The CAM/IMPACT/CoCiP Coupled Climate Model: Radiative forcing by aircraft in spreading contrails and large-scale cirrus

Radiative forcing by aircraft soot in large-scale clouds has been estimated to be both positive and negative, while forcing by contrails and contrail cirrus (i.e. spreading contrails) is positive. Here we use an improved model to estimate the forcing in large-scale clouds and by spreading contrails and evaluate the effects of coupling the hydrological cycle within CAM with the CoCiP contrail model. The large-scale cloud effects assume that the fraction of soot particles that have been processed through contrails are good heterogeneous ice nuclei (IN). We explore the effect of sulfate deposition on soot in decreasing the ability of contrail-processed soot to act as IN. The calculated total all-sky radiative climate forcing with and without coupling of CoCiP to the hydrological cycle within CAM and its range is reported. We discuss what is needed to narrow the range.