The coupled ICON Ocean-Atmosphere System and its Use for Weather-Scale Forecasts

The Earth System Modelling at the Weather Scale (ESM-W) project, a collaboration between the German Weather Service (DWD) and GeoInfoDienst BW, aims to develop a coupled ocean-atmosphere forecasting system. This system utilizes the ICON-O ocean model and the ICON-NWP atmospheric model.

In this presentation, we will showcase the advancements made in the ICON-based coupled global forecasting system. Notably, a near-real-time (NRT) system has been established, comprising a time-critical weakly coupled data assimilation cycle and coupled forecasts with lead times of up to ten days. The ocean component uses a 20km resolution while the atmospheric component uses the operational 13km resolution system. The ocean component assimilates ARGO buoy data and satellite-derived sea surface observations using 3D-VAR(-FGAT) methods, while the atmosphere employs an EnVAR method leveraging ensemble information from DWD’s operational routine. This system has been generating daily analyses and forecasts since May 2025 and is improved continuously.

We will present the coupled system’s developments and performance, including evaluations for both the data assimilation system and the model components. Additionally, we show verification against non-assimilated ocean datasets, such as fixed buoys.