Remote sensing platforms have traditionally produced data which span widespread spatial scales but are temporally limited. A radiation budget data set that gives strong spatial coverage while simultaneously spanning the interdecadal timescale provides an important tool with which to study global change. In order to produce an extended data set by combining shorter data sets, such as the products of ERB and ERBE projects, it is crucial to verify that each data set is well calibrated throughout its lifetime, and to intercalibrate the data sets with each other.
The Nimbus-7 ERB outgoing longwave radiation and reflected solar data sets are nine years in length. Although the calibration removed most of the long term drift associated with the degradation of the reflected solar filter dome, there is a residual downward trend in OLR of up to 14 Wm-2decade-1 that appears over the Sahara desert. The trend is caused by the combination of the desert albedo reflecting more strongly with increasing wavelength and the severity of the filter dome degradation increasing with decreasing wavelength.
A preliminary calculation of the magnitude of the filter degradation effect is made. Laboratory and Thematic Mapper (TM) spectral reflectance data of desert sand are used to estimate the reflectivity of the desert surface as a function of wavelength. The degradation of the shortwave filter as a function of wavelength is estimated with a quadratic fit to the known deterioration rates of the reflected solar and near-infrared channels. Using these parameters, the magnitude of the trend explained by this effect is calculated to be 11 Wm-2decade-1, which is nearly as large as the trends in the data