Special Operations of CERES for Radiation Experiment Tests (SOCRATES) – ERB Instruments Comparison

The Clouds and Earth Radiant Energy System project flew a scanning radiometer (PFM) aboard the Tropical Rainfall Measuring Mission TRMM satellite in 1998, and then aboard the Terra and Aqua spacecrafts in 2000 and 2002, respectively. These scanners have taken radiation measurements for more than a dozen years, and if ERB instruments before the CERES launch are included, an Earth radiation dataset is shown to be two decades long. CERES scanners were already used several times to assure the continuity and consistency of this dataset. Comparisons of overlapping data sets are necessary to assure a seamless climate data record of earth radiation budget, else any change may be due to difference of instruments or to climate change. To enable comparison of instruments on different platforms, the Programmable Azimuth Plane Scan (PAPS) feature of the CERES instrument was used to match other satellite instruments viewing geometry. This paper presents special operations of CERES using the PAPS mode with the objective to collect data for comparison at the radiance level with other Earth Radiation Budget (ERB) instruments, and also shows numerical results of such comparisons. The following campaigns are covered in the paper: (i) In 1998, the CERES instrument (PFM) was rotated in azimuth so its scan plane coincided with the cross-track scan plane of the ScaRAB-2 instrument when the orbits of their spacecraft intersected. In this data set, both instruments viewed the same scenes from the same directions within a few minutes of each other, so the radiance measured by both instruments could be compared. (ii) In March of 2000, the scan plane of CERES Terra (FM1 and FM2) was rotated to coincide with the cross-track scan of the PFM aboard TRMM satellite. Data collected over up to 10 orbital crossings per day are used to compare radiance measurements of PFM and FM1 or FM2. (iii) In July of 2002, radiance measurements of scanners on Terra and Aqua satellites are compared. Since both satellites are in a polar orbit, the scan planes of both instruments were adjusted to align with the minor plane (orthogonal to the solar plane at local noon). Data for comparison were collected at each node with the focus on Greenland as the target for SW comparison. This experiment is repeated every year during summer solstice to monitor CERES Terra and Aqua consistency. (iv) Geostationary Earth Radiation Budget (GERB) instruments aboard the MeteoSat-8 and 9 spacecraft use arrays with 256 detectors each to measure the reflected solar radiation and Earth emitted radiation. It generates an Earth radiation snapshot every 15 minutes. Since 2004, CERES (FM2) scanner on Terra has made measurements of the same radiances by matching the GERB-2 and then GERB-1 Earth viewing geometry. These special data collection campaigns have been performed during summer and winter solstice periods. Daily, up to five Terra passes are in the view of GERB, and data collected by FM2 are used to compare the GERB detectors with each other using the CERES as a transfer radiometer. In addition, the CERES/GERB comparison is done on geolocated gridboxes.