Angular Distribution Models for Top-of-Atmosphere Radiative Flux Estimation from the Clouds and the Earth’s Radiant Energy System Instrument on the Terra Satellite. Part II: Validation

Abstract

Errors in top-of-atmosphere (TOA) radiative fluxes from the Clouds and the Earth?s Radiant Energy System (CERES) instrument due to uncertainties in radiance-to-flux conversion from CERES Terra angular distribution models (ADMs) are evaluated through a series of consistency tests. These tests show that the overall bias in regional monthly mean shortwave (SW) TOA flux is less than 0.2 W m?2 and the regional RMS error ranges from 0.70 to 1.4 W m?2. In contrast, SW TOA fluxes inferred using theoretical ADMs that assume clouds are plane parallel are overestimated by 3?4 W m?2 and exhibit a strong latitudinal dependence. In the longwave (LW), the bias error ranges from 0.2 to 0.4 W m?2 and regional RMS errors remain smaller than 0.7 W m?2. Global mean albedos derived from ADMs developed during the Earth Radiation Budget Experiment (ERBE) and applied to CERES measurements show a systematic increase with viewing zenith angle of 4%?8%, while albedos from the CERES Terra ADMs show a smaller increase of 1%?2%. The LW fluxes from the ERBE ADMs show a systematic decrease with viewing zenith angle of 2%?2.4%, whereas fluxes from the CERES Terra ADMs remain within 0.7%?0.8% at all angles. Based on several months of multiangle CERES along-track data, the SW TOA flux consistency between nadir- and oblique-viewing zenith angles is generally 5% (<17 W m?2) over land and ocean and 9% (26 W m?2) in polar regions, and LW TOA flux consistency is approximate 3% (7 W m?2) over all surfaces. Based on these results and a theoretically derived conversion between TOA flux consistency and TOA flux error, the best estimate of the error in CERES TOA flux due to the radiance-to-flux conversion is 3% (10 W m?2) in the SW and 1.8% (3?5 W m?2) in the LW. Monthly mean TOA fluxes based on ERBE ADMs are larger than monthly mean TOA fluxes based on CERES Terra ADMs by 1.8 and 1.3 W m?2 in the SW and LW, respectively.