Influence of Cloud Retrieval Errors Due to Three Dimensional Radiative Effects on Calculations of Broadband Cloud Radiative Effect

Abstract

We investigate how cloud retrieval errors due to the three dimensional (3D) radiative effects affect broadband cloud radiative effects (CRE). A framework based on the combination of large eddy simulations (LES) and radiative transfer (RT) models was developed to simulate both one dimensional (1D) and 3D radiance, and shortwave (SW) broadband fluxes. Results show that the broadband SW fluxes reflected at top-of-the-domain, transmitted at the surface, and absorbed in the atmosphere, computed from the cloud retrievals using 1D-RT (called “1D-RT+retrieved clouds”) can provide reasonable broadband radiative energy estimates in comparison with those derived from the true cloud fields using 1D-RT (called “1D-RT+true clouds”). The difference between these 1D-RT simulated fluxes (1D-RT+retrieved clouds simulations, 1D-RT+true clouds simulations) and the benchmark 3D-RT simulations from the true cloud field (called “3D-RT+true clouds”), depends primarily on the horizontal transport of photons in 3D-RT, whose characteristics vary with the Sun’s geometry. When the solar zenith angle (SZA) is 5°, the domain-averaged fluxes simulated based on the 1D-RT+retrieved clouds are in excellent agreement with the 3D-RT+true clouds results, all within 7 % relative CRE bias. When the SZA is 60°, the differences between the results from the 1D-RT+retrieved clouds and 3D-RT+true clouds are determined by how the cloud side-illumination and shadowing effects offset each other in the radiance, retrieval, and broadband fluxes. This study suggests that although the cloud property retrievals based on the 1D-RT theory may be biased due to the 3D radiative effects, they still provide an observational basis for the estimation of broadband fluxes.