Influence of three-dimensional radiative effects on the spatial distribution of shortwave cloud reflection

This paper examines how three-dimensional radiative effects influence the w ay cloud fields appear in high-resolution shortwave satellite images. To do so, it uses cloud reflectance fields simulated by a Monte Carlo radiative transfer model. This study examines the influence of the two counteracting three-dimensional mechanisms: the smoothing effect of radiative diffusion, which can reduce brightness variations, and the sharpening effect caused by thick areas intercepting extra radiation through their sides and casting s hadows on the thin areas behind them, which can enhance brightness variabil ity. The findings suggest that current high-resolution retrievals of cloud structure can be significantly biased because they do not take these effect s into account. For oblique sun. high-resolution retrievals can overestimat e both the scene-averaged optical thickness and the magnitude of cloud vari ability, and yield systematically distorted cloud shapes and artificially a nisotropic cloud structures. It is shown that the biases are especially lar ge when cloud-top height variations are present because cloud-top variation s can cause a much stronger sharpening effect than internal cloud variabili ty. To prevent erroneous interpretation of retrieval results, an algorithm is proposed to determine whether retrievals based on any satellite image ar e affected significantly by these biases.