Evaluating the Clouds and the Earth's Radiant Energy System (CERES) Instrument Calibration Improvements

The Clouds and the Earth's Radiant Energy System (CERES) mission is instrumental in examining the role of clouds and radiation in the Earth's climate system. Two identical CERES instruments are deployed aboard NASA's Earth Observing System (EOS) Terra and Aqua satellites. Each CERES instrument consists of scanning thermistor bolometer sensors that measure broadband radiances in the 0.3 to 5μm (shortwave), 0.3 to < 100 μm (total) and 8 to 12 μm (window) radiometric regions. CERES instruments have the capability of scanning in either the cross-track or rotating azimuth plane (RAP) scan modes. Cross-track scanning, the primary mode of CERES operation, allows for the geographical mapping of the radiation fields. Operation in RAP mode enables the acquisition of data over a more extensive combination of viewing configurations, which leads to vastly improved angular distribution models used in radiance to flux conversion.

To evaluate, achieve and maintain radiometric stability, it is imperative that these instruments undergo rigorous radiometric calibration and validation. Calibration and validation studies have indicated spectral changes in the reflected solar spectral regions of the shortwave and total sensors. Spectral darkening is detected in the shortwave channel optics, which is more prominent while the instrument operates in RAP mode. This degradation is likely due to instrument scan plan being aligned with the spacecraft direction of motion during part of the RAP scan cycle, which makes the optics more susceptible to increased UV exposure and molecular contamination. Additionally, systematic daytime-nighttime longwave top-of-atmosphere (TOA) flux inconsistency was also detected during validation, which points to the changes occurring in the reflected solar spectral region of total sensor. Sensor corrections are derived and incorporated in the data processing stream in order to account for these changes.

This paper briefly describes the strategy to correct for the sensor response changes and presents the improvements in CERES Edition3 data products, which incorporates these sensor response changes in the computation of shortwave and longwave radiances.