Feedback Coupling of Absorbed Solar Radiation by Three Model Atmospheres with Clouds

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Abstract
A study of the amount of solar radiation absorbed by the earth-atmosphere system as a function of the surface temperature is made comparing three model atmospheres with clouds. The atmospheres are generated as previously reported by Weare and Snell. This involves a quasi-isentropic expansion of moist surface air of given relative humidity. “Rainout” of condensate and the lapse rate are parameterized. The three atmospheres to be compared are a horizontally homogeneous diffuse thin cloud structure, a half-cloud .half-clear structure, and a variable fractional cloud cover, each normalized to give the proper albedo at a reference point representative of global annual average conditions. Radiative transfer calculations are made using the modified two-stream approximation and/or the Eddington approximation. The results indicate that with the diffuse thin cloud the magnitude of the feedback coupling of solar radiation absorbed to surface temperature lies intermediate to that of the other structures, with the variable fractional cloud showing the largest negative feedback. The negativity decreases with increasing surface reflectivity and may become positive at reflectivities representative of snow or ice. The negativity also decreases slightly with a decrease in zenith angle of the sun and with an increase in surface relative humidity. Implications of these results in global climatic modeling are discussed.

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