a given evening, helping them determine a cloud's reflectance and therefore how much moisture it holds. Forecasters can use this information to predict how clouds will affect nighttime temperatures on the ground and to assist pilots in avoiding hazardous icing conditions on aircraft. The data can also improve day-to-day weather forecasts for communities in high latitudes that endure perpetual darkness for months on end without critical nighttime information about the changing weather mix around them.

Only one Day Night Band sensor is aloft today. Suomi-NPP, operated by the National Oceanic and Atmospheric Administration, flies in an orbit 500 miles high, synchronized to the sun, so it only passes over any given location at a local time of about 1:30 P.M. and again at about 1:30 A.M. If such sensors were included on satellites that hover in geostationary orbit, scientists could record continuous movies of the world's lights instead of snapshots.

One possible platform for this night vision is a future series of Geostationary Operational Environmental Satellites (GOES) now being planned by NOAA for launch in the 2030s. If they carried a sensor like the Day Night Band, researchers could determine the changing character of lights on land and sea and monitor clouds, rain, oil slicks, fires, smoke, dust storms, volcanoes and ice all night long. They could also track boats fishing illegally in restricted waters and help to locate downed aircraft such as those in the recent Malaysia Airlines, Air Algérie and AirAsia disasters.  

Hurricane hunter

Knowing the exact position of a hurricane's eye is crucial because the most intense winds and highest storm surges occur right around it. More accurate tracking can save lives, influence decisions by local emergency managers about evacuations, and spare millions of dollars by optimizing the deployment of safety personnel and disaster relief resources.

On July 28, 2013, Hurricane Flossie was bearing down on the Big Island of Hawaii. Weather forecasters were charting its movement closely, but as night fell, they lost sight of the eye. High cirrus clouds obscured the storm's lower-level center of circulation from infrared sensors on satellites above (black-and-white image). As night wore on, the forecasters became increasingly uncomfortable about a possible “sunrise surprise”—when they think they are tracking a hurricane's nocturnal path correctly, only to realize at sunrise that its center is displaced from the deepest clouds because of changes in upper-level winds that sheer and steer such storms.

Luckily, the satellite carrying the new Day Night Band sensor flew overhead in the predawn hours. It peered right through the high, thin clouds (blue in color image) and showed the hurricane's near-surface circulation (yellow). The imagery revealed that the center of the storm was farther north than expected (northwestern shift in blue circle), posing less of a threat to the island. Forecasters at the National Weather Service office in Honolulu quickly released a 5:00 A.M. forecast notifying emergency personnel of the revised storm path, preventing unnecessary evacuation and saving thousands of dollars in the process.

Rescue at sea

The fishing vessel Kiska Sea is an American member of the Bering Sea crabbing fleet, which was followed in the hit television series Deadliest Catch. In February 2014 strong northerly winds descended on the central Bering Sea, rapidly pushing free-floating sea ice into a region where the fleet had deployed traps called pots. On February 10 the crew on the Kiska Sea, the northernmost vessel at the time, contacted the National Weather Service Sea Ice Program in Anchorage, Alaska, to ask about the status of ice near its string of 150 pots, each as big as a queen-size bed.

The weather service confirmed that ice was encroaching. The Kiska Sea headed in to retrieve the pots, maintaining contact with ice program personnel. But on February 13 the vessel found itself surrounded by ice, some of it greater than three feet thick. To avoid being capsized or crushed, the Kiska Sea had to quickly get out, but the short day and moonless night made navigation treacherous. The ice program used Day Night Band data to find the ship's lights, accurately pinpointing its location (white dot in center of image at left; other ships are visible in lower right corner). The sensor also outlined the current ice edge, illuminated by the atmosphere's faint “nightglow” (jagged lines moving down from top right).

With this fine-tuned information, the weather service personnel helped the ship's captain chart a safe path to the west-southwest, out of the shifting ice pack. Earth's atmospheric “night-light” helped to guide the Kiska Sea to safety.

Peering through the smoke screen

Wildfires are on the rise in the western U.S., in part because of multiyear droughts. Firefighters who battle these violent conflagrations during the day often lose ground overnight, when it is difficult to track dangerous smoke and smoke-obscured fire lines. Wildfires can also create strong, shifting winds that modify the speed and direction of the blaze, suddenly putting firefighters in harm's way. The temperature of smoke rapidly cools to that of the surrounding atmosphere, making these plumes of small particles nearly invisible to infrared satellite sensors at night. The sensors also often miss small flare-ups along fire lines.

The inability to combat blazes at night is frustrating because cooler temperatures, higher humidity and lighter winds make the nocturnal hours ideal for gaining an advantage. Low-light sensors can help, as seen in images of the 2013 California Rim Fire (above). First, when moonlight is available, the sensors can often show smoke plumes clearly, providing reliable warning to firefighters (upper right-hand image; smoke is missing from lower left-hand image, which is infrared). Second, the sensors can more accurately pinpoint the fire lines, including any small flare-ups (greater detail in upper right-hand image). The plumes also contain valuable information about near-surface winds that are fanning flames. The right-hand image shows strong southerly winds carrying the smoke northward; firefighters would be well advised to attack the burn from its southern flanks.

Our atmospheric night-light

Even on a moonless night, far away from any lights, you can see a vague silhouette of your hand against the “black” sky. That is because complex chemical reactions in the upper atmosphere give off faint light. Astronauts on the International Space Station document this nightglow regularly, but its detailed structure has been elusive. Researchers working with the Day Night Band were astounded when they realized that features of nightglow seemed to be showing up in the data gathered one night near a thunderstorm. The imagery revealed characteristic ripples in the glow. Energy released within thunderstorms launches atmospheric waves that propagate upward. When these waves reach 55 to 60 miles up, near the top of the mesosphere, an hour or two later, they disturb the nightglow layer, creating glowing, concentric ripples. As on other occasions, the sensor captured this effect during a massive Texas thunderstorm in 2014 (right).

The waves are more than curiosities; they carry energy that drives the circulation of the upper atmosphere. The Day Night Band's ability to detect the waves and ripples is filling a gap in models of upper-atmosphere dynamics, helping researchers better predict weather and understand climate change. Surface-based observations have also linked nightglow waves to major earthquakes, including the one that generated the devastating 2011 tsunami near Tohoku, Japan. It appears that the seismic motions create upward-moving pressure waves in the atmosphere. It is possible that the Day Night Band could help scientists identify tsunamis as they cross an ocean basin by tracking the atmospheric waves riding above them.