They are called hurricanes in the Atlantic, typhoons in
the West Pacific, and tropical cyclones worldwide; but wherever
these storms roam, the forces that determine their severity now
are a little less mysterious. NASA scientists, using data from
the Tropical Rainfall Measuring Mission (TRMM) satellite, have
found “hot tower” clouds are associated with tropical cyclone
intensification.
Owen Kelley and John Stout of NASA’s Goddard Space Flight
Center, Greenbelt, Md., and George Mason University will
present their findings at the American Meteorological Society
annual meeting in Seattle on Monday, January 12.
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 altitude due to the large amount of
latent heat.Water vapor releases this latent heat as it
condenses into liquid.
A particularly tall hot tower rose above Hurricane Bonnie in
August 1998, as the storm intensified a few days before
striking North Carolina. Bonnie caused more than $1 billion
damage and three deaths, according to the National Oceanic and
Atmospheric Administration National Hurricane Center.
Kelley said, “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.” The pioneering work of Joanne Simpson, Jeffrey
Halverson and others has already shown hot towers increase the
chance a new tropical cyclone will form. Future work may use
this association to improve forecasts of a cyclone’s
destructive potential.
To achieve their goal, Kelley and Stout needed to compile a
special kind of global statistics on the occurrence of hot
towers inside tropical cyclones. The only possible data source
was TRMM satellite, a joint effort of NASA and the Japan
Aerospace Exploration Agency. “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 said. To compile global
statistics, the radar needs to be orbiting the Earth.
After compiling the statistics, Kelley and Stout found a
tropical cyclone with a hot tower in its eyewall was twice as
likely to intensify within the next six hours than a cyclone
that lacked a tower. The “eyewall” is the ring of clouds around
a cyclone’s central eye. Kelley and Stout considered many
alternative definitions for hot towers before concluding the
nine-mile height threshold was statistically significant.
Funding for the research was provided by NASA’s Earth Science
Enterprise. The Enterprise strives to advance Earth System
Science and to improve the prediction of climate, weather and
natural hazards from the unique vantage point of space.
For more information about the research and images on the
Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/0112towerclouds.html
For information about the TRMM Satellite on the Internet,
visit:
http://www.eorc.nasda.go.jp/TRMM
For information about NOAA’s National Hurricane Center, visit:
For information about Hurricane Bonnie, visit:
