September 9, 2002
There’s no Food and Drug Administration approval for satellite data, no consumer-style rating. So, how do the people who want to use data from a new instrument know they can trust it?
In the case of NASA’s newest weather instrument, the Atmospheric Infrared Sounder flying on the Aqua satellite, they’ll be relying on scientists and technicians from many different institutions working around the globe. From their various perches, among them a lighthouse in the Atlantic, an icebreaker in the Arctic, and a weather station in the Galapagos, they’re helping make sure that the sounder and its two sister instruments are doing what they are designed to do--provide a three-dimensional picture of Earth’s weather.
The Atmospheric Infrared Sounder together with the Advanced Microwave Sounding Unit and the Humidity Sounder for Brazil continuously measure global temperature, humidity and greenhouse gases. It would take hundreds of thousands of weather balloons daily to get the same information
“We say our data products represent the atmosphere,” says JPL meteorologist Dr. Eric Fetzer, who helps coordinate the instruments’ validation activities. “How do we prove that? We compare our measurements with those taken within the atmosphere. We’ve got twelve teams out in the field now using a variety of instruments to measure temperature, humidity, ozone, and radiation--the light, both visible and invisible, that Earth’s surface and atmosphere give off. Our initial validation efforts started in July and will run through November.”
Putting some light on the subject
Dr. Wallace McMillan, a physics professor at the University of Maryland Baltimore County, heads one of these validation teams. While the new sounding system peers down from Aqua’s orbit 705 kilometers (438 miles) above Earth, his instrument is looking up at the atmosphere from a lighthouse off Chesapeake Bay.
Built in 1965 by the U.S. Coast Guard in a style called “Texas platform,” the structure could double as an offshore oil platform. McMillan’s instrument, an interferometer, hangs from a steel I-beam 80 feet above the ocean. “We measure heat radiation that the atmosphere and the ocean give off,” he says. In addition, his group uses a lidar, a laser light that measures temperature and water vapor in the atmosphere, and launches weather balloons when Aqua passes overhead.
Though the lighthouse boasts seven bedrooms and satellite television (no premium channels), it’s not the most comfortable place to be. “It’s a very wet environment,” says McMillan. “There’s lots of rain. It’s cold in the winter and hot in the summer. We hit a high of 92 degrees a few weeks ago, and it was warmer than that in the living quarters. That does not make for happy scientists.” A helicopter flies in once a week bringing in fresh supplies and ferrying team members back and forth.
Dr. Peter Minnett, a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science, and his group are doing some of their validation work in more luxurious surroundings—onboard Royal Caribbean’s Explorer of the Seas. One of Minnett’s three interferometers looks at the ocean from a berth on the cruise ship’s sun deck. “From space, the Atmospheric Infrared Sounder sees radiation coming from both the atmosphere and the surface,” says Minnett. “By having a very precise reading of the radiation from the ocean, you can sort out just how much is coming from the atmosphere.”
While Minnett’s instrument has been onboard the Explorer of the Seas since November 2000, he’s never sailed on the ship. Technicians and post-graduate students take that duty. He is headed for colder climates with one of his other interferometers. “I’m going on the Arctic cruise next month on the Canadian ice breaker Pierre Raddison. We’ll be sailing along the Northwest Passage taking measurements in open water.” His third interferometer is scheduled to go to the western Mediterranean on another research vessel in late September.
On the graveyard shift
On the other side of the world in the Galapagos Islands, Dr. Holger Vömel, a research scientist at the University of Colorado, has been checking up on the Atmospheric Infrared Sounder by launching a kind of super-sized weather balloon when the satellite passed overhead at 1:30 a.m.
Standard weather balloons are about 1 meter (3 feet) in diameter and radiosondes attached to them measure temperature and humidity as they rise. Vömel's balloons, about 4 meters (12 feet) in diameter at the ground, carry additional instruments to measure water vapor and ozone. They can also travel much higher-- 30 to 35 kilometers (about 18 to 22 miles) rather than 12 kilometers (8 miles) for most radiosondes.
"The Galapagos are right at the equator in the Pacific Ocean," says Vömel, "and a great spot to watch interesting climate features such as El Niño." But being at the equator is no guarantee of heat. "They have a strong annual cycle, now it is cold and cloudy there," he says. "Later in the year, it will be warmer and clear." He'll be going back next year to make the same measurement through cloud-free skies. In the meantime, he and his water vapor/ozone balloons are off to Oklahoma for several weeks, "I hope I don't have the night shift." Then, he is on to Hawaii.
Other validation team members will be working in Antarctica, South American, Europe and Australia.
"We’ve been looking at all the data as we go along, and so far, everything looks really good," says Fetzer.
Contact: JPL/Alan Buis (818) 354-0474