Parameterization of the solar radiative characteristics of low clouds and studies with a general circulation model

Abstract

A broadband parameterization that improves the quantitative estimates of the solar radiative characteristics of low clouds is developed using reference solutions. The accuracy of the parameterization in determining the shortwave cloud absorption for a wide variety of low‐cloud conditions is better than 20%. Other broadband treatments, which do not adequately account for the influences due to above‐ and in‐cloud water vapor and water drop extinction, are also considered to investigate the sensitivity to these factors. The computed northern hemisphere summertime fluxes reveal that (1) the absorbed solar flux in low clouds (Fabs) is overestimated at high latitudes if the effect of attenuation by the above‐cloud vapor is ignored in the determination of the water drop absorption, (2) Fabs is underestimated in the tropical regions if in‐cloud vapor absorption is not considered, and (3) the conservative scattering assumption for drops yields a substantial underestimate of Fabs at most latitudes. General circulation model simulations with fixed sea surface temperatures and cloud amounts further highlight the significance of the vapor and drop optical properties. Differences in the broadband treatment of the radiative interactions with vapor and drops in low clouds introduce changes in the solar fluxes absorbed by the atmosphere and the surface; for the cases considered here, the solar flux change at the top of the atmosphere differs in sign from that at the surface. The flux differences bring about changes in vertical motion and precipitation; these, in turn, are accompanied by perturbations in the various components of the land surface heat (e.g., latent and sensible heat losses) and moisture (e.g., soil moisture, evaporation) budgets. For approximately similar solar flux differences the changes in the vertical motion, precipitation, and land surface parameters are dissimilar in the tropical and the midlatitude continental regions. Thus because of the adjustments in the atmosphere and the coupling between the atmosphere and the land surface processes, solar flux differences due to biases or deficiencies in the radiative treatment of vapor and drops affect the simulation of the hydrologic fields and the heat balance, including the atmospheric and land surface temperatures.