Compensation for spectral darkening of short wave optics occurring on the cloud's and the Earth's radiant energy system

Abstract

Cloud's and the Earth's Radiant Energy System (CERES) is an investigation into the role of clouds and radiation in the Earth's climate system. Four CERES scanning thermistor bolometer instruments are currently in orbit. Flight model 1 (FM1) and 2 (FM2) are aboard the Earth Observing System (EOS) Terra satellite and FM3 and FM4 are aboard the EOS Aqua satellite. Each CERES instrument measures in three broadband radiometric regions: the shortwave (SW 0.3 − 5µm), total (0.3− > 100µm), and window (8 − 12µm). It has been found that both CERES instruments on the Terra platform imply that the SW flux scattered from the Earth had dropped by up to 2% from 2000 to 2004. No climatological explanation for this drop could be found, suggesting the cause was a drift in both the Terra instruments. However, the onboard calibration lamps for the SW channels do not show a change in gain of this magnitude. Experience from other satellite missions has shown that optics in the orbital environment can become contaminated, severely reducing their transmission of ultra-violet (UV) radiation. Since the calibration lamps emit little radiance in the UV spectral region it was suggested that contaminates could be responsible for an undetectable 'spectral darkening' of the CERES SW channel optics and hence the apparent drop in SW flux. Further evidence for this was found by looking at the comparison between simultaneous measurements made by FM1 and FM2. The proposed mechanisms for contaminant build up would not apply to a CERES instrument operating in the normal cross track scan mode. Indeed it was found from the comparison between CERES instruments on Terra that the response of the instrument operating in rotating azimuth plane (RAPS) mode consistently dropped relative to the other cross track instrument. Since at all times one of the instruments operates in cross track mode, where it is not subject to spectral darkening, it allowed that unit to be used as a calibration standard from which the darkening of the other RAPS instrument can be measured. A table of adjustment coefficients to compensate for this spectral darkening are therefore derived in this paper. These figures are designed to be multiplied by SW fluxes or radiances produced in the climate community using Edition 2 CERES data. SW CERES measurements that have been revised using these coefficients are therefore to be referred to as ERBE-like Edition2 Rev1 or SSF Edition2B Rev1 data in future literature. Current work to fully characterize the effect of spectral darkening on the instrument spectral response before the release of Edition 3 data is also described.

Advances in Understanding Top-of-Atmosphere Radiation Variability from Satellite Observations

“…Indeed, early analyses of the CERES SW data revealed significant decreases in albedo that were more pronounced for clear ocean scenes than cloudy scenes. Further, direct comparison of coincident CERES nadir radiances from CERES instrument pairs on both platforms were found to depend strongly on the instrument scan mode (Matthews et al 2005). The primary mode of operation of CERES instruments is the crosstrack mode (i.e., perpendicular to the groundtrack), but CERES instruments can also be operated in a rotating azimuth plane mode (RAP), where the scan head rotates in azimuth as it scans in elevation.…”

CERES Top-of-Atmosphere Earth Radiation Budget Climate Data Record: Accounting for in-Orbit Changes in Instrument Calibration

“…2016, 8,182 10 of 14 flux difference that is statistically significant at the 95% significance level (Figure 7b; Table 4). Regionally, the most pronounced decreases in daytime-nighttime LW TOA flux difference occur over the west and central Pacific Ocean and over the Indian Ocean east of Madagascar (not shown).…”

“…Early analyses of the CERES SW data revealed significant decreases in albedo that were more pronounced for clear ocean scenes than for cloudy scenes. In addition, direct comparison of coincident CERES nadir radiances from CERES instrument pairs on both platforms depended strongly upon the instrument scan mode, with more degradation occurring when the instruments were operated in RAP mode [8]. This degradation is likely due to the instrument scan plane being aligned with the spacecraft direction of motion during part of the RAP scan cycle, which makes the sensor more susceptible to increased UV exposure and molecular contamination.…”

“…However, direct comparisons of nadir radiances from instruments on the same satellite showed that the RAP instrument was degrading relative to the crosstrack instrument [8], and the magnitude of the degradation depended on scene type. To provide users a means of correcting for this prior to release of Edition 3, user applied revisions were produced for clear ocean and all-sky scenes [8]. In Edition 3, temporal changes in the RAP instrument's SW SRF were included in the processing, removing the need for user-applied revisions.…”

Radiation budget estimates over Africa and surrounding oceans: inter-annual comparisons

“…The accuracies of CERES are 0.5 and 1% respectively (Wielicki et al, 1996). For CERES, a SW calibration drift of 1.5% has been detected and seems to have occurred when the instrument operated in rotating scan mode (Matthews et al, 2005). This drift is corrected in our datasets.…”