Seasonal prediction is one important element in a seamless prediction chain between weather forecast and climate projections. After several years of common development in collaboration with Universität Hamburg and Max Planck Institute for Meteorology, the Deutscher Wetterdienst performs operational seasonal forecasts since 2016 with the German Climate Forecast System, now in Version 2 (GCFS2.0). Here, the configuration of previous system GCFS1.0 and the current GCFS2.0 are described and the performance of the two systems is compared over the common hindcast period of 1990-2014. In GCFS2.0, the forecast skill is improved compared to GCFS1.0 during boreal winter, especially for the Northern Hemisphere where the Pearson correlation has doubled for the North Atlantic Oscillation index. During boreal summer, overall a similar performance of GCFS2.0 in comparison to GCFS1.0 is assessed. Future developments for climate forecasts need a stronger focus on the performance of seasonal dependent processes in a model system.

The domain nesting of the icosahedral non-hydrostatic (ICON) model has been used operationally at Deutscher Wetterdienst for several years. Now it was also made available for the atmospheric part of the ICON Earth system model. With this new climate configuration, regionally higher resolved simulations without the additional use of a separate regional climate model (RCM) are possible. Simulations were performed for the years 1979-2010 at a global resolution of about 80 km and a subdomain over Europe at 40 km resolution. Two simulations with this setup were evaluated and compared: one with a feedback from the regional subdomain to the global domain (two-way nesting) and one without feedback (one-way nesting). The mean atmospheric state of both simulations on the global scale is only slightly different compared to a reference experiment. However, comparisons to reanalyses show regionally distinct biases. The feedback from the subdomain to the global domain has a similar impact over Europe as a globally higher resolution, indicating a stronger North-Atlantic Oscillation at higher horizontal resolution. Over Europe, the skill is higher in the subdomain than in the global domain, but no systematic advantages can be attributed to the feedback. Artifacts at the lateral boundaries of the regional subdomain, as they are known from RCM simulations, also occur strongly in the simulation without feedback and are eliminated by allowing the feedback. A further reduction of resolution dependency of model physics is supposed to improve particularly the simulation with feedback.