NASA working to take the guesswork out of long-term drought prediciton

It’s tricky, this weather business — predicting drought, floods, rain
or
snow, especially months in advance. But NASA scientists at the National
Space Science and Technology Center in Huntsville, Ala., are working to
take
the guesswork out of long-term prediction.

“We’re researching methods to predict precipitation a season or more in
advance,” said Dr. Bob Oglesby, a senior atmospheric scientist at the
research center. The key, he said, is understanding how the atmosphere
interacts with the land — sometimes in a way that completely alters the
expected climate of a geographic area.

“The Gulf of Mexico, for example, is what keeps the Southeast from
becoming
semi-arid, or in the worst-case scenario, a big desert,” he said,
explaining
that atmospheric flow sweeps primarily from west to east. Without the
gulf,
states like Alabama, Tennessee and Georgia would be forced to seek
moisture
from the Pacific Ocean. “But a series of mountain ranges blocks the
way,”
he said. “If it weren’t for the nearby gulf, the lush, green landscapes
of
Southeast might more closely resemble the semi-arid landscapes of the
great
plains.”

Just as mountain ranges can block moisture from an entire region,
ridges of
atmospheric pressure act as similar obstacles — blocking out
much-needed
moisture where conditions are dry, resulting in a “thermal mountain
effect,”
first identified in 1953.

“Droughts are self-perpetuating,” Oglesby said. “If an area is already
experiencing drought conditions, it is more likely to continue in a
drought.
Similarly, if an area is experiencing extremely wet conditions, that
trend
is also likely to continue.”

This self-perpetuating cycle is due to the interaction of moisture in
the
soil with the atmosphere. If the amount of rain or snowfall drops below
average, the soil becomes dry. Then, as the Sun heats the Earth, less
moisture is available for evaporation.

With the resulting reduction in evaporation — and its cooling effects
—
the surface of the Earth warms, heating the atmosphere. As the
atmosphere’s
temperature increases, air rises. This cycle reinforces the ridge of
high
pressure, enhancing its abilities to block the flow of moisture from
bodies
of water as well as reducing the likelihood of thunderstorm formation.

Drought conditions also may be predicted by studying other factors,
including sea surface temperature variations such as those associated
with
ocean warming effects from El Niño, and ocean cooling effects from La
Niña;
north Atlantic oscillation, or air flow; and snow cover in surrounding
regions.

Oglesby’s research uses computer models to simulate and predict weather
conditions, using data such as soil moisture, precipitation and Earth’s
surface temperature. The biggest challenge in making long-term
predictions,
he said, is a lack of sufficient data on soil moisture, especially
moisture
in lower layers of the soil.

“If someone could provide us with the state of soil moisture over a
sufficiently large area, we can begin to predict its impact on
precipitation
over the next season or two,” he said. Oglesby sees hope for better
data in
the future from NASA remote sensing technology that gleans information
using
satellite or flights over select areas.

Above-average rainfall or snow in the winter or spring can increase soil
moisture to levels needed to help break the cycle of drought. But an
average
series of short, light rain showers — common in much of the South —
are
not generally enough, said Oglesby. “Even though surface soil may be
wetted
periodically, light rains may not drop enough moisture to reach lower
soil
areas. These perpetually dry areas, in turn, cause top soil to dry more
quickly — once again hindering the Earth’s natural cooling process.”

But there is hope even in the midst of drought conditions. “Even in a
dry,
Alabama summer, it rains,” said Oglesby, noting that large-scale
circulation
and thunderstorms in the summer can also break the cycle of drought.
“The
trick,” he said, “is replenishing the moisture in the soil before it’s
too
late.”

> From NASA’s Marshall Space Flight Center in Huntsville, Oglesby has
co-authored three research papers since 2001, published in the Journal
of
Climate and the Journal of Geophysical Research. Topics include
diagnosing
warm season precipitation, thresholds in atmosphere-soil moisture
interactions and the predictability of winter snow cover over the
Western
United States.

Oglesby has a bachelor’s degree in physical geography from the
University of
California in Davis and a doctorate in atmospheric dynamics from Yale
University in New Haven, Conn. He is based at the Global Hydrology and
Climate Center, one of seven science research centers at the National
Space
Science and Technology Center, a partnership with NASA’s Marshall Space
Flight Center, Alabama universities, industry and federal agencies.