Evaluating the Relative Stability of CERES, MISR and MODIS Radiances Over the Terra Record

The consistency of the calibration of instruments in orbit is essential to accurately determine any trends in the reflected short-wave (SW) radiation and in physical parameters retrieved using radiances. The 14-year record of measurements from the CERES, MISR and MODIS instruments onboard NASA’s Terra satellite allows us to evaluate the long-term relative stability of the radiances measured by these instruments. To examine the relative stability of the instruments’ calibration we use the CERES Single Scanner Footprint – MISR (SSFM) dataset. This dataset collocates radiances from CERES (Edition 3A), MISR (version F03_0024) and MODIS (Collections 4 and 5) and averages the narrowband radiances from the four MISR bands (446, 557, 672 and 868 nm) and two of the MODIS bands (650 and 858 nm) over the broadband CERES footprint. This provides us with a dataset of radiances matched in time, space and viewing geometry which we have extended to cover the 14 years of CERES Ed3A data currently available.

To evaluate the relative stability of the radiances we use a regression-based approach to calculate the bias between the different radiances relative to a reference year. The change in this bias over the record allows us to determine how the radiances from the different instruments are drifting relative to one another. Over all-sky ocean scenes, we find that the relative drift between the MISR narrowbands and CERES SW broadband radiance is within 1% per decade. The trend is larger when comparing the 866 nm (0.9±0.2% per decade), 672 nm (0.9±0.1%), and 557 nm (0.7±0.4%) bands than when comparing the 446 nm (0.0±0.2%) band. The MODIS 650 nm and 858 nm bands show drifts relative to CERES of 2.1±0.5% and 1.1±0.8% per decade. However, a portion of those trends appears to be related to a step-change of approximately 1.5% between 2008 and 2009. MODIS and MISR have a relative trend of 1.1±0.7% per decade between the 650 nm and 672 nm bands and no trend between the 858 nm and 868 nm bands. Overall, the relative stability between the different instruments is good, largely within the range of ±1.0% per decade, and all within the range ±2.1% per decade. These results lend support to the validity of time-series of radiation and physical parameters derived from these instruments.