Data assimilation (DA) is a collective term for mathematical methods that consistently combine models with observations. A great advantage of DA is that the information contained in the observations can be used to derive knowledge about unobserved, e.g., unobservable, quantities. This can be achieved by exploiting physical or statistical links between observed and unobserved variables (Fig. 1).
DA in our section focuses mainly on geodetic observations, e.g., Earth rotation, satellite gravimetry and satellite altimetry [Saynisch et al., 2011a, 2015]. In addition, new technologies are tested in so called Observing System Simulation Experiments (OSSE) for their potential benefit to the respective research community, as e.g., the newly emerging GNSS-Reflectometry technology.
In general, the observations are assimilated with state-of-the-art numerical models of Earth’s sub-systems, e.g., atmosphere, oceans, mantle and cryosphere [Neef and Matthes, 2012, Irrgang et al., 2017, Bernales et al., 2017].
Prior to a successful assimilation, realistic error budgets have to be derived. On the one hand, these budgets describe realistic statistics of the observations ,e.g., from GRACE [Dobslaw et al., 2016] or GNSS-R [Semmling et al., 2016]. On the other hand, these budgets have to describe the uncertainties and sensitivities of the used numerical models, see Fig. 2 [e.g., Zhang et al., 2017, Dill et al., 2015].
For the model error estimation numerically expensive ensemble calculations are required [e.g., Irrgang et al., 2016]. As part of benchmark experiments, these ensembles are ideally based on very different models [Saynisch et al., 2017, Sachl et al., 2017]. The ensemble information is subsequently used in state-of-the-art Ensemble Kalman Filters [Irrgang et al., 2017]. Due to the typical high-dimensionality of models of Earth System components, the ensemble generation and the Kalman filters have to operate on small yet dynamically optimal subspaces [e.g., Nerger et al., 2005].
In addition to Kalman filter techniques, Section 1.3 also uses a range of variational or adjoint techniques [Saynisch et al., 2011b, 2015]. Based on the specific research question, the models and the formulation of the assimilation method can be regional or global, statistical or variational. In particular cases the same observations are assimilated with both adjoint and Kalman-filter techniques to increase the robustness of the results [cf., Saynisch and Thomas, 2012, Saynisch et al., 2011b].
As part of the international GEROS-ISS project, we could for the first time demonstrate the information gain of GNSS-Reflectometry measurements for the oceanographic community and could give specific recommendations for undecided questions of observations density and precision [Saynisch et al., 2015, Wickert et al., 2016]. Within the SMART-Cables project, which aims to fit telecommunication cables with oceanographic sensors, a cable deployment strategy could be proposed that bases on the respective cable’s assimilation impact. As part of the Dynamic Earth-SPP, pioneering work is done in studying the potential of the currently ongoing satellite magnetometer mission Swarm for oceanic assimilation purposes [Irrgang et al., 2017, Saynisch et al., 2017].
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