By Candace Stuart
Small Times Senior Writer
JUNE 12, 2001 — Meteorologists had been tracking the storm since Aug. 18, watching it grow from a tropical depression over the Bay of Campeche off Mexico into a hurricane that teetered toward the coast of Texas. Strapped in seats of a P-3 weather reconnaissance aircraft, hurricane trackers were getting a first-hand look as the plane plowed through the roiling clouds.
But back at National Hurricane Center in Miami, meteorologist James Franklin had the best view of all – better than the pilots in the cockpit, better than the satellites in space and the radars on land. He had precise data from small tech devices that had been dropped from the plane to relay temperature, pressure, humidity and wind speed readings before plunging into the sea.
While others thought they were seeing a Category Three hurricane that August day in 1999, Franklin observed something even worse. Near sea level, Hurricane Bret had picked up wind speed while pressure continued to drop. It had advanced to a Category Four, with winds whipping at 144 miles per hour. Earlier predictions put the coastal cities Corpus Christi and Brownsville in its path.
“Aircraft (using traditional observation techniques) didn’t indicate Category Four until 12 hours later,” Franklin said. “Ours gave us 12 hours advance warning.”
Increasingly accurate weather predictions and better emergency warning systems have reduced hurricane deaths in the United States from averages of nearly 9,000 in the 1900s to 100 or so in the 1990s, although many more people now live in coastal cities, according to the National Oceanic and Atmospheric Association’s National Weather Service (NWS).
Researchers such as Franklin and National Center for Atmospheric Research engineer Terrence Hock have made those systems even better by designing tools using MEMS and microsystems. The two helped create the latest generation of dropsondes that provide real-time measurements of a storm’s vital signs. On May 21, Franklin received the National Weather Service’s highest honor, the Isaac M. Cline Award, for his part in the project.
A typical hurricane season has eight to 11 tropical storms, with about half at the severity of Allison, the first storm of the 2001 season. Allison dropped 32 inches of rain on parts of Houston this past week, causing excessive flooding, at least 18 weather-related deaths and losses estimated as high as $1 billion.
Normally, five to seven tropical storms develop into hurricanes, with two or three of those reaching Category Three classification. A tropical storm has sustained winds of 39 to 73 miles per hour; Category Three winds exceed 110 mph and Category Four start at 131 mph.
While weather-related deaths have plummeted, damage has risen steeply each decade as more businesses and homeowners build on and near shorelines. Property loss in the last decade topped $27 billion, more than $10 billion more than the previous decade, according to the NWS.
Weather researchers have used dropsondes since the 1970s to study climate conditions over oceans, where severe weather such as hurricanes develops. The cylindrical dropsondes are attached to parachutes and contain a variety of instruments that measure and transmit data on temperature, pressure, humidity and wind speed.
Scientists used data from the first dropsondes to design computer models for forecasting hurricanes and other severe weather. But the devices had many limitations; for instance, one type worked fine when dropped in the calm eye of a hurricane but often failed in the turbulent eyewall. None provided precise wind measurements.
In the mid-1990s, Hock and Franklin teamed up to make a much more reliable dropsonde, incorporating Global Positioning System (GPS) technology and improved sensors for readings that are fed into the forecasting models. The upgrade improved hurricane forecasting by 30 percent, according to one study.
The dropsondes contain three MEMS sensors, each as small as 100 microns, all housed together in a single module. They are made by the Finnish company Vaisala Inc., the leading supplier of sensors in radiosondes used in weather balloons.
Hock, who engineered the new dropsondes, chose the MEMS devices because they work quickly, have a long shelf life and are reliable and inexpensive. “We’re trying to use parts that are mass-produced so the cost is low,” he said.
Minuteness has its benefits, too. The surface area on a temperature sensor will absorb energy as it is exposed to the sun in higher altitudes. That skews its reading. But a MEMS temperature sensor has such a small surface area that the problem doesn’t occur.
But launching a sensor by balloon into the atmosphere is a lot different from dropping one into the eyewall of a hurricane, Hock said. Neither the temperature nor the pressure sensor survived a first trial. The first got crushed while the second split apart during the fall.
For the temperature sensor, Hock reverted to an older but sturdier model by Vaisala. He determined the pressure sensor was coming unglued, peeling from its substrate when ejected from a plane. Vaisala modified the design, which eliminated the problem.
The sensors take a reading about every five to seven meters during their fall, much more frequently than the 250-meter intervals of their predecessors, Hock said. A microprocessor made by Motorola Inc. controls the sensor module and sends the sensor data through a transmitter to the aircraft.
The GPS receiver placed in the dropsonde helps researchers accurately measure wind speed. To keep costs down, they use an eight-channel “codeless” receiver, which doesn’t compute distances or velocities. Instead it uses GPS satellite signals to track relative Doppler frequencies, which are based on the combined motion of the dropsonde and satellites. The frequencies are converted into digital signals, sent to the aircraft and processed to determine wind speed.
The sensor and GPS information reaches the airplane almost instantly and can be processed and transmitted to the National Hurricane Center in Miami in 20 minutes. The hurricane center feeds that information into its computer models to forecast the storm’s path and issue warnings.
The center remains on a 24-hour-a-day tropical storm watch from May 15 through Nov. 30. Typically, reconnaissance planes will drop one or two sondes a day into storms brewing in the Atlantic, Pacific and Gulf of Mexico. If a tropical storm begins to threaten land, they might increase that to 50 dropsondes for precise predictions.
“This technology allows us to make measurements that we never could before,” Franklin said. “We can drop it in the roughest part of a hurricane and every five meters get detailed measurements all the way down to the surface.”
It is those surface speeds that are of most interest to him when a storm approaches land, Franklin said. A hurricane’s winds are fiercest near the surface, he said, and “everyone lives at the surface.”
In the case of Bret, everyone turned out to be no one. On the morning of Aug. 22, Bret began threatening Brownsville, a city of about 100,000, with winds still raging at 144 mph. Residents already had been warned it was a Category Four, which meant it potentially could gust up to 155 mph and cause extensive damage.
By 6 p.m., Bret began to weaken, veering northwest. When it nicked Padre Island around midnight, its winds had slowed to 110 mph. On the 23rd, it made landfall in Kennedy County in south Texas, where it dropped 30 inches of rain – mostly on the cattle that far outnumbered the county’s few residents.
Brownsville and Corpus Christi were spared. The hurricane caused extensive beach erosion but claimed no lives.
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CONTACT THE AUTHOR:
Candace Stuart at [email protected] or call 734-994-1106, ext. 233.