All-Sky Surface Shortwave Downward Radiation Retrieval: Emphasizing Direct-Diffuse Separation and Adjacency Effect Correction

Surface shortwave downward radiation (SWDR) is a key parameter in the Earth's energy budget. Accurate estimation of SWDR is essential for understanding the interactions between the Earth-atmosphere system and global climate change. In particular, the direct and diffuse components of SWDR are critical for applications such as vegetation modeling, carbon cycle simulations, surface albedo estimation, and shortwave radiation correction in complex terrain areas. However, high-precision separation of direct and diffuse SWDR remains lacking. Additionally, current satellite-based SWDR studies often show limited accuracy, especially over highly reflective surfaces such as polar regions, where SWDR is frequently underestimated. This underestimation is attributed to the adjacency effect caused by highly reflective surfaces, which has rarely been quantitatively modeled.

Therefore, this study proposes a framework for shortwave radiation estimation that emphasizes the direct-diffuse SWDR separation and the adjacency effect correction. First, a unified shortwave radiation estimation algorithm is developed, allowing for the simultaneous estimation of total, direct, and diffuse SWDR. Second, an adjacency effect correction scheme is developed, which separately accounts for the influence of direct and diffuse SWDR components. This approach effectively mitigates the underestimation caused by adjacency effects over highly reflective surfaces. The proposed framework is simple and efficient, utilizing only satellite-observed radiance, angle, and elevation as inputs to achieve accurate inversion of total, direct, and diffuse SWDR. Validation using ground-based measurements from global observation networks demonstrates that this framework not only enables high-precision retrieval of SWDR components but also significantly reduces the underestimation caused by the adjacency effect over highly reflective surfaces. This scheme shows substantial potential for generating high-accuracy multi-component shortwave radiation products, advancing global energy budget analysis and related applications.