Effects of cloud shape and water vapor distribution on solar absorption in the near infrared

Abstract

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 nonflat-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.