HOUSTON, June 9, 2003 – University of Houston scientists are testing the waters – literally – with a new salt-detection device specially designed to collect data from rain and water vapor in tropical cyclones, all in an effort to better understand how tropical storms form and intensify into hurricanes.
The instrument may begin flying into storms on board a research plane as early as August, pending approval from the National Oceanographic and Atmospheric Administration.
James Lawrence, UH associate professor of geosciences, has studied hurricanes for years, from the ground and from the sky. He says one of the key elements powering a hurricane is how heat is transferred from the ocean's surface into the air, and if his detector is worth its salt, it should shed some light on the engine driving hurricanes.
"How heat is transferred from the ocean's surface into the air is a fundamental question, and understanding the mechanisms of heat transfer will help us make better models of hurricane formation, including models of how they grow in intensity," he says.
Lawrence says studies such as his are key to understanding the dangers of hurricanes. "Intensification of a hurricane is a very important issue, and sudden intensification is very dangerous. Unfortunately, this is what forecasters are least able to predict, along with how much rainfall will be produced by a storm."
One of the ways heat can be transferred from the ocean is by the wind churning up the sea and throwing droplets of sea spray up from the surface.
"Theorists think that there is a significant change in heat transfer as the winds pick up in the interior of the hurricane because you get a lot of sea spray," Lawrence says. His results from previous data gathering and research suggest that under the eye wall, which surrounds the center of a hurricane, a significant amount of sea spray is being uplifted. Perhaps as much as 30 percent of the total water and water vapor in the eye wall is being directly ripped off the sea surface, but Lawrence says he needed a new method to prove that hypothesis.
"The suggestion is there, but it's not easy to prove. For salt, there's no question that that water is coming from sea spray," he says. "Those droplets of water get carried up in the eye wall and into rain bands, and they take salt with them. By measuring the salt content, or salinity, of the rain, we should then be able to infer what's happening at the sea surface in terms of sea spray and the transfer of heat from the surface."
When the first version of Lawrence's device flew on board a research aircraft in the late 1990s, he realized he hadn't taken into account environmental factors that could skew the experiment's results – in this case, a dirty plane. Using a rain collector on the fuselage over the wing of the plane, Lawrence found that as it flew into a hurricane, the first rain collected was very salty. As it flew farther through the storm, the salinity decreased.
"We discovered that we were basically washing the airplane," Lawrence says. The plane had been sitting on the ground in Tampa, not far from the Gulf, so it was covered with salt. "As we started to go through the hurricane, the salt began to wash off the plane and into our collector, giving us those huge numbers."
To design a new salt detector, Lawrence worked with Hans Hofmeister, director of instrumentation for the UH Department of Chemistry. Hofmeister built a novel device that can be mounted on the very front, or nose, of the plane, where it should avoid detecting water that has splashed off the plane.
"That's our hypothesis, and we hope we can achieve that. Hans has come up with a very clever way of doing this so that we can get continuous, very sensitive, and real-time data over a whole possible range of salt content."
The UH device has already passed a few tests on the ground – first strapped to the front a Lawrence's pick-up truck while they drove on the freeway, then in a wind tunnel at Texas A&M University.
Lawrence, who has been funded by NASA, is currently awaiting final approval to fly the newly-designed instrument on board a hurricane research plane beginning in August. Scientists from the hurricane research division of NOAA determine what flies on the plane, which is operated by NOAA's Aircraft Operations Center in Tampa, Fla.
The UH detector is a small cylinder that is flat on the forward end and shaped to attach to the aircraft on the rearward end. Mounted on the front end is the heart of the device, an array of tiny wires lined up parallel with one another. The wires are connected to sophisticated controls and a voltmeter via cables that run through an opening at the rear of the cylinder. Once the device is mounted to the plane, those cables run inside the craft to a control console.
Typically, to detect salinity of water, researchers dip two electrodes into a container of solution and measure the amount of current that flows between the wires. But if you want to measure salt levels instantaneously, you can't wait for rain water to collect in a jar as a plane flies through a storm, Hofmeister says.
In his design, the wires are exposed to the air as the plane passes through rain and clouds.
"It's the same principle as when you're driving through a rainstorm and water piles up on your windshield," Hofmeister says. "Rain piles up on this slate of wires as it flies through a hurricane. The more salt that's in the water, the easier it is for current to flow between the wires, so the current is higher." The salinity detector actually measures voltage, he notes – the higher the current, the lower the voltage reading.
Another challenge Hofmeister faced in designing the instrument was making it capable of withstanding a high-voltage shock and still function. As a plane flies through the air, static electricity is generated, which discharges in huge high-voltage sparks.
"This instrument is a very delicate circuit, so I built surge suppressors that would take all that static electricity and get rid of it in order to prevent damage to the circuit," Hofmeister says. In his lab, the voltmeter connected to the UH device measures to one millionth of a volt difference in voltage readings, but a shock of 15,000 volts to the salinity detector has no effect – the device still functions normally.
"Our experiments probably won't be used to predict anything about an individual hurricane and its potential strength, but the better our understanding of how hurricanes in general work, the more you may be able to predict where one will go or how powerful it may become," Lawrence says.
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