A 50-member Ensemble of Data Assimilations
Lang, Simon ; Hólm, Elías ; Bonavita, Massimo ; Trémolet, Yannick
Since 2010, ECMWF has run an Ensemble of Data Assimilations (EDA) to help determine the initial conditions for its ensemble forecasts and its higher-resolution deterministic forecast. The EDA is an ensemble of 4D-Var data assimilations that reflects uncertainties in observations; atmospheric boundary conditions, such as sea-surface temperature; and the model physics. The role of the EDA is twofold: it contributes to the high-resolution initial conditions (the high-resolution analysis) by providing flow-dependent estimates of the errors in the short-range forecasts (the background) used in the data assimilation system; and it helps to determine the initial conditions for ensemble forecasts by providing EDA-based perturbations to the high-resolution analysis. In 2013, the number of EDA members was increased from 10 to 25. We have now developed a new, optimised 50-member EDA configuration that has a comparable computational footprint to the current operational 25-member configuration. The increase in ensemble size improves both the high-resolution analysis and the EDA-based perturbations to the initial conditions for the 50-member ensemble forecast. The change is due to be implemented in the next upgrade of ECMWF's Integrated Forecasting System (IFS Cycle 46r1), scheduled for 2019.
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