A 3D Monte Carlo radiative transfer model is used to demonstrate the importance of cloud shape and water vapor distribution on narrow‐band solar absorption at 0.93 and 2.0 µm. Diurnally averaged absorption for wavy‐topped broken cloud fields can exceed that based on conventional climate model assumptions (plane‐parallel cloud geometry and an unsaturated water vapor distribution in gaps between cloud elements) by 2–10% of the top‐of‐atmosphere insolation. Plane‐parallel clouds often underestimate the absorption by non‐flat‐top clouds, particularly at 2.0 µm and large solar zenith angles. Ambiguities in assigning the above‐cloud water vapor profile create uncertainties in the absorption comparisons between the plane‐parallel and non‐flat‐top clouds, which increase with solar zenith angle and may be as large as 5 to 8%. A thin saturated water vapor layer (0.4 km) above the cloud top systematically enhances column absorption, the magnitude depends on cloud altitude and wavelength. Thus, realistic 3‐D distributions of cloud shape, brokenness and water vapor are needed to quantify the role of clouds in excess absorption.
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Podgorny et al. (1998) studied this question.