Spectral balancing of a broadband Earth observing radiometer with co-aligned Short Wave channel to ensure accuracy and stability of broadband daytime Outgoing Long-Wave Radiance measurements: Application to CERES

摘要

In order to best detect real changes in the Earth's climate system, it is estimated that in space based instrumentation measuring the Earth Radiation Budget (ERB) must remain calibrated with a stability of 0.3Wm~(-2) per decade and reach an absolute accuracy of 1Wm~(-2). Such stability is beyond that specified by existing ERB programs such as the Clouds and the Earth's Radiant Energy System (CERES, using three broadband radio-metric scanning channels: the shortwave (SW 0.3 - 5μm), Total (0.3- ＞ 100μm), and window (8 - 12μm)). The CERES measurement of daytime outgoing longwave radiance (OLR) is obtained using subtraction of the SW channel signal from that of the co-aligned Total channel telescope. This requires precise balancing of the estimated response of the Total channel optics with those of the SW only channel when viewing daytime Earth scenes. Any post ground calibration contamination of Total channel optics that reduces its response to SW radiance can therefore upset this balancing process, introducing biases and trends in measurements of daytime LW radiance. This paper presents a new methodology used for balancing Total and SW channel spectral responses for all daytime Earth scenes using a model of contaminant spectral darkening. The results of the technique when applied to both CERES units on Terra are shown to remove significant trends and biases in measurements of daytime LW radiance.