A new collection of Earth science data is now publicly available to advance
global studies of how our planet’s lands, oceans, atmosphere and life all
interact to define our world’s water cycle, carbon cycle and climate system.
These data are courtesy of the Moderate-resolution Imaging Spectroradiometer
(MODIS), flying aboard NASA’s Terra satellite. Whereas good MODIS data have
been available since November 2000, the newly released data set, called
Collection 3, marks a significant step forward in data quality and

“MODIS achieved a steady-state level of operations in November (2000) and
since that time we have continued to make every attempt to improve the
algorithms,” said Team Leader Vince Salomonson, of NASA’s Goddard Space
Flight Center in Greenbelt, Md. “In June, we decided to provide the Earth
science community with a set of consistently processed data products based
upon our best efforts at that time.”

Like a highly advanced digital camera in space, MODIS has been measuring
visible and infrared wavelengths from all over our world at multiple scales
of time and space since February 2000. MODIS extends the measurement
heritage begun by CZCS and AVHRR, and now provides exciting improvements in
spectral detail, spatial resolution and accuracy over previous sensors.

Collection 3 currently includes three months of data products – March, April
and May of 2001 – and is now being expanded to encompass a one-year span
from November 2000 to November 2001. These advanced, scientifically useful
products are an intermediate step between the earliest beta-quality products
and formally validated global data sets. Yet even in this intermediate
stage, the team finds its MODIS data are of superior quality and provide an
excellent new data set for global change studies.

This interdisciplinary collection of Earth science data includes information
on more than 40 meteorological, biological and hydrological characteristics
of the Earth, including some of the first ever routine, global observations
of key Earth science characteristics, such as aerosol concentration over
land and enhanced estimates of the effective leaf area of Earth’s
vegetation. Measurements of ocean temperatures and chlorophyll
concentrations reveal more detail and variability than previously seen,
while MODIS’ chlorophyll fluorescence product adds a whole new dimension to
studies of the marine biosphere.

MODIS’ data products are available at spatial resolutions ranging from 250
meters per pixel, to 1 kilometer, to 1 degree; and range over time from
daily, to weekly and monthly global composites. Production of MODIS’ data
products is divided among the team’s three discipline groups: Atmosphere,
Land and Ocean. Atmosphere Group Leader Michael King is excited by what he
sees in the MODIS data.

“We now have the ability to track and characterize aerosol optical
properties over land and ocean, globally,” he said. “And we have an
unprecedented ability to separate small particles that result from human
activities and biomass burning from coarse particles that result naturally,
such as sea salts and desert dust.”

According to King, MODIS allows scientists to measure the differences in
cloud optical properties for both water and ice clouds. The sensor can
characterize the optical thickness of clouds and quantify the size of the
particles within them. Moreover, MODIS is the first space-based sensor with
a 1.37-µm channel, which is particularly sensitive to high, thin cirrus

Complementing its measurements of the atmosphere, MODIS carries a suite of
channels that are particularly well suited for studies of the Earth’s land
and ocean surfaces. The idea is to observe and measure the interactions of
the atmosphere with lands, oceans and life on our world. Another copy of the
MODIS instrument will fly aboard Aqua in spring 2002 with an afternoon
equatorial crossing to capture the daily variability of the Earth system.

According to Land Group Leader Chris Justice, MODIS is the first space-based
sensor that enables scientists to remove comprehensively the effects of the
atmosphere for relatively unobstructed measurements of the surface. Thus,
MODIS’ measurements of surface reflectance and radiance are used to generate
higher-order data products, such as spectral albedo, enhanced vegetation
indices, leaf area index and the fraction of photosynthetically active
radiation absorbed by plants, land use and land cover change, fires and burn
scars, snow and ice cover and land surface temperature.

MODIS sets a new precedent in terms of radiometric data quality, integration
of many different algorithms for producing a suite of data products, data
validation and quality assurance and improved resolution. “MODIS is capable
of showing land cover dynamics – on a global scale, every day, at up to 250
meter resolution – that were not possible before,” Justice noted. “It is
becoming apparent that MODIS 250-meter data can help advance land research
that requires higher resolution and frequent measurements.”

Among the new products generated from MODIS, the Enhanced Vegetation Index
(EVI) product and the Spectral Albedo Product are noteworthy and unique to
MODIS. The EVI offers an alternative to the Normalized Difference Vegetation
Index (NDVI) measurements and appears to respond better to changes in
vegetation as a function of changing climate, changing seasons, land use and
differing plant structures.

According to Ocean Group Leader Wayne Esaias, MODIS’ global 1-kilometer
resolution sea surface temperature and ocean color (chlorophyll) products
are an order of magnitude better than the same measurements made by
precursor sensors.

“The details and patterns in MODIS’ bio-optical and sea surface temperature
distributions is astounding,” Esaias stated. “For the first time, we can see
correlations between the physics and biology of the ocean at spatial scales
that just weren’t available to us before. MODIS also offers improved
approaches for chlorophyll measurements, color dissolved organic matter,
coccolithophore calcite and ocean productivity.”

Additionally, MODIS measures ocean parameters together with important
atmospheric variables, like aerosols and clouds, to give scientists a much
more complete understanding of the ocean-atmosphere climate interactions.

NASA’s Terra satellite is the flagship of the Earth Observing System series
of satellites, part of NASA’s Earth Science Enterprise, a long-term research
program dedicated to understanding how human-induced and natural changes
affect our global environment.

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