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Data assimilation refers to the problem of finding trajectories of a prescribed dynamical model in such a way that the output of the model (usually some function of the model states) follows a given time series of observations. Typically though, these two requirements cannot both be met at the same time–tracking the observations is not possible without the trajectory deviating from the proposed model equations, while adherence to the model requires deviations from the observations. Thus, data assimilation faces a trade‐off. In this contribution, the sensitivity of the data assimilation with respect to perturbations in the observations is identified as the parameter which controls the trade‐off. A relation between the sensitivity and the out‐of‐sample error is established, which allows the latter to be calculated under operational conditions. A minimum out‐of‐sample error is proposed as a criterion to set an appropriate sensitivity and to settle the discussed trade‐off. Two approaches to data assimilation are considered, namely variational data assimilation and Newtonian nudging, also known as synchronization. Numerical examples demonstrate the feasibility of the approach. Copyright © 2011 Royal Meteorological Society

Sensitivity and out‐of‐sample error in continuous time data assimilation