The CERES project has advanced the state-of-the-art in Earth Radiation Budget (ERB) observations through improved accuracy of the CERES instruments and extensive use of coincident higher spatial resolution spectral imager measurements on both low-Earth orbit and geostationary platforms. CERES involves a high level of data fusion. During the CERES period, the team has processed data from 7 CERES instruments, 2 MODIS, 2 VIIRS and 20 geostationary imagers, all integrated to obtain climate accuracy in radiative fluxes from the top to the bottom of the atmosphere. Over 90% of the CERES data product volume involves two or more instruments.
Seven CERES instruments on five satellites have been launched (TRMM, Terra, Aqua, S-NPP, NOAA-20). Six of the seven are currently operational. Each CERES instrument is a narrow field-of-view scanning radiometer with nadir footprint size of 10 km (TRMM), 20 km (Terra, Aqua), or 24 km (S-NPP, NOAA-20). A narrow FOV enables: higher spatial resolution, clear-sky observations for cloud feedback studies, frequent space views to correct thermal offsets, intercalibration with other instruments, and lunar/solar calibrations.
CERES instruments measure broadband radiances in 0.3-5 µm (SW), 0.3-200 µm (TOT) and 8-12 µm (WN) channels. On NOAA-20, the FM6 instrument replaces the WN channel with a LW channel (5-35 µm).
Each CERES instrument can scan in three principal modes: fixed azimuth plane (FAP or crosstrack), rotating azimuth plane (RAP), and programmable azimuth plane (PAP).
In crosstrack mode, CERES scans from limb-to-limb perpendicular to the groundtrack. This mode provides global coverage daily. In RAP mode, CERES scans in elevation as it rotates in azimuth. RAP data are used to develop empirical angular models used in CERES processing. In PAP mode, the CERES instrument is sent commands from Earth to align its scan plane with that of other instruments (including CERES instruments on different platforms) so that different instruments can be compared.
Onboard calibration sources include a solar diffuser, a tungsten lamp system with a stability monitor, and a pair of blackbodies that can be controlled at different temperatures. Cold space looks and internal calibration are performed during nominal Earth scans. The CERES instruments also periodically scan the moon, which is used as an additional check on instrument radiometric stability.
Imager measurements provide detailed information about the scene observed by CERES. The imager spectral radiances are used to infer cloud, aerosol and surface properties. These data are used in CERES processing to convert measured CERES radiances to TOA radiative fluxes. The imager retrievals are also used to determine surface radiative fluxes and in process studies involving clouds-aerosols-radiation interactions.
Geostationary imager measurementsare used to infer TOA and surface radiative fluxes and cloud properties betweenCERES observation times. To maintain calibration traceability, GEO imagerradiances are calibrated against polar orbiter imager radiances and the derivedGEO fluxes are normalized to the CERES measurements.
GEO imager characteristics vary with time. Accounting for these changes and removing artifacts due to instrument anomalies/limitations is a necessary step toward producing a consistent record of ERB throughout the CERES time period.
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