Wednesday, 5 June 2002: 9:45 AM
Enhanced absorption of solar radiation by 3d mixed phase clouds
For several years a potential discrepancy between theoretically expected and observationally derived solar broadband cloud absorption termed 'anomalous cloud absorption' has been discussed. The existence of such large absorption is questionable but so is the strongly simplified treatment of solar radiative transfer in most atmospheric models. The present paper is exploring the potential of more realistic 3d inhomogeneous clouds to provide more absorption then their homogeneous counterparts. A large number of fully 3d inhomogeneous clouds (3d extinction coefficient, 3d scattering and absorption properties) with strong variability in their mixed phase microphysical properties have been taken into account. Solar broadband radiative fluxes have been calculated by means of a Monte Carlo radiative transfer code. While previous work was concentrated on variable cloud extinction coefficients only, our results demonstrate that local positive correlation between cloud extinction and and particle size (i.e. absorptivity) leads to an enhanced absorption exceeding 15 Wm^-2 compared to the same clouds with horizontally averaged cloud particle scattering and absorption properties. These 3d effects of the particle scattering and absorption properties are strongest at small solar zenith angles and for the most inhomogeneous convective clouds types. Averaged over all cloud realizations under consideration, the enhanced absorption ranges from 1 Wm^-2 at 75 degree solar zenith angle to 4 Wm^-2 at 15 degree solar zenith angle.