Microphysical and radiative properties of stratocumulus clouds: the EUCREXmission 206 case study

In this conclusion paper, remote sensing retrievals of cloud optical thickn ess performed during the EUCREX mission 206 are analyzed. The comparison wi th estimates derived from in situ measurements demonstrates that the adiaba tic model of cloud microphysics is more realistic than the vertically unifo rm plane parallel model (VUPPM) for parameterization of optical thickness, The analysis of the frequency distributions of optical thickness in the clo ud layer then shows that the adiabatic model provides a good prediction whe n the cloud layer is thick and homogeneous, while it overestimates signific antly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thic kness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect Of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent a nd lifetime, is likely to be more significant than the Twomey effect. (C) 2 000 Elsevier Science B.V. All rights reserved.