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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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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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Paolo Ghinassi, Georgios Fragkoulidis, and Volkmar Wirth

; Asselin 1972 ) to control the growth of the computational mode. As an initial condition we specify a purely zonal background flow with a wavelike perturbation superimposed. For the background flow we follow Held and Phillips (1987) and set with , , and m s −1 . This flow resembles the climatology observed in the upper troposphere, with westerly subtropical jets at and easterlies in the deep tropics. Onto this background flow we superimpose two localized wave packets, which are specified in

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Christian Euler, Michael Riemer, Tobias Kremer, and Elmar Schömer

-study framework also: the ET of Tropical Storm Karl (2016) in the North Atlantic. The ET of Karl occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX; Schäfler et al. 2018 ) and has been identified as one of the “triggers” for midlatitude impact during this period ( Schäfler et al. 2018 ). In the extratropical phase of ET, Karl merged with a weak, preexisting cyclone and formed an unusually strong jet streak downstream, which initiated further development that resulted in a

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Tobias Kremer, Elmar Schömer, Christian Euler, and Michael Riemer

the midlatitude jet. Fig . 1. Synoptic overview of Karl’s evolution depicting θ e at 3 km (colors) and wind speed at 10 km height (gray shading) using COSMO data at (a) 2100 UTC 24 Sep (defined as IS stage) and (b) 1200 UTC 25 Sep (defined as XT stage). The official NHC track between 1800 UTC 23 Sep and 1800 UTC 25 Sep is shown in black, with dots every 6 h. The respective track in the COSMO simulation is shown in red. The white squares highlight the center times of the four stages that are

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

gradual development, the transformation stage ( Klein et al. 2000 ; Ritchie and Elsberry 2001 ) can be divided into three phases with the following key elements. The TC first encounters a decrease of intensity due to lower sea surface temperatures (step 1); second, the vortex tilts by the influence of vertical wind shear by the midlatitude jet stream (step 2); and third, the interaction of the TC circulation with cold dry midlatitude air north of the baroclinic zone induces a frontal structure (step

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

-0315.1 Moncrieff , M. W. , D. E. Waliser , and J. Caughey , 2012 : Progress and direction in tropical convection research: YOTC International Science Symposium . Bull. Amer. Meteor. Soc. , 93 , ES65 – ES69 , https://doi.org/10.1175/BAMS-D-11-00253.1 . 10.1175/BAMS-D-11-00253.1 Nie , Y. , Y. Zhang , G. Chen , and X.-Q. Yang , 2016 : Delineating the barotropic and baroclinic mechanisms in the midlatitude eddy-driven jet response to lower-tropospheric thermal forcing . J. Atmos. Sci

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Michael Maier-Gerber, Michael Riemer, Andreas H. Fink, Peter Knippertz, Enrico Di Muzio, and Ron McTaggart-Cowan

. IEEE Trans. Acoust. Speech Signal Process. , 26 , 43 – 49 , https://doi.org/10.1109/TASSP.1978.1163055 . 10.1109/TASSP.1978.1163055 Shapiro , M. , and D. Keyser , 1990 : Fronts, jet streams, and the tropopause. Extratropical Cyclones: The Erik Palmén Memorial Volume , C. W. Newton and E. Holopainen, Eds., Amer. Meteor. Soc., 167–191. 10.1007/978-1-944970-33-8_10 Shapiro , M. , and Coauthors , 1999 : A planetary-scale to mesoscale perspective of the life cycles of extratropical cyclones

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