干旱区生态环境知识资源中心

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 approximate to0.4% or approximate to1 W m(-2) equivalent flux), less than 0.2% for LW radiances (std dev approximate to0.1% or approximate to0.3 W m(-2) equivalent flux), and less than 0.2% (std dev approximate to0.1%) 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 degreesS and 20 degreesN 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 approximate to7.5% (approximate to3 W m(-2) equivalent diurnal average flux) lower than ERBS over clear ocean, as compared with approximate to1.7% (approximate to4 W m(-2) equivalent diurnal average flux) for deep convective clouds and approximate to6% (approximate to4-6 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 approximate to1.7%, and the CERES radiances are now consistently approximate to3.5%-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 approximate to0.1% (approximate to0.3 W m(-2)) of ERBS over clear ocean and approximate to0.5% (approximate to1.5 W m(-2)) over clear land and desert.