In-Flight Spectral Characterization and Calibration Stability Estimates for the Clouds and the Earth’s Radiant Energy System (CERES)

Abstract

It is essential to maintain global measurements of the earth radiation budget (ERB) from space, the scattered solar and emitted thermal radiative fluxes leaving the planet. These are required for the purpose of validating current climate model predictions of the planet's future response to anthropogenic greenhouse gas forcing. The measurement accuracy and calibration stability required to resolve the magnitude of modelsuggested cloud-climate feedbacks on the ERB have recently been estimated. The suggestion is for ERB data to strive for a calibration stability of 60.3% decade 21 for scattered solar, 60.5% decade 21 for emitted thermal, and an overall absolute accuracy of 1 W m 22 . The Clouds and the Earth's Radiant Energy System (CERES) is currently the only satellite program to make global ERB measurements, beginning in January 1998. However, the new climate calibration standards are beyond those originally specified by the NASA CERES program for its edition 2 data release. Furthermore, the CERES instrument optics have been discovered to undergo substantial in-flight degradation because of contaminant issues. This is not directly detectable by using established calibration methods. Hence, user-applied revisions for edition 2 shortwave (SW) data were derived to compensate for this effect, which is described as ''spectral darkening.'' Also, an entirely new in-flight calibration protocol has been developed for CERES that uses deep convective cloud albedo as a primary solar wavelength stability metric. This is then combined with a sophisticated contamination mobilization/ polymerization model. The intention is to assign spectral coloration to any optical degradation occurring to the different CERES Earth observing telescopes. This paper quantifies the stability of revised edition 2 data. It also calculates stability, which the new protocols could give CERES measurements if used. The conclusion is that the edition 2 revisions restore the originally specified stability of CERES SW data. It is also determined that the climate calibration stability goals are reachable by using the new in-flight methodologies presented in this paper. However, this will require datasets of longer than approximately 10 yr. It will also require obtaining regular raster scans of the Moon by all operational CERES instruments.

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

“…Based upon data collected by prior missions, Matthews (2009) assumed the following expression for the loss of transmission with wavelength, or spectral degradation, in the SW:…”

Examination of long‐wave radiative bias in general circulation models over North Africa during May–July

“…GERB SW was found to be substantially larger than CERES, by 7.5% (Clerbaux et al, 2009); this discrepancy may be partially explained by CERES ground calibration issues (Matthews, 2009). Clerbaux et al (2009) also found LWc to be around 2% or 5 W m −2 lower than CERES.…”

“…The main source of uncertainty in the CERES and ERBS radiative fluxes is absolute calibration which is quoted as being 2% in the short-wave (SW) channel and 1% in the total channel (Loeb et al, 2009); the LW error is estimated by Loeb et al (2009) to be 1.5% of the mean flux, or around 4.5 W m −2 for a LW flux of 300 W m −2 , typical of desert conditions. (Matthews, 2009) argues, based upon consideration of ground calibration techniques and vicarious calibration using deep convective cloud tops, that current CERES products underestimate daytime LW by around 0.75% and SW albedo by 1-3.5%. We assume a monthlymean LW uncertainty of around 5 W m −2 for ERBS and CERES datasets.…”

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

“…We derive α for each month of the Terra and Aqua missions assuming spectral darkening in the SW only occurs for the instrument in RAP mode. During RAP mode, the instrument scans in the ram direction of travel and is therefore more susceptible to molecular contamination [14]. The RAP instrument's α in a given month is selected from a set of candidate values, after accounting for time varying gain coefficient changes in filtered radiances.…”

“…Those results showed a consistency of 0.3 Wm´2 per decade at the 95% confidence level. molecular contamination [14].…”