Spectral Unfiltering of ERBE WFOV Nonscanner Shortwave Observations and Revisiting its Radiation Dataset from 1985 to 1998

Wide-field-of-view (WFOV) nonscanner instruments were onboard NASA's Earth Radiation Budget Satellite (ERBS) and the NOAA-9 and NOAA-10 satellites, and provided broadband shortwave (SW) and longwave (LW) irradiances from 1984 to 1999. However, Lee et al. (2002) noted degradation in the WFOV SW dome transmissivity. To account for this degradation, these SW instruments were calibrated with the spectrally flat gray assumption. More recently, Loeb et al. (2012) showed higher degradation in the transmissivity of shorter wavelengths suggesting a need for both temporal and spectral dependent corrections for better calibration. Such an approach may also eliminate an additional adjustment that was applied to irradiances in the existing products to remove the observed trend of day-minus-night longwave irradiances (Wong et al. 2006). We plan to reprocess the ERBE WFOV nonscanner record by characterizing the spectral degradation of the SW dome transmissivity over time. Solar data observed by the WFOV SW nonscanner during calibration days are used to estimate a time and spectral dependent spectral response function (SRF). Coefficients derived from this SRF are then used to improve the irradiance estimate. In addition, since the spectrum of reflected irradiance depends on scene type, ISCCP-derived cloud properties and surface type are used. Preliminary results indicate that taking account of spectral degradation reduces the observed day-minus-night longwave irradiance trends in the tropics (20ON and 20OS) by similar to 34%, while almost all of the trend is removed in the region between 60ON and 60OS. This presentation explains the reprocessing approach and compares the existing and reprocessed ERBE dataset. Once ERBS measurements are calibrated against CERES instruments, this work allows for the generation of a long-term radiation datasets consistent with those provided by CERES.