Hurricane prediction: catching the waves.

Hurricane prediction: Catching the waves

Hundreds of tropical weather disturbances form each year, but only about one in 10 gathers enough power to become a hurricane. New research supports the theory that a developing storm must encounter huge atmospheric pressure "waves" 7 to 9 miles above the sea to gain hurricane force.

"Ultimately, we expect that this could lead to a better forecasting of hurricane development and hurricane intensification," says atmospheric scientist Richard L. Pfeffer of Florida State University in Tallahassee.

The theory, which Pfeffer proposed in 1980, holds that well-organized, wave-like pressure disturbances in the upper troposphere help to spin rising air columns into hurricanes. He likens the action to stirring a cup of coffee, creating a depression in the center. The tropospheric waves whirl the air up and out of the top of a column, lowering air pressure at the sea surface. The low pressure draws more air and moisture into the system. Condensing water vapor releases heat, which causes air in the center to rise faster, fueling a hurricane.

The waves are only one ingredient in the hurricane recipe, but Pfeffer thinks they're essential. Other ingredients include warm seas and a site at least 5[degree] latitude from the equator, because the Earth's rotation helps spin up the storm.

In the early 1980s, tests using two-dimensional computer models supported Pfeffer's theory. But those models neglect many variations in wind, temperature and pressure that play an important part in Pfeffer's theory. Now Pfeffer and Malakondayya Challa have tested the theory with a three-dimensional computer model of hurricanes developed by Rangarao V. Madala and others at the Naval Research Laboratory in Washington, D.C.

The computer model averaged data collected from real hurricanes and storm systems in the Atlantic. Pfeffer and Challa found that cloud clusters and depressions required tropospheric waves in order to form into hurricanes in the model. When the researchers removed the effects of the waves, the weather system models didn't develop into hurricanes. Their findings will appear early next year in the JOURNAL OF THE ATMOSPHERIC SCIENCES. Next, Pfeffer hopes to test his theory against specific data collected from individual storms.

Atmospheric scientist John Molinari at the State University of New York at Albany has already applied Pfeffer's theory to satellite data collected as Hurricane Elena churned the Gulf of Mexico in 1985. In the April 15 JOURNAL OF THE ATMOSPHERIC SCIENCES, Molinari reports the hurricane indeed encountered a trough of low pressure in the upper troposphere 30 hours before it intensified. But while he thinks such troughs do boost hurricane intensity, he remains unconvinced that they are essential for hurricane genesis. Using Pfeffer's model to predict whether a storm will become a hurricane may be difficult, he says, because the tropospheric waves are too high for tracking planes to measure and may not show up in satellite data.