Spotlight on Research: Hot Tower Discovery Can Show Hurricane Power

Posted: September 17, 2004 at 1:00 am, Last Updated: November 30, -0001 at 12:00 am

By Robin Herron

Two George Mason researchers in the School of Computational Sciences are watching with particular interest as storms develop this fall, having discovered that a “hot tower,” a rain cloud that rises unusually high, is associated with hurricane, typhoon, or tropical cyclone intensification.

In other words, if there’s a hot tower above the eye of a hurricane, watch out.

For example, a particularly tall hot tower rose above Hurricane Bonnie in 1998 as the storm intensified a few days before striking North Carolina. Bonnie caused more than $1 billion damage and three deaths.

Owen Kelley
Owen Kelley with a model of a “hot tower.”
Photo courtesy Owen Kelley

Owen Kelley, a PhD student in Computational Sciences and Informatics, and John Stout, research scientist at Mason’s Center for Earth Observing and Space Research, are also scientists at NASA’s Goddard Space Flight Center in Greenbelt, Md. They used data from the Tropical Rainfall Measuring Mission (TRMM) satellite for their research. Their findings, which they presented at the American Meteorological Society last January, caused a stir among scientists as well as in the media.

Kelley, who earned a master’s in applied physics from George Mason in 1997, says, “The motivation for this new research is that it is not enough to predict the birth of a tropical cyclone. We also want to improve our ability to predict the intensity of the storm and the damage it would cause if it struck the coast.”

John Stout
John Stout
Photo courtesy John Stout

Kelley and Stout define a “hot tower” as a rain cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere. It extends approximately nine miles (14.5 km) high in the tropics. These towers are called “hot” because they rise to such a high altitude due to the large amount of latent heat. Water vapor releases this latent heat as it condenses into liquid.

The researchers needed to compile a special kind of global statistic on the occurrence of hot towers inside tropical cyclones. The only possible data source was the TRMM satellite, a joint effort of NASA and the Japan Aerospace Exploration Agency (JAXA). “Many satellites can see the top of a hot tower, but what’s special about this satellite’s precipitation radar is that it gives you ‘x-ray vision’ so you can see inside a hot tower,” Kelley says. To compile global statistics, the radar needs to be orbiting the Earth.

After compiling the statistics, Kelley and Stout found that a tropical cyclone with a hot tower in its eyewall—the ring of clouds around a cyclone’s central eye—was twice as likely to intensify within the next six hours than a cyclone that lacked a tower. The team considered many alternative definitions for hot towers before concluding the nine-mile height threshold was statistically significant.

Kelley explains that both he and Stout, who has a master’s degree in meteorology, developed the ideas in the hurricane research. Kelley wrote the software that did the actual calculations. “John has a deeper knowledge of meteorology than I do, and that is one of his major contributions to our research.”

Their research is ongoing. They have expanded the results that they presented in January, looking at six years of TRMM data, rather than just the first four, and “we are refining our statistical methods,” Kelley says.

In August, NASA announced that it will extend operation of TRMM through the end of the 2004 hurricane season. Launched in 1997, TRMM was originally designed as a three-year research mission. Following four years of extending TRMM, NASA and JAXA had planned to decommission the satellite, but decided to extend the mission at the request of the National Oceanic and Atmospheric Administration (NOAA).

This fall, the hurricane season has been unusually active and Kelley and Stout have been watching for hot towers. For example, they looked into the heart of Hurricane Ivan on Sept. 15, one day before Ivan struck Alabama. The TRMM satellite revealed a tall rain cloud in Ivan’s eyewall, but it was only 7.5 miles high, not tall enough to be considered a “tower.” As their research would suggest, Ivan’s surface winds held steady immediately after this satellite overflight, but the winds did not become more intense.

TRMM is the first satellite to provide detailed measurements of rainfall over the tropics, allowing scientists to study the transfer of water and energy among the global atmosphere and ocean surface that form the faster portion of the Earth’s climate system. Because TRMM’s radar enables it to “see through” clouds, it allows weather researchers to make the equivalent of a CAT-scan of hurricanes and helps weather forecasters to use TRMM data to improve prediction of severe storms.

Funding for the research was provided by NASA’s Earth Science Enterprise. For more information about the research, visit the Goddard Space Flight Center web site.

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