Coupled assimilation developments for operational NWP at ECMWF
This paper presents coupled assimilation developments at the European Centre for medium-Range Weather Forecasts (ECMWF) for operational Numerical Weather Prediction (NWP) and future operational reanalyses. ECMWF strategy is to move toward an Earth system approach, where coupled assimilation provides consistent initial conditions to a coupled forecast model, including atmosphere, land, ocean, wave and sea ice components.
Recent improvements in Weakly Coupled Data Assimilation (WCDA) between land and atmosphere consist of making use of the Ensemble Data Assimilation to compute the Jacobians in the weakly coupled extended Kalman filter (EKF) soil moisture analysis. This approach enables a dynamic link to the meteorological conditions, providing a new component to land-atmosphere coupling. It also reduces the cost of the EKF, which opens the perspectives for a stronger outer-loop coupling (quasi-strongly coupled data assimilation - Quasi-SCDA) between land and atmospheric data assimilation systems.
For ocean, sea-ice and atmosphere different coupling approaches have been evaluated, ranging from weakly to quasi-SCDA. WCDA was implemented in operations for sea ice in 2018 and it is considered for the ocean in 2019. Quasi-SCDA ocean-atmosphere is under evaluation for NWP and demonstrated promise in tropical areas, but exposed some challenging issues in the extratropics. Combined WCDA and Quasi-SCDA approaches are also under investigation, to address the ocean observation latency issue in an operational context.
Moving toward operational coupled assimilation also requires major work on system infrastructure, as well as on ocean, sea ice and land observing systems at all levels (governance, acquisition, pre-processing, archiving, monitoring, etc…) to ensure that they satisfy operational NWP and reanalysis systems requirements and constraints.
Dr. Patricia de Rosnay ECMWF