Performance of two GCM borne radiation schemes in calculating contrail radiative forcing

Contrail radiative forcing is difficult to determine even if the numerous key parameters like coverage, ice water content, crystal size etc. are known. One reason is the considerable amount of cancellation between the positive (warming) component from the contrails' greenhouse effect and the negative (cooling) component from backscattering of solar irradiance. This sets high demands on the abilities of the radiative transfer model used to calculate the forcing, and a substantial respective uncertainty has been documented in literature. To complicate things further, the longwave/shortwave cancellation depends on some ambient parameters like ambient temperature, co-existing natural clouds, surface albedo and some others. \nClimate models are optimally suited to provide a representation of the required variety of ambient parameters for a climatological estimate of contrail radiative forcing. However, comprehensive global climate models have to use simplified radiative transfer schemes for reasons of computational economy. Hence, a dedicated test of these schemes is always indicated. We present a comparison of contrail radiative forcing between two contrail radiative forcing estimates from global climate models. The first is yielded with the ECHAM4 model frequently used for this purpose over the last ten years, the second by the more recent ECHAM5/EMAC model to be applied in the coming years. Use is made of the so-called "Myhre benchmark test" with specified contrail parameters. The ratio of longwave/shortwave cancellation for various seasons and the daytime/nighttime difference are features of particular focus. \nContrail radiative forcing is difficult to determine even if the numerous key parameters like coverage, ice water content, crystal size etc. are known. One reason is the high degree of cancellation between the positive (warming) component from the contrails' greenhouse effect and the negative (cooling) component from backscattering of solar irradiance. This sets high demands on the abilities of the radiative transfer model, and a considerable uncertainty of respective calculations has been documented in literature. Furthermore, the longwave/shortwave cancellation is sensitive to several ambient parameters like temperature, co-existing natural clouds, and surface albedo.