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June
19, 2007 NEW
TOOLS TO FORECAST HURRICANE RAINFALL INLAND All
eyes are on where hurricanes make landfall, but the massive storms
actually
cause the most deaths inland, where severe flooding often surprises
residents. Now,
researchers are learning how to predict where tropical storms and
hurricanes
will dump the most rain — even days after — and
hundreds of miles away from —
landfall. In
a paper in the current issue of the journal Professional Geographer,
Corene
Matyas, an assistant professor of geography at the University of
Florida,
outlines new tools to predict how the storm’s intensity,
distance it has moved
inland and landscape topography alters its “rain
shields” — the bands of heavy
rain so visible in Doppler radar images. Among other things, her tools
proved
adept at modeling observations that when hurricanes or tropical storms
encounter the Texas hill country or the Appalachian Mountains, their
rain
shields tend to line up in the same direction and with the same
orientation as
the underlying topography. “There
are a lot of different things that can affect where the rainfall can
occur in
the storm and how heavy that rainfall will be,” Matyas said.
“Our goal is to
work toward predicting how those factors will determine the rainfall
pattern.” Historically,
hurricanes have proven most fatal at landfall, with coastal residents
overcome
by storm surge and high winds. But over the past four decades,
forecasters have
become more skilled at predicting hurricanes’ tracks over
open water, enabling
most coastal residents to flee or prepare for the storms well in
advance. As
a result, the highest proportion of hurricane and tropical fatalities
has
shifted inland. One study cited in the Matyas paper found 59 percent of
deaths
from tropical storms or hurricanes between 1970 and 1999 occurred
because of
heavy rainfall rather than wind or storm surge. As storms track inland,
they
inevitably ensnare more cities and towns. In 1998, Tropical Depression
Charley
left 20 people dead near Del Rio, Texas, more than 200 miles from where
the
storm made landfall, Matyas notes. Researchers
are developing some models for forecasting inland rain patterns, but
they have
difficulty accounting for the lopsided or elongated shape the pattern
often
takes, with most if not all rain falling on one side of the storm. A
common
assumption is that rainfall will decrease as the hurricane moves away
from the
ocean, which is generally true but may be obviated by other weather
systems and
local landscape. Matyas’
goal was to find new tools to improve the models. She
studied radar data from 13 U.S. storms that made landfall between 1997
and
2003, then used a common tool in geography — geographical
information systems,
or GIS — to measure how rainfall patterns changed. GIS is a
computer system
that makes it possible to analyze spatial patterns of data. It is often
used to
track things such as voting patterns, but using GIS in meteorology
— where
spatial patterns change — is relatively new, Matyas said. Matyas
outlined the edge of the rain shields using radar data, then measured
their
shapes by calculating characteristics such as the position of their
center of
mass. She repeated the analysis for each hour that the storms were over
land.
She then used a statistical technique, discriminant analysis, to
determine
which shape and size best place the storms into groups based on their
intensity, how far they travel inland and the topography they
encounter. The
success of the discriminant analyses indicates that these shape
measures could
serve as predictive tools for future rainfall models. In
a demonstration of the potential, the shape measures helped to confirm
that
that the orientation of storms’ rain shields corresponds
closely to the
orientation of the land topography. With
hurricanes crossing Texas hill country, the rain shields tend to line
up
parallel to the main axis of the hills, running west to east. Storms
near the
Appalachians also line up parallel to the mountains, whose axis runs
southwest
to northeast, with the heaviest rain consistently occurring to the west
of the
track. This is due to a combination of the mountains and a wedge of
cold and
dry continental air forcing the moist air upward, causing the water
vapor to
condense and fall to the ground as rain. This phenomenon does not
happen with
the Texas storms, as the dry continental air masses over Texas are
similar in
temperature to tropical moist air masses that accompany hurricanes. Frank
Marks, a research meteorologist and director of the National Oceanic
and
Atmospheric Administration’s Hurricane Research Division,
said Matyas’
conclusions “have a lot of merit in terms of understanding
the structure, size
and shape of the rain shield.” He
said the next step is to add rainfall amount to the variables. The end
goal: a
model that will provide inland residents with the same targeted advance
warnings
and watches that coastal residents get today — but for heavy
rainfall rather
than wind or storm surge. ##
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