A comparison of two major Earth radiation budget data sets

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Abstract
The Earth radiation budget data obtained from the Nimbus 7 Earth Radiation Budget (ERB) and the Earth Radiation Budget Experiment (ERBE) spacecraft missions are compared for several overlapping months during the period November 1984 to January 1986. The Nimbus 7 ERB data set starts in November 1978 and extends through October 1987 (9 years). The ERBE data set starts in November 1984 with the ERBS satellite, which was supplemented by the NOAA 9 in January 1985 and the NOAA 10 in September 1986. Data from both the ERBS and the NOAA 9 are presently being released to the scientific community. The principal aim of the ERBE project is to obtain improved measurements of the diurnal variations in the Earth radiation budget in order to derive accurate radiation budget products. On the Nimbus 7 ERB, only the wide field of view sensors have been active since July 1980. During the comparison period, both the ERBS and NOAA 9 had active wide field of view, medium field of view, and narrow field of view (scanner) sensors. Top of the atmosphere, colocated wide field of view (WFOV) measurements are compared to estimate relative biases. Then all the time‐ and space‐averaged products are compared. Each WFOV instrument has a shortwave and a total spectral channel. The outgoing longwave radiation is taken as the difference between the total and shortwave readings. The Nimbus 7 WFOV shortwave sensor reads about 2.5% higher than the ERBS sensor, while the Nimbus 7 total channel reads about 1% below the ERBS channel near midnight and about 1% above it near noon. A comparative study of the time‐ and space‐averaged data products for April, July, and October 1985 and January 1986 shows that the Nimbus 7 ERB global averages agree in the mean with the combined ERBS/NOAA 9 scanner values to within 0.16% in the outgoing longwave radiation and 0.03% in the albedo. The agreement of the combined ERBS/NOAA 9 WFOV products to the ERBS/NOAA 9 scanner products is 0.17% in the outgoing longwave radiation and 3.55% in the albedo. Certain fortuitous approximations in the time‐ and space‐averaging algorithms appear to have helped the agreement in the final mean Nimbus 7 and ERBE products. Monthly and regional differences are somewhat larger than for the 4‐month mean global values.