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Lars Wiegand, Arwen Twitchett, Cornelia Schwierz, and Peter Knippertz

assessing such limitations than deterministic forecasts alone ( Tracton and Kalnay 1993 ). To represent initial condition uncertainties, different methods are used. The European Centre for Medium-Range Weather Forecasts (ECMWF) uses singular vectors to achieve maximum perturbation growth for a given (48 h) optimization time ( Palmer et al. 1992 ). The National Centers for Environmental Prediction (NCEP) use the ensemble transform technique ( Wei et al. 2008 ), which is an improved version of the

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Paraskevi Giannakaki and Olivia Martius

introduce a tool for the quantification and characterization of errors in the representation of synoptic-scale Rossby waveguides, that is, areas of elongated strong PV gradients that are collocated with the jet streams and the dynamical tropopause in forecast data. Numerous object-based verification techniques have been developed in recent years [see Gilleland et al. (2009) and Jolliffe and Stephenson (2012) for details]. Here, we modify the Method for Object-based Diagnostic Evaluation (MODE

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Lisa-Ann Quandt, Julia H. Keller, Olivia Martius, and Sarah C. Jones

1. Introduction As atmospheric blocking may cause high-impact weather like heat waves and flooding (e.g., Matsueda 2009 ), there is a special interest in its predictability and the physical processes that limit forecast quality. Blocking is a large-scale flow pattern that decelerates arriving eddies, as well as the background flow, and forces them to follow a more meridional direction (e.g., Rex 1950 ; Arakawa 1952 ; Sumner 1954 ). Atmospheric blocking is persistent and self-sustaining (e

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