NASA TRMM Sees Rain From Hurricanes Fall Around the World

Since rain and freshwater flooding are the number one
causes of death from hurricanes in the United States over the
last 30 years, better understanding of these storms is vital
for insuring public safety. A recent study funded by NASA and
the National Science Foundation offers insight into patterns
of rainfall from tropical storms and hurricanes around the
world.

Researchers at the University of Miami’s Rosenstiel School of
Marine and Atmospheric Science, Miami, and the National
Oceanic and Atmospheric Administration Atlantic Oceanographic
and Meteorological Laboratory’s Hurricane Research Division,
Miami, used data from NASA’s Tropical Rainfall Measuring
Mission (TRMM) satellite to show how rain falls at different
rates in different areas of a storm. The results were
published in the July issue of the journal Monthly Weather
Review.

The results are already being used in a model developed at the
Hurricane Research Division to estimate rainfall accumulation
related to tropical cyclones. The findings are important
because they may help in the development of better forecasts.

The TRMM satellite offers the best measurements of how and
where rain falls around tropical cyclones. This is because its
orbit is low to the Earth, allowing more detailed information
on storms, and it was designed to cover the tropics.

Tropical cyclones consist of winds rotating around low-
pressure centers in the tropics that can develop into
everything from tropical storms to Category 5 hurricanes.

From 1998 through 2000, TRMM observed 260 tropical cyclones in
six major ocean basins. Researchers found that the rainfall
intensity and where the heaviest rains fell varied depending
on a storm’s wind speeds, its location and the environment of
each basin.

Scientists looked at three types of tropical cyclones, based
on a standard system for classifying these storms. Tropical
storms have wind speeds of less than 73 miles per hour (mph).
Category 1 and 2 hurricanes blow with winds of 74 to 110 mph,
and Category 3 to 5 hurricanes’ winds range above 110 mph.

“This study is important because we know very little about the
rainfall distribution in tropical cyclones,” said lead author
of the study, Manuel Lonfat, a University of Miami researcher.
“It revolutionizes our understanding of the distribution of
rain in tropical cyclones,” he added. Lonfat is a NASA Earth
System Science Fellowship recipient.

“More than 50 percent of deaths in the U.S. from tropical
cyclones over the last 30 years are related to freshwater
flooding. So this is currently a very large problem for the
forecasting community,” Lonfat said.

When all storms were averaged together the most intense
rainfall occurred within 50 kilometers (about 31 miles) of a
storm’s center, with evidence of very large rain rates as far
as 300 to 400 kilometers (about 186 to 250 miles) from the
center.

When all storms were averaged and analyzed basin by basin,
storms in the North Indian basin were the wettest, and East-
central Pacific storms were the driest. The Atlantic and West
Pacific storms showed similar rain rates: this at first
surprised the researchers since Western Pacific storms tend to
be bigger and were presumed to be wetter.

Researchers also found that the storms were not symmetric,
meaning that rain fell at different rates in different areas
of a storm. If a round storm were divided into four equal
parts through the center, called quadrants, in general it was
found that the heaviest rainfall occurred in one of the front
quadrants. However, the heaviest rainfall shifted from the
front-left to the front-right quadrant as a tropical cyclone’s
intensity increased.

Tropical storms were less symmetric, while stronger hurricanes
had a more symmetric inner core. In the Southern Hemisphere,
the heaviest rain occurred to the front-left of the storm’s
path, while in the Northern Hemisphere the heaviest rainfall
peaked in the front-right quadrant.

Normally, the only way to accurately measure rain falling from
a hurricane is when it gets close enough to the coast to be
picked up by National Weather Service radars, or by rain
gages. Since TRMM is space-based, researchers can assess the
rainfall over vast tracts of ocean, where these storms spend
most of their lives.

For more information and images on the Internet, please visit:

http://www.gsfc.nasa.gov/topstory/2004/0817trmmhurricane.html