Understanding top-of-atmosphere flux bias in the AeroCom Phase III
models: a clear-sky perspective
Abstract
Biases in aerosol optical depths (AOD) and land surface albedos in the
AeroCom models are manifested in the top-of-atmosphere (TOA) clear-sky
reflected shortwave (SW) fluxes. Biases in the SW fluxes from AeroCom
models are quantitatively related to biases in AOD and land surface
albedo by using their radiative kernels. Over ocean, AOD contributes
about 25% to the 60°S-60°N mean SW flux bias for the multi-model mean
(MMM) result. Over land, AOD and land surface albedo contribute about
40% and 30%, respectively, to the 60°S-60°N mean SW flux bias for the
MMM result. Furthermore, the spatial patterns of the SW flux biases
derived from the radiative kernels are very similar to those between
models and CERES observation, with the correlation coefficient of 0.6
over ocean and 0.76 over land for MMM using data of 2010. Satellite data
used in this evaluation are derived independently from each other,
consistencies in their bias patterns when compared with model
simulations suggest that these patterns are robust. This highlights the
importance of evaluating related variables in a synergistic manner to
provide an unambiguous assessment of the models, as results from single
parameter assessments are often confounded by measurement uncertainty.
We also compare the AOD trend from three models with the
observation-based counterpart. These models reproduce all notable trends
in AOD (i.e. decreasing trend over eastern United States and increasing
trend over India) except the decreasing trend over eastern China and the
adjacent oceanic regions due to limitations in the emission dataset.