In the framework of the EU FP7-funded project UERRA, the regional ensemble reanalysis system COSMO-REA12-ENS has been developed based on the COSMO-REA6 system. The continuous nudging scheme is extended through the assimilation of perturbed observations to account for the uncertainty in the measurements. Currently,  the production of a 5-year period and the evaluation of the system's performance is under way.