Effect of clouds on direct aerosol radiative forcing of climate

Abstract

The effect of a cloud layer on top-of-atmosphere (TOA) aerosol radiative forcing is examined by means of a one-dimensional vertical column simulation. To span the range between nonabsorbing and strongly absorbing particles, (NH4)2SO4 and soot aerosols are considered individually and in internal and external mixtures. For a cloud layer embedded within an aerosol layer it is shown that direct aerosol radiative forcing still occurs. For a nonabsorbing aerosol a maximum in (negative) forcing actually occurs for a thin cloud layer (100 m thickness for the set of parameters considered). The presence of an embedded cloud layer enhances the heating effect of soot aerosol, producing, for thick clouds, forcing values as much as a factor of three over those under cloud-free conditions. An absorbing aerosol layer can lead to an increase of in-cloud solar heating rates by up to 3% for the parameter values considered here. A cirrus cloud layer above an aerosol layer leads to only modest changes of TOA aerosol forcing from those in the absence of the cloud layer; thus aerosol forcing in the presence of typical cirrus clouds cannot be neglected.