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

NASA's Clouds and the Earth's Radiant Energy System (CERES) mission provides highly accurate radiance measurements that are critical to the long-term monitoring of the Earth's radiation budget. These measurements are made from instruments deployed aboard the Earth Observing System (EOS) Terra and Aqua satellites and the Suomi National Polar-orbiting Partnership (S-NPP) satellite. Each CERES instrument is equipped with three scanning thermistor bolometer sensors that measure broadband radiances in the shortwave (0.3 to 5.0 μm), total (0.3 to <100 μm) and water vapor window (8 to 12μm) regions. In-flight calibration sources are used to identify post-launch changes in the sensors' gains while rigorous calibration and validation protocols that are being implemented have been instrumental in detecting changes in spectral responsivity in the reflected solar spectral regions of the shortwave (SW) and total (TOT) sensors.

Calibration and validation studies have indicated the presence of spectral darkening in the SW channel optics that is strongly correlated to the instrument's operational modes. In the absence of a climatological explanation for the darkening, the changes likely occurred during part of the rotating azimuth plane (RAP) scan cycle when the scan plane was aligned with the direction of motion, causing increased UV exposure and molecular contamination on the optics. Systematic daytime-nighttime longwave top-of-atmosphere (TOA) flux inconsistency was also detected during validation, highlighting changes in the shortwave region of the total sensor.

The strategy used to characterize the changes in the SW spectral region for CERES Terra and Aqua instruments are discussed in this paper. CERES Terra and Aqua Edition-4 data, which incorporate the most recent calibration changes, show better consistency throughout the mission and improvement in flux anomaly trends over preceding CERES data versions. Comparisons made between Terra and Aqua Edition-4 and S-NPP radiative fluxes will also be presented.