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Marlene Baumgart, Michael Riemer, Volkmar Wirth, Franziska Teubler, and Simon T. K. Lang

1. Introduction Numerical weather prediction has improved remarkably over the last decades (e.g., Bauer et al. 2015 ). Occasionally, however, very poor medium-range forecasts do still occur ( Rodwell et al. 2013 ). Forecast errors arise due to errors in the initial conditions and due to model deficiencies (e.g., Palmer and Hagedorn 2006 ). After 1–2 forecast days, localized errors may form that start to affect the synoptic-scale flow (e.g., Davies and Didone 2013 ; Martínez-Alvarado et al

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Kirstin Kober and George C. Craig

physically based stochastic perturbations will be introduced as well as the mesoscale weather prediction model used in this study and the verifying observational data. In section 3 , the method will be applied in several case studies representing two different weather regimes to assess the structure of the perturbations, their impact on the precipitation fields, and the sensitivities in parameter settings of the perturbations scheme. Additionally, forecast quality of other variables will be assessed

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Hilke S. Lentink, Christian M. Grams, Michael Riemer, and Sarah C. Jones

1. Introduction The extratropical transition (ET) of tropical cyclones (TCs) is associated with high-impact weather, both locally and in downstream regions ( Jones et al. 2003 ; Evans et al. 2017 ). The local direct impact is mainly caused by strong wind gusts and excessive precipitation. These are not always well forecasted because a numerical weather prediction model is prone to small errors that evolve during the complex interaction between a poleward-moving TC and its environment. The

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Julia H. Keller, Christian M. Grams, Michael Riemer, Heather M. Archambault, Lance Bosart, James D. Doyle, Jenni L. Evans, Thomas J. Galarneau Jr., Kyle Griffin, Patrick A. Harr, Naoko Kitabatake, Ron McTaggart-Cowan, Florian Pantillon, Julian F. Quinting, Carolyn A. Reynolds, Elizabeth A. Ritchie, Ryan D. Torn, and Fuqing Zhang

processes associated with mesoscale convective systems over the Great Plains has been identified as a source of short-term skill degradations (busts) in ECMWF forecasts for Europe ( Rodwell et al. 2013 ). In such cases, diabatic processes act to decelerate the eastward progression of a synoptic-scale trough over the Rocky Mountains, similar to the processes described in section 2b . Errors in the representation of these diabatic processes and their impact on the midlatitude flow may lead to large phase

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Volkmar Wirth, Michael Riemer, Edmund K. M. Chang, and Olivia Martius

et al. 2008 ; Majumdar et al. 2010 ; Chang et al. 2013 ; Zheng et al. 2013 ). c. Upscale error growth affecting RWPs The predictability of RWPs depends on the upscale growth of errors and uncertainties from the convective scale to the synoptic scale (e.g., Zhang et al. 2003 ). Figure 13 illustrates such upscale error growth from the PV perspective, which emphasizes the tropopause waveguide and RWPs as focal points. At forecast day 2, PV errors exhibit localized, mesoscale maxima near the

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