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MISR Science Goals and Objectives
MISR's Study of Earth's Surface
By John Martonchik, MISR Science Team
What effect does Earth's surface have on Earth's energy balance?
Land surface processes are important components of the terrestrial climate
system. These processes include the a two-way exchange of radiation (in the form
of sunlight and heat) and matter (particularly gases such as water vapor and
carbon dioxide) between the surface and atmosphere. Scientists measure these
exchanges in terms of flux, which is the rate of flow of radiation or matter
across a unit surface area. Different surface types (rocky surfaces, bare soils,
vegetative canopies, etc.), defined by their physical, chemical, and structural
properties, will have a direct but varying influence on these fluxes.
What will MISR measure about Earth's land surface?
Mathematical descriptions of the flux of energy and material between Earth's
surface and atmosphere, form the basis of physical models that are used to study
Earth's energy cycle. Such models require quantitative information that can be
provided by instruments such as MISR.
A key land surface climatological parameter, based on radiative fluxes, is
the albedo. The albedo is defined as the ratio of the solar flux scattered from
the surface to the solar flux incident on the surface. It is a measure of the
fractional amount of solar radiation absorbed by the surface. Theoretically,
albedo can range from unity (a surface that does not absorb any of the incident
radiation at the wavelength(s) of interest) to zero (a "black" surface that
absorbs all incident radiation at the wavelength(s) of interest).
Natural land surfaces display a wide range of albedo values. In the
climatologically important polar regions, albedos of the ice-and-snow-covered
areas (known as the cryosphere) are continuously modified by natural processes
and by human sources of pollution. This affects the amount of solar energy
reflected by the surface. A "feedback mechanism" between the atmosphere and the
cryosphere results -- if a snow-covered surface is partly blackened by soot
particles, for example, the surface will absorb more solar energy, which may
melt the snow, and darken the surface further. Conversely, if a surface is
whitened by a deposit of fresh snow, it will reflect more sunlight than before,
helping to preserve the snow cover.
For vegetated terrain, knowing the albedo more accurately may lead to
improved estimates of photosynthesis, transpiration rates, and the amount of
absorbed photosynthetically active radiation (PAR). These parameters play an
important role in models of the way surface vegetation and the atmosphere
interact.
Most satellite instruments look at Earth only in a single direction, usually
vertically downward. But, except for movie-screen material, common surfaces
reflect different amounts of radiation in different directions. So to make good
measurements of the total reflected flux, Earth's surface must be viewed
from many directions. This is one way the multiple view angles of MISR will
improve the our knowledge of Earth's albedo.
In addition to telling about biophysical fluxes, the albedos of vegetation
canopies may contain some information about the structural state of the
vegetation, such as: the amount of leaf area, leaf orientation statistics, the
percentage of stems, branches, trunks, etc. Researchers have argued, on the
basis of field measurements and 3-dimensional canopy modeling, that the
directional reflectance characteristics (i.e., how much solar radiation is
scattered from the canopy in a particular direction), is diagnostic of such
canopy structure variables. MISR will provide data sets of these angular
reflectance "signatures" for many classes of surface cover. These angular
"signatures" have the potential for improving the process of classifying and
monitoring various "biome types" on a global basis.
To determine surface albedo and angular reflectance properties, corrections
must be made for the effects of scattering and absorption by atmospheric
aerosols. Estimated aerosol optical properties derived from MISR data will be
used for this purpose.
What will MISR measure about the ocean surface?
The concentrations of chlorophyll "a" and phaeophytin "a" pigments in
phytoplankton have been used to estimate the rate of biological productivity in
ocean waters. The determination of phytoplankton pigment concentration is based
on water-leaving radiances, known as ocean color, in several spectral bands in
the visible and near-infrared. The primary instrument for assessing ocean
productivity on the EOS spacecraft is MODIS. However, due to sun glint over a
portion of the MODIS swath as the satellite passes over the equator, some
imagery will be lost. This gap in the ocean color data will be partially filled
by MISR.
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