Spectral Extension of the CPF‐Measured Earth‐Reflected Solar Radiances for CPF‐CERES Intercalibration

Abstract The Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder (CPF) mission is set to launch an SI‐traceable reflective solar (RS) spectrometer aboard the International Space Station to measure Earth‐reflected solar radiation with a radiometric uncertainty of 0.3%–0.6% ( k = 1). This paper introduces a fast and accurate spectral extension algorithm designed to expand CPF‐like spectral radiances beyond the CPF spectral range (0.35–2.3 μm) to align with the CERES SW spectral range for its filtered (0.2–5 μm) and unfiltered (0.2–15.0 μm) radiances, facilitating accurate CPF‐CERES intercalibration. The algorithm demonstrates excellent accuracy in generating the missing energy within the UV and IR segments of the CERES SW range. The standard deviation is 0.2% for the integrated unfiltered radiances and 0.05% for the integrated filtered radiances. The bias is only −0.01% for the unfiltered case and −0.003% for the filtered case. The methodology was validated using measured EMIT radiance data. We employed EMIT radiances within the wavelength range of 0.43–2.25 μm to generate radiances for both the shorter (0.381–0.43 μm) and longer wavelength range (2.25–2.493 μm). The standard deviation in the integrated broadband radiances is about 0.1%, and the bias is less than 0.004% for over 1.5 million EMIT samples. These statistics suggest that the spectral extension algorithm is robust and effective in substantially reducing the spectral difference‐induced uncertainty in the CPF‐CERES intercalibration.