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We’ve all seen the satellite pictures and maps of developing hurricanes, but is there a way to spot storms even sooner?
As this ScienCentral News video reports, a new eye in the sky can "see" the winds of a developing storm.
Earlier Warning
The satellite imagery that brings TV news viewers those menacing spirals of clouds during a hurricane warning are what meteorologists at NOAA’s National Hurricane Center (NHC) watch around the clock throughout hurricane season. The images from the satellites called GOES tell scientists whether there is enough "organized convection"—when the cloud spiral is a closed circle-- to "call" a tropical depression or storm.
But this season the NHC has a new tool that can detect potential hurricanes as much as four days before they get organized. A NASA satellite called QuikSCAT carries a device known as "SeaWinds" that can see through the clouds and detect the swirling of winds at the ocean’s surface.
SeaWinds measurements of wind speeds and direction are used to calculate vorticity, a measure of the telltale swirling. "It’s able to pick up the vorticity at the surface of the ocean very, very early, even when there’s no clouds above, which is the usual indication there’s a storm there," says Jim O’Brien, Director of Florida State University’s Center for Ocean-Atmosphere Prediction Studies (COAPS). COAPS scientists are also on NASA’s Science Team for SeaWinds.
At NASA’s request, after the satellite’s launch in 1999, the team used SeaWinds data from the 1999, 2000 and 2001 hurricane seasons to compare how well it "could have" detected storms that ultimately were detected using conventional methods. They report in the June Bulletin of the American Meteorological Society that SeaWinds could successfully spot developing tropical storms from several hours to as much as four days earlier. A recent study done by NOAA experts reported similar success.
Seeing Winds
Since its radar uses microwaves, SeaWinds is able to "see" through the clouds. Then an instrument called a scatterometer senses ripples on the ocean’s surface. The technique has its limitations, though. The SeaWinds scatterometer may work well for locating potential hurricanes, but it is not so useful once they’ve actually developed. "It can’t really measure winds much more than 60 miles an hour," says O’Brien. "And the other thing is where there’s lots of rain—for example when a hurricane gets really strong—we really can’t see it at all."
Another issue is "false alarms"—when it spots a potential hurricane that doesn’t ever turn into one. "But that’s OK," says O’Brien. "We’re getting all the ones that do pop up and become important for people."
Before this, the only methods for detecting winds far out in the Atlantic ocean were ships and buoys. "[SeaWinds] covers about 92 percent of the globe in one day, which compared to conventional observations is terrific," says Mark Bourassa, assistant professor COAPS and co-author of the NASA study. "We get about as many observations in one day as you get... from ships and buoys in a year."
And in the Pacific Ocean (where tropical storms are called typhoons), there are far fewer ships and buoys, making SeaWinds even more valuable to Pacific nations. "Other countries had virtually nothing else until QuikSCAT," says Peter Black, a scientist in NOAA’s Hurricane Research Division (HRD). Black points out that while SeaWinds has its limitations, spotting hurricanes wasn’t even its biggest objective. "Its number-one application is climate modeling. Its use in tropical storms is kind of a spin-off," he says. "In our view, a big spin-off."
With SeaWinds on QuikSCAT now an operational part of forecasters’ arsenal, they are eager to increase their coverage of ocean winds. "We only have one scatterometer—we really need two," says O’Brien. "A lot of times we don’t really see the actual formation of the beginning tropical depression because we’re just not hitting it."
A second SeaWinds instrument, called SeaWinds on ADEOS II is expected to launch sometime in 2003, O’Brien says.
"By combining the two satellites, we’ll have fantastic coverage over the ocean."
