The influence of neighbouring clouds on the clear sky reflectance studied with the 3-D transport code RADUGA

Operational remote sensing of terrestrial atmosphere is heavily based on the 1-D radiative transfer equation. However, cloudy scenes are influenced by 3-D effects (e.g., illumination from cloud sides, etc.). This leads to biases in aerosol/cloud/land/ocean retrieval schemes for scenes with clouds. These biases can be understood and quantified only with the use of the 3-D radiative transfer theory, which allows to account for arbitrary spatial variation of atmospheric parameters. The task of this paper is twofold. First of all we introduce a novel technique for the solution of the 3-D radiative transfer equation based on the grid approximations and the straightforward iteration procedure realised on supercomputers with parallel architecture. We study the performance of our technique comparing with the solutions obtained by the Monte-Carlo code. A close correspondence is found. Secondly, we quantify the influence of neighbouring clouds on the clear sky reflection function at the nadir observation depending on the solar illumination conditions. We find that the influence of cloud on the clear sky reflectance function is not negligible (even outside the cloud geometrical shadow). Thus, the peculiar inner boundary layer arises in the sky reflectance function with shadowing and brightening effects.