Effect of cloud inhomogeneity on direct radiative forcing due to aerosols

Abstract

The effect of including horizontal cloud inhomogeneity on the direct radiative forcing due to sulphate and soot aerosols is explored. Cloud inhomogeneity is represented using the gamma independent pixel approximation for optical depth. Using a two-stream radiation model, the assumption of plane-parallel clouds normally used in climate models is shown to systematically underestimate the magnitude of the negative radiative forcing due to sulphates and systematically overestimate the positive forcing due to soot aerosol. For overcast skies and a Northern Hemisphere mean aerosol profile, these biases can reach as much as 0.1 W m-2, representing up to 30% of the forcing for sulphates and 5% for soot. The bias introduced in forcing due to sulphate aerosol is much larger than would be expected from the effect of the different albedo produced by altering the treatment of cloud optical depth. For "global mean" conditions, considering both clear and cloudy regions, the biases are between 2 and 4% for all three quantities (around 0.04 W m-2 in radiative forcing). This bias can generally be reduced further by constraining the albedo to a fixed (observed) value, thereby using the plane parallel homogeneous approximation with a different (but incorrect) cloud optical depth. The sensitivity of these results to solar zenith angle, cloud properties, surface reflectance, the number of streams used in the radiative transfer model, and the relative humidity is also investigated. For regions with coincident overcast marine stratocumulus, high aerosol loading and high relative humidity, and an external mixture of soot and sulphate aerosol, absolute values of the bias in forcing due to sulphates could reach as much as 1.5 W m-2 for a solar zenith angle of 60°, this being around 15% of the total forcing. Copyright 2000 by the American Geophysical Union.