Determining the Relationship between the Total and Window Channel Nighttime Radiances for the Ceres Instrument

Observing Earth’s radiant energy budget from space is critical to improving our understanding of Earth’s climate system. The Earth Radiation Budget Experiment (ERBE) (Barkstrom 1984; Barkstrom and Smith 1986) was the first initiative to provide simultaneous observations of Earth’s radiant energy with identical instruments flying aboard separate satellites. The design of the ERBE instrument was based upon three complementary broadband radiometers which measured the shortwave (< 5 μm), longwave (> 5 μm), and total regions of the spectrum. Since any two of the ERBE radiometers could be used to simulate the third, a three channel intercomparison, based on redundancy, was available to uncover any changes in the relative sensitivities of the individual radiometers. Such a three channel intercomparison thus provided confidence in the application of the ERBE measurements over the lifetime of the instrument while mitigating the concern over instrument degradation. When the Clouds and the Earth’s Radiant Energy System (CERES) (Wielicki et al. 1996) instrument was designed, the longwave channel was replaced by an 8 to 12 μm infrared window channel. While such a substitution allowed for improved measurements of the radiant energy budget near the Earth’s surface, the straightforward ERBE-like three channel intercomparison was sacrificed. Characterizing the stability of the CERES instrument, however, is critical to understanding the measured radiances from CERES. Thus, we have undertaken an investigation to determine whether conditions exist wherein the CERES window channel measurements can accurately yield a one-to-one representation of broadband longwave radiances. We initiated the present study by obtaining CERES nighttime (i.e., darkside of the earth) measured radiances for the window and total channels for the first seven months of 1998. The nighttime total channel radiances are, of course, directly related to broadband longwave radiances. An examination of the CERES data reveals a highly correlated linear relationship between the window and total channel measurements for conditions corresponding to deep convective clouds. With the establishment of that relationship between the CERES window and total channel measurements, and thus a relationship between the CERES window channel measurements and broadband longwave radiances, we have been able to formulate a three channel intercomparison for the CERES instrument which is analogous to the ERBE three channel intercomparison.