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.