Earth radiation budget measurements: pre-ERBE, ERBE, and CERES

Experiments of increasing sophistication have intensively observed the Earth''s radiation budget over the last 2 decades. During the first years of these measurements there were many types of sensors and satellite orbits. The earliest measurements used very simple thermal devices and assumed that the Earth reflected and emitted isotropically. Including crude bidirectional reflectances and limb-darkening models improved the results. Likewise calibration improved. The Nimbus-7 Earth Radiation Budget (ERB) measurements used pyroelectric and thermopile detectors with on-board calibration. All of the measurements showed that the Earth was darker than expected. In the most recent observations by the Earth Radiation Budget Experiment (ERBE) there have been three major improvements. First ERBE measurements were better calibrated than previous ones. Second the experiment used systematic observations of angular distributions and scene identification from the Nimbus-7 data to improve estimates of instantaneous flux. Third the ERBE algorithms for time interpolation and averaging were more sophisticated than the measurements made before. In the closing years of this century we expect to produce a new set of measurements of the Earth''s radiation budget. This set will come from an investigation of " Clouds and the Earth''s Radiant Energy System" (CERES) a part of the Earth Observing System (EOS) . The CERES investigation will use the ERBE heritage of high-accuracy scanner measurements. CERES will also improve our understanding of the role of clouds in the radiation budget by retrieving