Contrails in a comprehensive global climate model: Parameterization and radiative forcing results

[1] A parameterization of contrails for use in comprehensive global climate models is introduced. It is based on the thermodynamic theory of contrail formation, which has been applied in a consistent way with the cloud parameterization scheme of the version 4 European Center/Hamburg General Circulation Model. Both the coverage and the optical properties of contrails are calculated as functions of instantaneous values of atmospheric variables as they are provided by the climate model. The resulting spatial distributions of contrail parameters prove to be useful for explaining observed differences between contrails in different geographical regions. The time mean properties of the simulated contrails are in fair agreement with observations, though the values of ice water path and optical depth tend to be somewhat lower than those reported from in situ measurements. The radiative forcing of contrails resulting from the climate model experiments is substantially lower than estimated in a previous study, where mean parameters for contrails and the ambient atmosphere were prescribed in a radiative transfer model. One contribution to this disagreement arises from the smaller mean ice water content in the climate model simulations. However, the largest part must be related to a different treatment of the interference of contrails with natural high clouds. The sensitivity of the contrail radiative forcing to systematic errors in simulated ambient atmospheric variables (like temperature, humidity, and natural clouds) as well as to the parameterization of cloud and contrail overlap needs to be investigated further.