Next-generation angular distribution models for top-of-atmosphere radiative flux calculation from CERES instruments: Validation

Abstract

Radiative fluxes at the top of the atmosphere (TOA) from the Clouds and the Earth's Radiant Energy System (CERES) instrument are fundamental variables for understanding the Earth's energy balance and how it changes with time. TOA radiative fluxes are derived from the CERES radiance measurements using empirical angular distribution models (ADMs). This paper evaluates the accuracy of CERES TOA fluxes using direct integration and flux consistency tests. Direct integration tests show that the overall bias in regional monthly mean TOA shortwave (SW) flux is less than 0.2 Wm -2 and the RMSE is less than 1.1 Wm -2. The bias and RMSE are very similar between Terra and Aqua. The bias in regional monthly mean TOA LW fluxes is less than 0.5 Wm -2 and the RMSE is less than 0.8 Wm -2 for both Terra and Aqua. The accuracy of the TOA instantaneous flux is assessed by performing tests using fluxes inverted from nadir- and oblique-viewing angles using CERES along-track observations and temporally and spatially matched MODIS observations, and using fluxes inverted from multi-angle MISR observations. The averaged TOA instantaneous SW flux uncertainties from these two tests are about 2.3 % (1.9 Wm -2) over clear ocean, 1.6 % (4.5 Wm -2) over clear land, and 2.0 % (6.0 Wm -2) over clear snow/ice; and are about 3.3 % (9.0 Wm -2), 2.7 % (8.4 Wm -2), and 3.7 % (9.9 Wm -2) over ocean, land, and snow/ice under all-sky conditions. The TOA SW flux uncertainties are generally larger for thin broken clouds than for moderate and thick overcast clouds. The TOA instantaneous daytime LW flux uncertainties derived from the CERES-MODIS test are 0.5 % (1.5 Wm -2), 0.8 % (2.4 Wm -2), and 0.7 % (1.3 Wm -2) over clear ocean, land, and snow/ice; and are about 1.5 % (3.5 Wm -2), 1.0 % (2.9 Wm -2), and 1.1 % (2.1 Wm -2) over ocean, land, and snow/ice under all-sky conditions. The TOA instantaneous nighttime LW flux uncertainties are about 0.5-1 % (< 2.0 Wm -2) for all surface types. Flux uncertainties caused by errors in scene identification are also assessed by using the collocated CALIPSO, CloudSat, CERES and MODIS data product. Errors in scene identification tend to underestimate TOA SW flux by about 0.6 Wm -2 and overestimate TOA daytime (nighttime) LW flux by 0.4 (0.2) Wm -2 when all CERES viewing angles are considered.