Determination of Unfiltered Radiances from the Clouds and the Earth’s Radiant Energy System Instrument

Abstract

A new method for determining unfiltered shortwave (SW), longwave (LW), and window radiances from filtered radiances measured by the Clouds and the Earth?s Radiant Energy System (CERES) satellite instrument is presented. The method uses theoretically derived regression coefficients between filtered and unfiltered radiances that are a function of viewing geometry, geotype, and whether cloud is present. Relative errors in instantaneous unfiltered radiances from this method are generally well below 1% for SW radiances (std dev ≈0.4% or ≈1 W m?2 equivalent flux), less than 0.2% for LW radiances (std dev ≈0.1% or ≈0.3 W m?2 equivalent flux), and less than 0.2% (std dev ≈0.1%) for window channel radiances. When three months (June, July, and August of 1998) of CERES Earth Radiation Budget Experiment (ERBE)-like unfiltered radiances from the Tropical Rainfall Measuring Mission satellite between 20°S and 20°N are compared with archived Earth Radiation Budget Satellite (ERBS) scanner measurements for the same months over a 5-yr period (1985?89), significant scene-type dependent differences are observed in the SW channel. Full-resolution CERES SW unfiltered radiances are ≈7.5% (≈3 W m?2 equivalent diurnal average flux) lower than ERBS over clear ocean, as compared with ≈1.7% (≈4 W m?2 equivalent diurnal average flux) for deep convective clouds and ≈6% (≈4?6 W m?2 equivalent diurnal average flux) for clear land and desert. This dependence on scene type is shown to be partly caused by differences in spatial resolution between CERES and ERBS and by errors in the unfiltering method used in ERBS. When the CERES measurements are spatially averaged to match the ERBS spatial resolution and the unfiltering scheme proposed in this study is applied to both CERES and ERBS, the ERBS all-sky SW radiances increase by ≈1.7%, and the CERES radiances are now consistently ≈3.5%?5% lower than the modified ERBS values for all scene types. Further study is needed to determine the cause for this remaining difference, and even calibration errors cannot be ruled out. CERES LW radiances are closer to ERBS values for individual scene types?CERES radiances are within ≈0.1% (≈0.3 W m?2) of ERBS over clear ocean and ≈0.5% (≈1.5 W m?2) over clear land and desert.