Determination of wavelength-dependent spectral darkening occurring on a broadband Earth observing radiometer: application to clouds and the Earth's radiant energy system (CERES)

It is estimated that in order to best detect real changes in the Earth's climate system, space based instrumentation measuring the Earth Radiation Budget (ERB) needs to remain calibrated with a stability of 0.3% per decade. This stability is beyond the specification of existing ERB programs such as the Clouds and the Earth's Radiant Energy System (CERES, using three broadband radiometric scanning channels: the shortwave 0.3 - 5μm, total 0.3- > 100μm, and window 8 - 12μm). It is known that when in low earth orbit, optical response to blue/UV radiance can be reduced significantly due to UV hardened contaminants deposited on the surface of the optics. Typical onboard calibration lamps do not emit sufficient energy in the blue/UV region, hence this darkening is not directly measurable using standard internal calibration techniques. This paper details a study using a model of contaminant deposition and darkening, in conjunction with in-flight vicarious calibration techniques, to derive the spectral shape of darkening to which a broadband instrument is subjected. The model ultimately uses the reflectivity of Deep Convective Clouds as a stability metric. The results of the model when applied to the CERES instruments on board the EOS Terra satellite are shown. Given comprehensive validation of the model, these results will allow the CERES spectral responses to be updated accordingly prior to any forthcoming data release in an attempt to reach the optimum stability target that the climate community requires.