Parameterization of cloud optical properties for semidirect radiative forcing

Abstract

[1] A parameterization of liquid cloud optical properties with a mixture of black carbon is proposed. It is found that the changes in cloud optical properties due to mixture of black carbon can be treated as a perturbation to existing cloud optical property parameterizations in climate models. The advantage of the proposed scheme is that current cloud optical property parameterizations used in climate models can be kept. It is shown that the dominant factor with respect to radiative forcing due to the inclusion of black carbon in cloud droplets is the resulting change in single-scattering albedo values. Therefore, a simple scheme to modify only the single-scattering albedo is considered. The additional consideration of the modification of asymmetry factor only applies to the case of very large black carbon volume fraction. The results in a one-dimensional radiation model show that internal mixtures of black carbon can have a significant impact on solar flux and heating rates. For a black carbon volume fraction of 10−7, the reduction in solar flux at the top of the atmosphere can be over 0.5 Wm−2 and the heating rate can increase by about 0.08 Kd−1 for a solar zenith angle of 53°.