The Clouds and Earth's Radiant Energy System (CERES) scanning radiometer has the capability of rotating in azimuth so as to scan at any angle relative to the orbit track. Thus, in addition to the usual cross-track scan, it can rotate in azimuth to scan in the orbit plane. When scanning in this manner, all of the footprints of the scan lie along the groundtrack of the spacecraft, and any scene along the groundtrack is viewed at a number of zenith angles. Used in this mode, the CERES scanning radiometer provides data which are very well-suited for computing limb-darkening functions, which describe the variation of Earth-emitted radiance with view zenith angle. The CERES scanning radiometer includes an 8-12 micron channel for measuring the radiance in the atmospheric window. This paper uses alongtrack data from the CERES instrument aboard the Tropical Rainfall Measuring Mission (TRMM) to compute limb-darkening models for broadband Earth-emitted radiation and for 8-12 micron radiation. Data from the CERES instrument aboard the TRMM spacecraft are limited by the orbit to regions within 37 degrees from the equator. Limb-darkening functions for the window channel are found to be very similar to those for broadband Earth-emitted radiation for most scene types.
Measured results are compared with MODTRAN calculations based on meteorological parameters from European Centre for Medium-Range Weather Forecasts (ECMWF). This comparison for the 8-12 micron window radiance for clear ocean shows the effects of variation of sea-surface emissivity with view angle. This effect is not discernible for broadband Earth-emitted radiance, for which the effects of surface details are greatly diminished by the atmosphere.