Forecasters buoyed as new satellite breezes past old technology


Monday, July 24th 2000, 12:00 am
By: News On 6


The water darkens. Tiny waves tiptoe across the surface. The mainsail stiffens. To sailors, signs of an approaching storm. But to scientists, the call of drifting icebergs and vanishing forests.

Thanks to a new piece of NASA technology, scientists can now monitor winds over most of the world's oceans. The satellite-borne radar, called SeaWinds, has already improved weather forecasts and helped scientists better understand hurricanes. Now SeaWinds is proving useful for studying changes on land and over ice as well.

SeaWinds, launched in June 1999 on NASA's QuikSCAT satellite, is a specialized radar called a scatterometer. It determines wind speed and direction over the oceans by measuring the roughness of the water's surface from its perch 500 miles above Earth. A transmitter on the radar sends down short pulses of microwave radiation and measures the amount that is bounced back. Over water, the amount that is bounced back depends on how fast the wind is blowing.

"The radiation is scattered off real short waves on the surface – the cat's paws you can see when a little gust of wind blows over a water surface and makes the water darker," says oceanographer Michael Freilich of Oregon State University, leader of the SeaWinds project.

Faster wind means rougher seas, which reflect more energy. By looking at the same spot from several angles, the radar can also tell which direction the waves are moving.

Every day, SeaWinds provides wind speed and direction over 90 percent of the ocean surface.

"This is a major improvement in coverage," says Dr. Freilich. NASA's previous scatterometer monitored just 70 percent of the ocean surface in a day. A similar European satellite has roughly 40 percent coverage.

SeaWinds is also highly detailed. For an area of ocean the size of Texas, the best that computer models can do is estimate winds at about 70 locations in a 24-hour period. SeaWinds makes 2,000 actual wind observations in the same time.

But coverage and detail don't mean anything unless you can see the surface. Over vast stretches of the ocean, it's cloudy almost all the time. Traditional weather satellites can't see through these clouds, but microwave radars can.

The idea behind scatterometry has been around for almost 35 years, but the first reliable data from this kind of instrument didn't appear until the early 1990s. Up to that point, the only sources of wind observations over the ocean were ships and weather buoys.

"Ship measurements are notoriously inaccurate," says Dr. Freilich. "You often find ships reporting a location in the middle of a continent because they get the latitude and longitude mixed up." As a result, forecasters don't put much faith in ship observations.

There are also large regions of the ocean – like the wind-whipped Southern Ocean surrounding Antarctica – where ships never venture.

"You've got to be crazy to take a ship down there," says Dr. Freilich.

These large data gaps pose a real challenge to weather forecasters trying to make predictions of weather over the ocean and along the coast.

"You can't predict the weather tomorrow if you don't know the weather today," says Dr. Freilich.

Filling in the ocean data gaps has been one of SeaWinds' greatest contributions, says Robert Atlas, head of the Data Assimilation Office at NASA's Goddard Space Flight Center in Greenbelt, Md.

Dr. Atlas has seen significant improvement in weather forecasts over the Atlantic Ocean since forecasters started receiving SeaWinds data a year ago. The data have been especially valuable in detecting and forecasting intense storms.

"The scatterometer really picks up where the high winds are. This is also where there are high waves," says Dr. Atlas. As a result, he says, forecasters have a better handle on the weather that ships should avoid.

Joseph Sienkiewicz, senior forecaster at the National Weather Service Marine Prediction Center, agrees: "There have been some west Pacific storms where SeaWinds has been very useful." SeaWinds also helped forecasters track Hurricane Cindy off the East Coast in 1999.

While studying climate is SeaWinds' primary mission, scientists have found all kinds of unanticipated uses for the radar's data.

"SeaWinds was originally intended for climate use," says Shuyi Chen, professor of atmospheric sciences at the University of Miami. "Later on, we realized the data could be used for other things like hurricanes."

Dr. Chen is using SeaWinds data to understand how hurricanes form, and to better predict where they will move.

"In the early stages of a hurricane, models have trouble locating where the circulation is," says Dr. Chen, referring to the spiraling winds at the center of the storm. "SeaWinds definitely gives you that." Dr. Chen is now using SeaWinds data to improve computer simulations of Hurricane Floyd, which ravaged the East Coast last summer.

While most SeaWinds scientists have focused on the 70 percent of the Earth covered by water, electrical engineer David Long of Brigham Young University wasn't content staring out to sea. From his landlocked office in Provo, Utah, Dr. Long developed a way to use the radar's data to study processes over land.

"Scatterometers measure the roughness of the ocean surface," says Dr. Long, "but vegetated areas are rough as well. Trees have all these leaves."

Dr. Long learned how to distinguish different types of vegetation in the SeaWinds data. This enabled him to spot highway construction and other development in the Amazon rain forest.

"I was surprised by the extent of the deforestation," says Dr. Long. "It has made quite a dent. ... Clearly, people are affecting the environment." Dr. Long also uses SeaWinds to map the melting of icecaps in Greenland and the Antarctic.

But perhaps Dr. Long's most intriguing use of SeaWinds came not as the result of scientific savvy, but serendipity. While making images of Antarctic ice, Dr. Long spotted a large iceberg drifting across the Drake Passage, a major shipping lane between the Antarctic Peninsula and the tip of South America.

"When I talked to the National Ice Center, they didn't know about it," says Dr. Long. The National Ice Center, in Washington, D.C., tracks and forecasts the movement of sea ice for the armed forces and other government agencies. At the time of Dr. Long's discovery, the ice center had been using visual satellite data to follow roaming icebergs. Scientists at the center lost track of Dr. Long's berg when it drifted under an extensive area of clouds. The center now uses SeaWinds to pinpoint iceberg locations with unprecedented accuracy.

The biggest challenge for SeaWinds, though, may not be finding new uses for its data, but keeping it alive. Putting the technology into action has been an exercise in frustration for NASA scientists.

The first experimental scatterometer was launched as part of NASA's Skylab mission in 1973. It worked great until an astronaut put his foot through it on a space walk less than a year later. A replacement was launched in 1978 but suffered "an untimely failure of its electrical system," Dr. Freilich recalls, after several months of high-quality data.

NASA tried for many years to fly another scatterometer and eventually got one up on a Japanese satellite, ADEOS, in 1996. Scientists said goodbye to that instrument a year later after the satellite lost power.

At 13 months, SeaWinds is the longest-lasting U.S. scatterometer in space.

"I'm probably going to regret saying this," says Dr. Freilich, "but there are no indications that there will be any problems." A replacement for SeaWinds is scheduled for launch in November 2001.