BATON ROUGE, La. — How much storm surge will a hurricane drive ashore? How much rain will a hurricane that lands on the Gulf Coast bring to places as far inland as Kentucky or the Northeast? When will a monster hurricane suddenly dissipate from a record-maker to a dud?
The answers to many of these questions often are tied to knowing how much moisture swirls in the atmosphere around a hurricane churning across the ocean.
This hurricane season, for the first time, researchers with the National Oceanic and Atmospheric Administration say they will use Global Positioning System technology to measure the dynamics of airborne moisture far offshore in the Gulf of Mexico, and track the fuel available to ramp up tropical systems moving through the Gulf.
“GPS has enabled a revolution in the way we do a lot of things,” said Seth I. Gutman, a physical scientist and chief of NOAA’s GPS-Met Observing Systems Branch in Boulder, Colo. Better use of GPS to measure moisture for hurricane forecasting has been “a long time coming,” he said at a hurricane conference at Louisiana State University on Tuesday.
NOAA and LSU researchers collaborated to install GPS weather stations on two offshore oil platforms in the Gulf of Mexico this winter. The stations will give meteorologists the first ever over-the-Gulf moisture readings, Gutman said.
This is done by measuring the time it takes radio signals transmitted by satellites 22,000 miles away to reach the GPS stations in the Gulf. Moisture causes a radio signal to bend and slow down.
“It’s kind of like launching a weather balloon every second of the day,” said Roy Dokka, the executive director of LSU’s Center for GeoInformatics and a collaborator on the experiment.
For hurricane forecasters, how much moisture or dry air a hurricane will suck in is a life-or-death question.
“The energy contained in a hurricane is related to the amount of moisture in the storm. We call it moisture flux,” Gutman said. “It’s the exchange of moisture in the open ocean, and the exchange of moisture between the oceans and the atmosphere that determines whether or not there is the energy to sustain a storm, to intensify it, to reduce it.”
Throw dry air in the mix, and a monster hurricane can get knocked out. Forecasts on a storm’s intensity have been far off the mark in the past because of a lack of information about moisture, officials said.
A good example occurred in 2002.
On Oct. 2, 2002, Hurricane Lili was within hours of striking the central Louisiana coast as a Category 4 hurricane with 145 mph winds. At that strength, Lili had the potential to cause catastrophic damage. But within hours, Lili swallowed dry air and came ashore as a Category 1 storm on Oct. 3.
“The ingestion of dry air is one of the harder things we can understand,” said Barry Keim, Louisiana’s state climatologist. “Understanding what’s out in front of a hurricane, and ultimately what will be ingested in a system, will play a big role in determining its intensity and how much rainfall it will produce.”
Until now, weather observations in the open ocean primarily were done via buoys and information collected from hurricane hunters that fly into tropical systems and deploy weather instruments. Meanwhile, GPS stations have been limited to locations close to shore, Gutman said.
Ideally, Gutman said, GPS stations should be mounted on many more oil platforms and on ships to give future forecasters a broad picture of the atmosphere over the oceans at any given time.
The GPS stations in the Gulf cost about $7,000 each and were placed on the West Cameron 560 and the Eugene Island 337, platforms owned by Devon Energy. The stations involve GPS receivers, a frisbee-sized antenna and basic weather instruments to measure temperature and pressure.