In-Flight Calibration of the Nimbus-7 Earth Radiation Budget (ERB) Sensors. Part III: Long-Term Changes

Abstract

Sensitivity changes in the four wide-field-of-view (WFOV) Nimbus-7 earth radiation budget (ERB) sensors were monitored over a 9-yr period (November 1978?October 1987) by use of a number of reference sources. The sun was the primary reference and was used to check the shortwave (SW; about 0.2?4 ?m) sensitivities on the twin total channels 11 and 12. The longwave (LW; greater than 4 ?m) sensitivity in channel 12 was checked by a time series analysis of the nighttime mean global terrestrial signal, but the method could not be usefully applied to channel 11 because it was shuttered too much of the time. The accuracy of this type of analysis was verified by comparing a similar shortwave time series analysis with the solar calibration results. It was also checked by comparing channel 12 nighttime measurements with those from the companion scanner. The scanner had a built-in blackbody for calibration, but the scanner failed after 20 months. As a result of this comparison, a bias adjustment of 12.6 W m?2 was made in the channel 12 measurements. In addition, channels 11 and 12 were compared to each other. The shortwave channels 13 (0.2?4 ?m) and 14 (0.7?2.8 ?m) were covered by Suprasil-W domes that blocked radiation greater than 4 ?m. A piece of red glass in channel 14 further restricted its spectral range to the near infrared. After launch, these domes fogged asymmetrically. For this reason, the effective sensitivity changes in these channels were monitored by comparison with channel 12 using the whole earth as a transfer target. The shortwave range mentioned above for channels 11 and 12 really refers to channel 13 and not to channels 11 and 12. By October 1987, the following sensitivity decreases had occurred: channel 11 (no observable change), channel 12 (LW 2.5%, SW 1.5%), channel 13 (13.3%), and channel 14 (6%). Corrections for these changes kept the calibrated signals stable to better than 0.5% over the 9-yr period. Year-to-year annual global mean longwave shifts of 0.1%?0.4% have been related to climate perturbations and appear real.