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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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Christian M. Grams and Heather M. Archambault

1. Introduction The large-scale midlatitude flow is dominated by the upper-level jet stream that serves as a waveguide for Rossby waves (e.g., Martius et al. 2010 ). Because their general evolution follows dry dynamics that can be represented at grid scale in numerical weather prediction (NWP) models, Rossby waves may be expected to feature a high degree of predictability ( Grazzini and Vitart 2015 ). However, major forecast uncertainty and error in the midlatitudes in current NWP models have

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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

1. Introduction During a stratospheric sudden warming (SSW) event, the polar stratospheric temperature increases accompanied by a weakening of the polar night jet. Major SSW events (MSSW) are defined as SSW events with a reversal of the zonal mean zonal wind from westerlies to easterlies at 60°N and a 10-hPa layer. Those events represent the greatest part of intraseasonal variability in the winter middle atmosphere and are associated with low predictive skill at lead times more than 10 days

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Julian F. Quinting and Sarah C. Jones

diabatic processes and upper-level divergent flow that impinges on the midlatitude waveguide (e.g., Riemer et al. 2008 ; Hodyss and Hendricks 2010 ; Torn 2010 ; Archambault et al. 2015 ; Torn et al. 2015 ). Negative PV advection by the divergent flow deflects PV contours poleward and strengthens the meridional PV gradient, which results in a downstream ridge amplification and a jet streak intensification, respectively. The ridge amplification and jet streak intensification may result in the

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Maxi Boettcher and Heini Wernli

” constitutes a clear example for a two-phase development. The precursor rapidly crossed the North Atlantic as a DRW before it intensified to one of the most harmful storms in central Europe in the last few decades. Wernli et al. (2002) identified an intensive straight zonal jet during the DRW propagation phase of Lothar far to the north of the low-level vortex and excluded a significant upper-level forcing of the surface low due to the absence of waves on the intense jet. They demonstrated that later in

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

its long persistence extending to mid-August is not ( Matsueda 2011 ). Thus, the question arises: what are the causes of this long-lasting heat wave or rather the blocking high? Blocking, in general, denotes the effect of a synoptic system acting as a barrier to the westerly flow splitting the jet stream ( Elliott and Smith 1949 ). The formation of a block over Europe can be ascribed to the convergence (or absorption) of wave activity density flux associated with an incoming anomalous quasi

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Maxi Boettcher and Heini Wernli

models were cloud-diabatic heating in a baroclinic background atmosphere producing a positive potential vorticity (PV) anomaly at low-tropospheric levels. The following basic conditions for DRW existence and propagation emerged from these simulations. The vortex of the positive low-level PV anomaly (that is accompanied by a weak SLP minimum) induces a poleward low-level jet of warm moist air at its downstream side. This stream ascends along the poleward-sloping isentropes until condensation occurs

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

density and ensure that the SAMURAI analysis weights the observational data more heavily than the background field. 3. Synoptic situation At 0600 UTC 20 September Sinlaku was located at 35.1°N, 144.7°E to the south of a nearly zonal upper-level flow that constituted the southern branch of a split jet stream ( Fig. 1 ). The two branches of the jet stream merged about 15°E of Sinlaku at the southern apex of an upper-level midlatitude trough. A low-level low pressure system associated with this trough

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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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