Haywood, J. M., and K. P. Shine, 1997: Multi-spectral calculations of the direct radiative forcing of tropospheric sulphate and soot aerosols using a column model. Quarterly Journal of the Royal Meteorological Society, 123(543/544), 1907-1930.

Abstract: A newly developed multi-spectral radiative-transfer code is used to investigate the important features of the direct radiative forcing of sulphate and soot aerosol; the indirect effect of both aerosol species is not investigated in this study. The direct radiative forcing is presented for different surface albedos, solar zenith angles, relative humidities and aerosol vertical profiles together with effects upon the surface irradiance and long-wave radiative forcing. The effect of subgrid-scale variations in relative humidity are examined using idealized relative humidity distributions. The results show that subgrid-scale variations in relative humidity and the spatial correlation between cloud and areas of high relative humidity should be considered in future general-circulation model calculations of the direct forcing due to sulphate aerosol. A comparison of the direct forcing obtained by adjusting the surface albedo to that using the full multi-spectral column calculation is performed; the results indicate that recent estimates of the climate response to the direct forcing of sulphate may be too large. The contribution to the direct forcing from cloudy-sky regions appears to be negligible for sulphate aerosol but there is a considerable enhancement of the forcing due to soot aerosol if soot exists within or above clouds. These calculations show that a small amount of soot, relative to the sulphate mass loading, can cause a significant positive direct forcing and emphasize that the vertical profile of soot aerosol relative to cloud must be established to enable accurate assessment of the direct radiative effects of anthropogenic emissions of soot aerosol.