Deciphering tropical cyclones: it's hurricane season, and scientists continue to search for the cold, hard facts about these fearsome storms

0 Comments | Insight on the News, July 1, 2002 | by Lidia Wasowicz

Microscopic ice particles can determine whether a storm unleashes hurricane-force fury or peters out meekly, say researchers who hope to turn weather forecasting into a science. The finding adds a critical piece to the most detailed picture of a hurricane ever assembled, pinpointing key factors that turn turbulence into tempest.

The number and distribution of ice particles within the tropical cyclone's imposing framework, known to exceed 65,000 feet in height, can signal its course, the investigators have found. If large numbers of small ice crystals are concentrated toward the hurricane's top, they will reflect more sunlight, cooling the storm's upper levels while its bottom remains relatively warm. This contrast of air temperatures between extremities might contribute to the storm's strength, just as a cold front moving into warm, moist air triggers thunderstorms.

"It turns out the temperature in the upper levels of the 'eye' of the hurricane is most critical to how low the surface pressure gets; the lower the pressure, the stronger the hurricane," says Jeff Halverson of NASA's Goddard Space Flight Center in Greenbelt, Md.

"The bottom line here is that by identifying where larger ice particles are in a hurricane, we are adding to the data we put in computer models simulating the storms, and that will improve predictions on whether a hurricane is strengthening or weakening" adds lead researcher Andrew Heymsfield of the National Center for Atmospheric Research (NCAR) in Boulder, Colo. He expects refined versions of the models to be in place within a year.

Infamous for their unpredictable, often deadly behavior, hurricanes--the name derives from the Caribbean god of evil Hurican--can bring roiling seas, roaring winds and relentless rains. By definition, a hurricane is a tropical cyclone with a warm core or eye and maximum sustained winds of at least 74 mph, occurring over the North Atlantic. The same phenomenon over the Pacific is called a typhoon.

Each year, an average of 10 tropical storms develop over the Atlantic Ocean, Caribbean Sea and Gulf of Mexico. Six of these, on average, become hurricanes. Many stay at sea, but those that touch land can wreak havoc with property and lives. On U.S. soil, hurricanes kill 50 to 100 people in an average three-year period. The deadliest of all hurricanes swept through Galveston, Texas, on Sept. 8,1900, claiming some 6,000 to 8,000 lives.

The end of last year marked the first time in 20 years in the United States that two hurricane seasons passed without a land strike. Nevertheless, the 2001 season saw an above-average 15 storms intense enough to warrant a name, nine classified as hurricanes and six as tropical storms.

The hurricane investigators base their conclusions on data gathered by instruments aboard three aircraft that flew below, through and above Hurricane Humberto during three days of the storm's semi-circular romp across the Atlantic west of Bermuda during five days in September 2001. (The violent, 105 mph winds never touched land, and caused no loss in life or property.) Bombarded with parachuting instruments, Humberto became the most intensely studied storm of all time. The research was part of the Convection and Moisture Experiment conducted by NASA, the National Oceanic and Atmospheric Administration and several universities.

The scientists were surprised at the large number of ice particles as big as 7 millimeters--one-third of an inch--across the upper reaches of the storm, some 39,000 feet aboveground. Since larger particles fall faster than smaller ones, they were not expected so high up, according to study coauthor Aaron Bansemer of NCAR. The Humberto data showed some of the larger particles were transported, or they may have started out as small particles that aggregated on their way up, a process thought reserved for warmer temperatures, usually at around 18,000 feet.

"The upper portion of a storm is cold, with ice particles so tiny you need a microscope to see them," says Halverson, whose own research last September took him inside Hurricane Erin, which streaked across the North Atlantic whipped by 120 mph winds. "We are looking at a storm from the microscopic perspective to the scale of the storm itself, from a micron--a millionth of a meter--to a couple of hundred kilometers, or tens of thousands of meters. This is a vast scale of science." With Erin, researchers got the first detailed look at the eye of a hurricane. "Humberto gave us the most complete all-around picture," Halverson says.

The latest research brings scientists to an important crossroads. "We're poised at last to answer some of the final outstanding questions of what makes a hurricane tick" Halverson says. "Just as detectives use fingerprints to conduct a postmortem on a body, we can use ice particles to conduct a postmortem on a hurricane. With our coasts built up and populated to capacity, it has been sheer luck that we've not had more disasters but, with hurricanes, as with earthquakes, the big one is coming, and we want to be prepared when it does."