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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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Florian Harnisch and Martin Weissmann

. 2005 ). Several studies showed that despite the increased use of satellite data in the analysis of numerical weather prediction models, additional dropsonde measurements of key variables such as wind, temperature, and humidity in the environment of TCs can lead to improvements of TC track forecasts of the order of 10%–20% ( Aberson 2003 ; Wu et al. 2007b ; Yamaguchi et al. 2009 ). Despite the average reduction of track forecast errors, single cases occur where additional observations do not

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Gabriel Wolf and Volkmar Wirth

1. Introduction Midlatitude weather is dominated by a succession of synoptic-scale cyclones and anticyclones. These, in turn, are often accompanied by longer-lived and larger-scale Rossby wave packets (RWPs) in the upper troposphere ( Chang 2005 ; Wirth and Eichhorn 2014 ). This suggests that the dynamics of such RWPs play an important role for the prediction of midlatitude cyclones and the associated weather, which arguably explains the recent interest in upper-tropospheric wave packets. An

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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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Julian F. Quinting, Michael M. Bell, Patrick A. Harr, and Sarah C. Jones

west and warm air to the east during the interaction of the TC and the midlatitude baroclinic zone. This interaction involves lower-tropospheric temperature advection, the deformation of the TC warm core, lower-tropospheric frontogenesis, and slantwise ascent (descent) to the east (west) of the TC in a baroclinic environment ( Harr and Elsberry 2000 ; Harr et al. 2000 ). Harr and Elsberry (2000) showed that the modification of the equivalent potential temperature gradient during the

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Olivia Martius and Heini Wernli

-scale modification of subtropical jets. The remainder of this paper is organized as follows: the details of the Lagrangian approach are specified in the next section. Section 3 discusses three episodes of subtropical jet formation, followed by generalization of the case study results in section 4 . The final section provides a discussion of the results and some concluding remarks. 2. Data and methods Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data ( Dee et al

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Andrea Schneidereit, Dieter H. W. Peters, Christian M. Grams, Julian F. Quinting, Julia H. Keller, Gabriel Wolf, Franziska Teubler, Michael Riemer, and Olivia Martius

frequency and WCB activity over the North Pacific are examined to reveal their contribution to the observed development of the anticyclone. The mechanisms of the amplification of the Alaskan and Scandinavian anticyclones are diagnosed next. The article is finalized by a summary. 2. Data and methods a. Data The ERA-Interim data of the European Centre for Medium-Range Weather Forecasts are used ( Dee et al. 2011 ). The present analysis includes the period from 1979 to 2012. If not stated otherwise, daily

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Julia H. Keller

representation in current weather forecasting models, may improve future ET event forecasts (e.g., Jones et al. 2003 ; Harr 2010 ). Diabatic processes associated with the transitioning cyclone or the precipitation along the baroclinic zone (e.g., Riemer et al. 2008 ; Torn 2010a ; Grams et al. 2011 ; Pantillon et al. 2013a ) reduce potential vorticity (PV) in the ascending air mass. The divergent upper-level outflow of the TC advects low-PV air toward the midlatitude baroclinic zone. This strengthens

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Julia H. Keller, Sarah C. Jones, and Patrick A. Harr

preexisting midlatitude wave pattern poses a challenge for numerical weather prediction (NWP) systems. Using various ensemble forecast systems, Harr et al. (2008) and Anwender et al. (2008) showed that ET events often coincide with increased forecast uncertainty in downstream regions. By conducting numerical experiments for the ET of Hurricane Helene (1996), Pantillon et al. (2012) further showed how forecast errors associated with the ET event propagate toward downstream regions and lead to

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