MEASURING
EARTHSHINE: HOW NEW TERRA DATA ARE IMPROVING WEATHER AND CLIMATE
FORECAST MODELS
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A
sensor aboard NASAs Terra satellite is helping scientists
map how much sunlight the Earths surface reflects back
up into the atmosphere, and this new detailed information
should help to greatly improve weather and forecast models.
The
Moderate Resolution Imaging Spectroradiometer (MODIS) now
routinely provides daily global and local measurements of
albedo, or the total amount of light reflected from Earths
surface out to space. These precise data may allow scientists
to better understand and predict how various surface features
absorb and reflect solar radiation, which influence both short-term
weather patterns and longer-term climate trends.
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In
a May 2002 issue of Geophysical Research Letters, a team of
scientists at Boston University reported that the new albedo
measurements match up well with the wide variance of geological
features found across the Earths barren landscapes.
Zooming
in on Africas Sahara Desert and the Arabian Peninsula,
for instance, MODIS observed considerable variability in reflectance
across the regionÑfrom the darkest volcanic terrains
to the brightest sand sheets, said Elena Tsvetsinskaya,
the papers lead author and a researcher at Boston University.
So we can relate specific soil groups and rock types
to MODIS-derived albedo measurements.
This
correlation is important because most current weather forecast
models treat this region as if the surface is uniform and
therefore reflects the same amount of light all across its
wide expanse. However, the terrain across the Sahara Desert
and Arabian Peninsula is actually quite varied. Darker surface
features (like rocks and plant canopies) absorb more light
than lighter surfaces (like sand) and therefore get hotter
in the afternoon. Over the course of a day, these heating
differences can set up atmospheric motions that influence
clouds and rain.
By
coupling the MODIS measurements with geologic information,
Tsvetsinskaya and her colleagues have provided weather and
climate modelers with a new map of albedo across Northern
Africa and the Arabian Peninsula that they can use to fine-tune
their models. The team classified the region into eight categories,
each of which has a distinct reflectivity range.
There
is a certain scientific beauty in deriving albedos from something
else in the model (such as geologic information), said
Robert Dickinson, project lead at the Georgia Institute of
Technology. But the more practical reason is for Ôwhat
if studies. For example, what if the wind in Africa
blew so hard it covered all the black rocks with white sand?
This
scenario hints at what both meteorologists and climate researchers
alike know well: albedo for a given region can change relatively
quickly. For example, the bright white snow- and ice-covered
landscapes of Canada and Siberia during the winter and early
spring reflect most incoming solar radiation back up into
space, thereby helping to keep the surface cold.
But
as the snow melts with the gradual onset of summer, the boreal
forest canopy is exposed to the sunlight. The vegetated surface
is much darker and strongly absorbs light, which helps to
warm the surface.
NASA
scientists discovered recently that once the snow melts in
the Earths boreal regions, densely-vegetated surfaces
begin to release a significant amount of heat into the overlying
atmosphere. Yufang Jin, a Boston University graduate student,
uses MODIS albedo data to document the difference between
snow covered and snow-free vegetated surfaces in a related
paper in another May issue of Geophysical Research Letters.
Over
the longer term, regular MODIS albedo measurements will allow
scientists to monitor how the Earths reflectivity changes
on a global scale. This will help us determine how the
Earths climate is changing, both globally and locally,
said Crystal Schaaf, co-author and research professor at Boston
University. When humans convert a vegetated region to
a more reflective surface type (such as an urban area), the
albedo changes.
Similarly,
Schaaf added, when places like the grasslands of the African
Sahel contract, they leave more reflective, barren deserts,
which increases albedo.
Launched
December 18, 1999, Terra is the flagship of the Earth Observing
System series of satellites and a central part of NASAs
Earth Science Enterprise. The mission of the Earth Science
Enterprise is to develop a scientific understanding of the
Earth system and its response to natural and human-induced
changes to enable improved prediction capability for climate,
weather, and natural hazards.
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