Coupled data assimilation at ECMWF: current status, challenges and future developments

This article presents coupled data assimilation (DA) activities at the European Centre for Medium-Range Weather Forecasts (ECMWF). Coupled DA is an essential component of the ECMWF Earth-system strategy. It aims at providing consistent initial conditions to the coupled atmosphere, land, and ocean forecast model. The article introduces the different DA systems and observing systems for each Earth-system component. It discusses challenges related to observation consistency, availability, and sustainability across the components. It gives a review of coupling methodologies and presents coupling methods in development at the ECMWF. The current ECMWF system relies on weakly coupled DA approaches between land and atmosphere, between wave and atmosphere, and between ocean and atmosphere. Research on coupled DA has been centred on outer loop coupling developments and evaluation, focused on ocean-atmosphere coupling for reanalysis and for numerical weather prediction. The latest configuration of the ocean-atmosphere outer loop coupling combines weak and outer loop coupling methods. The article discusses the challenges in assimilating surface-sensitive observations and it presents opportunities that coupled DA offers to enhance the exploitation of interface observations. It presents recent developments on land-atmosphere forward operator coupling and multilayer snow-emission modelling and shows the potential for coupled radiative transfer modelling at low-frequency passive microwave. The article discusses ongoing developments of in-house sea-surface temperature analysis based on coupled skin temperature assimilation. It presents preliminary results that demonstrate meaningful upper ocean temperature increments when assimilating skin temperature information from four-dimensional variational assimilation. Coupled assimilation results are illustrated and discussed in the context of several applications related to reanalysis, scatterometer impact on coupled ocean-atmosphere, tropical cyclone, impact of weakly coupled sea-ice-atmosphere data assimilation as well as land surface coupled DA case studies. These examples illustrate benefits and challenges of coupled DA developments. The article discusses future plans of coupled DA developments in support of ECMWF's Earth-system 2021-2030 strategy.