Abstract Art or Arbiters of Energy?
Two new sensors flying aboard NASAs Terra satellite, launched in December 1999, are designed to help scientists answer these questions. The Moderate-resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging Spectroradiometer (MISR) give scientists new capabilities for measuring the structure and composition of clouds. MODIS observes the entire Earth almost every day in 36 spectral bands ranging from visible to thermal infrared wave-lengths. With spectral and spatial resolutions superior to that of AVHRR (its heritage sensor), MODIS can measure a wide suite of clouds' physical and radiative properties. Specifically, MODIS can determine whether a cloud is composed of ice or water particles (or some combination of the two), it can measure the effective radius of the particles within a cloud, it can observe how much (or little) sunlight passes through a cloud, and it can measure the temperature and altitude of cloud tops. Moreover, with its unique 1.37-micron channel, MODIS observes thin cirrus clouds with unprecedented sensitivity. This channel not only enables scientists to quantify the impact of cirrus clouds on the radiation balance, but it also permits image analysts to correct for the presence of cirrus in remote-sensing scenes used to examine surface or lower-level features.
Complementing MODIS, the MISR instrument sees the Earth simultaneously in red, green, blue, and near infrared wavelengths at nine different anglesat four progressively more oblique angles ahead of Terra, four angles aft of the satellite, and one at nadir. Because it measures any given scene from multiple angles, MISR is ideally designed to help scientists better understand how clouds interact with radiant energy as both a function of their structure and type. CERES complements MODIS and MISR by providing measurements of the shortwave and longwave radiant energy that clouds reflect and emit back into space.
ESAs next-generation satellite missions for comprehensively examining Earths climate system will begin with the late 2002 launch of its first Environmental Satellite (Envisat). Similar to MODIS in the scope of its research applications, Envisat carries the Medium-Resolution Imaging Spectrometer (MERIS). Like Terras MODIS, MERIS has a wide viewing swath (1,500 km), with a morning equatorial crossing, and it can see the entire Earth within every 3 days. Scientists are using its data to derive cloud cover, cloud altitude, water vapor, and aerosol properties. Unlike MODIS (which uses a cross-track scan mirror), MERIS is a push-broom scanner based upon Charge-Coupled Device (CCD) technologies with gains and offset settings that can be optimized for observing specific targets. This is a similar technology to that used by MISR.
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