Towards a reliable GCM estimation of contrail radiative forcing

[1] Selected results of the ECHAM4 general circulation model are compared with sophisticated radiative transfer model calculations in order to investigate the performance of the ECHAM4 radiation schemes with respect to the radiative forcing (RF) of contrails. The mean shortwave RF is found to be in accordance with radiative transfer model results. For the longwave part it is shown that the effective emissivity approach combined with the assumption of maximum-random cloud overlap used in the standard ECHAM4 leads to a considerable underestimation of the longwave contrail RF. Hence, this approach cannot be recommended for calculating the longwave RF of optically thin clouds in general. As an alternative, more adequate results of a modified version of the ECHAM4 longwave scheme are discussed.