How cloud geometry and solar zenith angle control 3D radiative effects

Most atmospheric models treat radiation as a 1D process, creating biases called 3D radiative effects. Modeled shortwave 3D radiative effects are positive with the sun overhead (1D surface flux is artificially dim) and negative when the sun is near the horizon. Using a comprehensive sample of 3D radiative effects from shallow cumulus, deep convection, and stratocumulus LES cloud scenes, we decompose the 1D to 3D change in surface flux by cause: changes due to the amount of intercepted direct radiation and scattered light produced (cloud cover), changes due to the fate of scattered light (transmissivity), and changes due to their covariance. The decomposition reveals that cloud cover is the primary driver for how 3D cloud radiative effects change as the sun lowers. Using this framework, we develop a simple, quantitative model and find that the sign change is an inevitability: across all clouds scenes and the broader parameter space explored, 3D radiative effects always change from positive to negative as the sun lowers. The sign change occurs because transmissivity enhancement remains roughly constant with solar zenith angle while cloud cover expands super-linearly, causing diminishing positive effects to eventually be outpaced by growing negative effects. Higher cloud aspect ratios (defined height-to-width) accelerate this transition; higher initial coverage delays it due to cloud overlap. The model improves process-level understanding, revealing the importance of accurately representing how clouds interact with the direct beam.