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Volkmar Wirth and Christopher Polster

1. Introduction Upper-tropospheric flow in the extratropics is often characterized by a narrow band of strong westerlies, the so-called jet stream or, short, jet ( Glickman 2000 ; Vallis 2006 ; Galperin and Read 2019 ; Woollings 2020 ). Usually, there are undulations superimposed on this jet, which are referred to as Rossby waves and which sometimes occur in the form of Rossby wave packets ( Rossby et al. 1939 ; Rossby 1940 ; Rhines 2002 ; Wirth et al. 2018 ). The troughs and ridges

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Andreas Schäfler, George Craig, Heini Wernli, Philippe Arbogast, James D. Doyle, Ron McTaggart-Cowan, John Methven, Gwendal Rivière, Felix Ament, Maxi Boettcher, Martina Bramberger, Quitterie Cazenave, Richard Cotton, Susanne Crewell, Julien Delanoë, Andreas Dörnbrack, André Ehrlich, Florian Ewald, Andreas Fix, Christian M. Grams, Suzanne L. Gray, Hans Grob, Silke Groß, Martin Hagen, Ben Harvey, Lutz Hirsch, Marek Jacob, Tobias Kölling, Heike Konow, Christian Lemmerz, Oliver Lux, Linus Magnusson, Bernhard Mayer, Mario Mech, Richard Moore, Jacques Pelon, Julian Quinting, Stephan Rahm, Markus Rapp, Marc Rautenhaus, Oliver Reitebuch, Carolyn A. Reynolds, Harald Sodemann, Thomas Spengler, Geraint Vaughan, Manfred Wendisch, Martin Wirth, Benjamin Witschas, Kevin Wolf, and Tobias Zinner

diabatic processes are particularly active as a result of relatively high sea surface temperatures and the intensification of the jet stream as the high latitudes cool. Many of the weather phenomena central to the growth of disturbances on the jet stream and midlatitude predictability are active in fall, such as extratropical cyclones with intense fronts and warm conveyor belts (WCBs), carrying air from the oceanic boundary layer into ridges at the tropopause level. There is also the possibility of

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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

moves poleward and starts to interact with the midlatitude flow ( Fig. 1a ). This results in the formation of a jet streak ( Fig. 1b ) and a poleward deflection of the jet near the transitioning cyclone in conjunction with the development of a ridge–trough couplet ( Fig. 1b ). At the same time, a region of enhanced moisture flux—a so-called atmospheric river ( Zhu and Newell 1998 )—forms ahead of the downstream trough. The ridge–trough couplet continues to amplify, a new cyclone develops farther

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

gradual than that of the carrier wave (dotted) or the RWP signal (blue). Fig . 1. Schematic of a Rossby wave packet (RWP) at a specific time. The blue line represents , the black dotted line is the underlying carrier wave , and the two red lines depict plus (upper line) and minus (lower line) the amplitude . A real world example is presented in Fig. 2 . Figure 2a shows the midlatitude jet with large meridional undulations over North America. Over the rest of hemisphere, the jet is more zonally

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Georgios Fragkoulidis and Volkmar Wirth

the seasonal variability of RWP amplitude E and phase speed c p , respectively. For most regions in the extratropics a strong seasonality is apparent, associated with the maximization of wave activity during winter and its minimization during summer (see also Souders et al. 2014b ), as well as changes in strength and location of the jet streams ( Fig. S9 ). The bands of elevated E and c p values over the two ocean basins signify the areas where RWPs tend to attain large amplitudes and

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Joaquim G. Pinto, Florian Pantillon, Patrick Ludwig, Madeleine-Sophie Déroche, Giovanni Leoncini, Christoph C. Raible, Len C. Shaffrey, and David B. Stephenson

center. Some of the moisture transported by the feeder airstream is supplied to the base of the warm conveyor belt where it ascends to form precipitation, while the rest remains at low levels, forming the leading edge of an atmospheric river. Lea Eisenstein (KIT) presented a modeling study of the first detected sting-jet windstorm over continental Europe (“Egon” in January 2017). Devastating sting jets are associated with strong wind gusts lasting for a few hours over a distinct region located

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Jacopo Riboldi, Christian M. Grams, Michael Riemer, and Heather M. Archambault

tropical air masses, which surround recurving TCs, ascend in the vicinity of the jet stream, forming clouds and precipitation and leading to strong latent heat release ( Grams et al. 2013b ; Grams and Archambault 2016 ). The associated patterns of potential vorticity (PV) creation and destruction can influence the large-scale flow by enhancing low-level cyclonic circulations ( Stoelinga 1996 ; Čampa and Wernli 2012 ; Binder et al. 2016 ; Crezee et al. 2017 ) and upper-level anticyclonic

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Gabriel Wolf and Volkmar Wirth

wave activity continues to be transported eastward toward Europe. Both at this time and three days later ( Fig. 5d ), the vector F is slightly diffluent between Europe and Iceland. The RWP seems to interact with the larger-scale ridge over central and eastern Europe, which implies that the RWP is losing wave activity to the background flow or by dissipation. At the same time, a significant fraction of the wave activity is being transferred to the subtropical jet over the Mediterranean with

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Tobias Selz

investigated on a logarithmic x axis. 3. Results a. Time series of DKE As an example Fig. 2 shows the error growth of the geopotential of the July 2017 ensemble run over the Northern Hemisphere at four different forecast times. At time zero the ensemble members are identical and thus the five lines lie exactly on top of each other. After 7 days into the forecast one can see slight perturbation to the jet, which then grow further, and after about 14 days of simulation the ensemble members seem to be

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George C. Craig, Andreas H. Fink, Corinna Hoose, Tijana Janjić, Peter Knippertz, Audine Laurian, Sebastian Lerch, Bernhard Mayer, Annette Miltenberger, Robert Redl, Michael Riemer, Kirsten I. Tempest, and Volkmar Wirth

storms, Pantillon et al. (2018) showed that classical regression-based statistical postprocessing generally improves raw ensemble forecasts from a convection-permitting model. However, postprocessing fails to reduce forecast errors for some winter storms with uncharacteristic forecast errors, and even increase them for a rare sting-jet cyclone affecting western Europe in October 2013. For 2-m temperature forecasts for Germany, Rasp and Lerch (2018) proposed a novel postprocessing approach based

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