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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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Maxi Boettcher and Heini Wernli

) generalized the hypothesis that antecedent vorticity development should be relevant to subsequent explosive development by looking into a large sample of cyclone events. They established the concept of a “two-phase development” for cyclones showing a pronounced near-surface vorticity precursor. Simultaneously, theoretical studies using idealized two-dimensional models investigated the impact of diabatic processes on a baroclinic unstable environment ( Snyder and Lindzen 1991 ) and in mesoscale convective

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Martin Weissmann, Florian Harnisch, Chun-Chieh Wu, Po-Hsiung Lin, Yoichiro Ohta, Koji Yamashita, Yeon-Hee Kim, Eun-Hee Jeon, Tetsuo Nakazawa, and Sim Aberson

operational hurricane track forecast models. Bull. Amer. Meteor. Soc. , 77 , 925 – 933 . Chou , K.-H. , and C.-C. Wu , 2008 : Development of the typhoon initialization in a mesoscale model—Combination of the bogused vortex with the dropwindsonde data in DOTSTAR. Mon. Wea. Rev. , 136 , 865 – 879 . Fiorino , M. , 2008 : Record-setting performance of the ECMWF IFS in medium-range tropical cyclone track prediction. ECMWF Newsletter, No. 118, ECMWF, Reading, United Kingdom, 20–27 . Harnisch

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Kirstin Kober, Annette M. Foerster, and George C. Craig

evaluated in comparison to radar data. Groenemeijer and Craig (2012) showed that the stochastic scheme contributes significant variability in a mesoscale ensemble prediction system, but that the contribution varies strongly between different weather regimes. Here we investigated whether this additional variability produces a measurable improvement in various metrics of forecast quality. Two cases selected from the study of Groenemeijer and Craig (2012) were considered to illustrate the performance

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Tobias Selz and George C. Craig

convective-scale errors start to spin up balanced motions, which continue to grow at a slower rate through baroclinic instability (stage 3). This model describes how the intrinsic convective-scale uncertainty is able to grow upscale and contaminate the mesoscale and the large-scale forecast after a period of time. However, the relevance of this conceptual model is arguable because of the high level of idealization. The 10-km model resolution used in the main experiments of Zhang et al. (2007) was

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Julia H. Keller

. Atmos.–Ocean , 42 , 235 – 250 , doi: 10.3137/ao.420402 . 10.3137/ao.420402 Davis , C. , B. Brown , and R. Bullock , 2006 : Object-based verification of precipitation forecasts. Part I: Methodology and application to mesoscale rain areas . Mon. Wea. Rev. , 134 , 1772 – 1784 , doi: 10.1175/MWR3145.1 . 10.1175/MWR3145.1 ECMWF , 2008 : European Centre for Medium-Range Weather Forecasts Ensemble Prediction System—Operational archive Cycle35r3, Resolution T639L62, accessed April 2011

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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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Maxi Boettcher and Heini Wernli

1. Introduction Meteorological research on diabatic Rossby waves (DRWs) has been intensifying in recent years after a DRW was detected for the first time in numerical weather prediction (NWP) model output for a high-impact weather event. According to a mesoscale model hindcast simulation, a DRW served as an important precursor to the “Lothar” storm after Christmas 1999, which was one of the most harmful winter storms over Europe in the last few decades ( Wernli et al. 2002 ). The explosive

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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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