The Real Story of Precipitation in
the Southern Appalachian Mountains
6
NCRWA.COM |
Spring 2014
from the president
supply comes from southern Appalachian
headwaters. To answer some of these
big questions, scientists are increasingly
turning to satellite data.
Barros describes satellite-based precip-
itation instruments as a game changer for
scientists in how they study hydrology
and do science. NASA’s Tropical
Rainfall Measuring Mission (TRMM),
launched in 1997, and the upcoming
Global Precipitation Measuring (GPM)
mission that will launch in February
2014, see more of Earth than ground-
based instruments and from a vantage
point that is not impeded by topography
in mountainous regions.
While Barros and her team were sticking
sensors on mountain tops, they were
also downloading data from TRMM’s
precipitation measurement instruments
and data processing products. The
dual collection of data from the ground
and the sky allows scientists like
Barros to validate and improve those
satellite measurements, and make a few
discoveries along the way.
Many people don’t realize, Barros says,
the southern Appalachians have the
highest annual rainfall of the southeastern
United States, as much as humid Florida.
It rains on average 54 inches per year
in the Everglades, about 50 inches in
North Carolina, and about 58 to 60
inches a year in the Appalachians. The
Mountains contribute to the enhanced
weather: moist air bumps into them as it
moves over the land, and when it rises, it
cools and can’t hold on to its water vapor
which condensates and forms clouds.
This leads to as much rain falling in the
Appalachians as is delivered to the coast
by hurricanes like Francis or Ivan. And
in years without a hurricane or tropical
storm to factor in, it rains nearly the same
amount, on average, every month in the
southern Appalachians. Forty percent
in summer and up to 60 to 70 percent
in winter of all that rainfall is very
light rainfall.
That’s why the Smokies are called the
Smokies, Dr. Barros explains. “There’s
always a little bit of fog and low level
clouds and there’s always a little bit of
rain going in the air.” Until her team
measured it, scientists didn’t know how
important light rain was to the region.
At all times of the day, light rainfall is
the dominant type of precipitation. And
because light rainfall is the most reliable
and most frequent form of rainfall in the
region, Barros says, contributing 50 to
60 percent of total precipitation over a
year, it governs the regional water cycle.
Light rain is no less than the lifeline of
freshwater resources for the landscape’s
ecosystems. This is probably also true in
mountainous regions everywhere, Barros’
finds, which carried big implications for
communities worldwide.
Will Light Rain Become
a Misty Memory?
So what happens to light rain as global
climate shifts, and things grow warmer?
At lower elevations, more of the fine
drops from light rain will evaporate in the
air and fail to reach the ground. Lower
elevations will have to contend with not
only higher temperatures, but less cloud
cover-affecting a cycle in which plants
and forests and rain have this amazing
rhythm in which one feeds the other.
Depending on how daytime temperature
change, Barros’ thinks that morning light
rainfall would likely be more sustainable
than afternoon light rainfall.
“In North Carolina,” Dr. Barros says,
“different stakeholders are suing each
other regarding inter-basin water transfers
because of the high frequency of drought
we’re having. So this dependency on
It rains on average 54 inches per
year in the Everglades, about 50
inches in North Carolina, and
about 58 to 60 inches a year in
the Appalachians.
I
f you walk into a cloud at the top of a
mountain with a cup to quench your
thirst, it might take a while for your cup
to fill. The tiny, barely-there droplets are
difficult to see, and for scientists they,
along with rain and snow, are among
the hardest variables to measure in Earth
Science, says Ana Barros, professor
of engineering at Duke University. As
part of the Science Team for NASA’s
Precipitation Measurement Missions
(PMM) that measure rainfall from space,
Barros and her research team trekked
into the Great Smoky Mountains and
other areas of the southern Appalachian
Mountains, to learn more about where,
when and how rain falls in the rugged
terrain. What they found was eye-
opening: much of the water people
counted on falls as light rain, and no one
knew about it.
Understanding precipitation in the
mountains is one of the first steps to
understanding local ecology, regional
climate, and the effects of any changes
that might impact the nearly 30 million
people in Georgia, South and North
Carolina, and Tennessee whose water
