The CERA Project: Building of a Coupled Data Assimilation System for Reanalysis at ECMWF
De Boisseson, Eric; Balmaseda, Magdalena; Mogensen, Kristian; Laloyaux, Patrick

As part of the ESA Data Assimilation Projects, a coupled data assimilation system for reanalysis called CERA (Coupled ECMWF ReAnalysis) is being developed at ECMWF. The CERA project ultimately aims at generating a consistent climate system state for climate studies and forecast purposes. CERA is based on the ECMWF ocean-atmosphere coupled system. The atmospheric component is based on the IFS software from ECMWF and the oceanic component is based on the NEMO framework. In CERA, the assimilation of atmospheric and oceanic observations is done separately. The atmospheric assimilation is done through a 4D-Var technique while the ocean assimilation relies on the NEMOVAR system based on the 3D-Var FGAT technique. The building of the CERA system follows several intermediate steps. The first task (called CERA-S) consists in introducing a Sea Surface Temperature (SST) constraint in the coupled model to avoid the model drift while allowing the simulation of coupled processes. The second task, called CERA-A, is based on the existing EMCWF atmospheric data assimilation framework but uses a coupled model in the outer loops. Similarly, the third task, CERA-O, is based on the NEMOVAR ocean data assimilation framework but uses a coupled model in the outer loops. The CERA final product will consist in a merge of the CERA-S, CERA-A and CERA-O systems. This presentation will describe the concept of the different tasks and the first validation and results obtained from each of them.