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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

amplification of the ridge–trough couplet directly downstream of ET. More recent work has investigated the processes that determine downstream development following the onset of ET beyond one wavelength [see Fig. 2 and blue labels for orientation; section 3a(1) ] and identified a climatological signal of RWP development downstream of ET [ section 3a(2) ]. Furthermore, the development of high-impact weather in regions downstream of ET has been investigated more recently ( section 3b ). a. Modification of

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Christian M. Grams and Heather M. Archambault

modification during and after ET. In addition, the characteristics of the different weather systems involved in ET are contrasted and their modification by ET is explored. The study is organized as follows. A brief review of diabatic processes during TC–extratropical flow interaction is given in section 2 . The construction of a composite of 12 western North Pacific September ET cases used as initial data for numerical simulations with the mesoscale Consortium for Small-Scale Modeling (COSMO) model with

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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

observations. All of the mentioned studies were carried out with global model versions using three-dimensional variational data assimilation (3D-Var), whereas many of the leading NWP centers use four-dimensional variational data assimilation (4D-Var) nowadays. The use of 4D-Var has also enabled a drastic increase of satellite observations assimilated in NWP models. These modifications are likely to reduce the beneficial influence of additional observations. However, impact studies with 4D-Var assimilation

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

addressed the following important questions: Do these local modifications exhibit a downstream impact (i.e., do they modify the propagation of RWPs)? And how important are modifications by latent heat release compared to dry dynamics? Early studies demonstrated a clear impact of latent heat release on individual weather events in the downstream region [e.g., a cyclone as in Hoskins and Berrisford (1988) , or heavy precipitation as in Massacand et al. (2001) ]. More recent studies interpreted the

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Julian F. Quinting and Sarah C. Jones

packets (RWPs) (e.g., Harr et al. 2008 ; Reynolds et al. 2009 ; Anwender et al. 2010 ; Pantillon et al. 2013 ). Hence, it is of interest to investigate the excitation and modification of Rossby waves through TCs over various ocean basins in order to quantify how and to what extent recurving TCs impact the weather patterns in downstream regions. A key factor for the modification of the extratropical flow pattern through a transitioning TC is the reduction of potential vorticity (PV) through

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Gabriel Wolf and Volkmar Wirth

1. Introduction The statistics of midlatitude weather systems show significant deviations from zonal symmetry, especially in the Northern Hemisphere. This zonal asymmetry is at least partly caused by the uneven distribution of continental-scale orography and land–sea distribution (e.g., Swanson 2007 ). Close to the surface the asymmetry is associated with a zonal variation of storm tracks ( Hoskins and Valdes 1990 ; Chang and Orlanski 1993 ). In the upper troposphere it leads to a zonal

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Andrea Schneidereit, Silke Schubert, Pavel Vargin, Frank Lunkeit, Xiuhua Zhu, Dieter H. W. Peters, and Klaus Fraedrich

large-scale flow and in transients ( Blackburn et al. 2011 ). Two large-scale modes of variability characterize the summer 2010 affecting the circulation of the whole year: ENSO and the Arctic Oscillation (AO; Blunden et al. 2011 ) or North Atlantic Oscillation (NAO). While ENSO shows a transition from El Niño to La Niña conditions, the AO is in a strong negative phase. European weather and ENSO are connected due to teleconnections that, starting from the eastern North Pacific storm track, continue

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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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