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Christian M. Grams and Heather M. Archambault

of the spatially and temporally averaged 250–150-hPa-layer averaged negative PV advection by the irrotational wind. As noted in Archambault et al. (2013 , 2015) , strong interactions tend to be associated with Rossby wave amplification and dispersion across the North Pacific to North America. In the present study, the selection of cases is constrained to September, which is the most active month for western North Pacific ET and specifically for strong interactions. 1 Table 1. Characteristics

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

-persisting blocking situation causing severe droughts, crop loss, and numerous forest and peat fires. Anomalous high temperatures have been observed not only over western Russia but also over other regions of the world (e.g., in the northeastern part of North America; Blunden et al. 2011 ). The summer temperatures for western Russia reached a record maximum [since at least 1880; (National Oceanic and Atmospheric Administration) NOAA/National Climatic Data Center (2010); Grumm (2011)], which even exceeded the

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Julian F. Quinting and Sarah C. Jones

that TCs with a strong upper-level divergent flow amplify a preexisting RWP that disperses downstream across the North Pacific to North America. The importance of a preexisting RWP for the downstream flow evolution is confirmed by Torn and Hakim (2015) who investigate subsets based on the existence or nonexistence of an upstream trough. They conclude that TCs that interact with a preexisting midlatitude trough are associated with a higher-amplitude downstream response than TCs that do not

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Maxi Boettcher and Heini Wernli

in winter with a value of 3.2 Bergeron (corresponding to a pressure decrease of 53 hPa in 24 h). This event led to a major North Pacific storm in early December 2007. This storm became known under the name “great coastal gale of 2007” ( Crout et al. 2008 ; Hofmann 2010 ) and severely affected the North American west coast. The DRW aspects of this case will be briefly described in appendix B . Fig . 4. Histogram of DRW intensification (measured in Bergeron, see text for details) for the warm

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Yannick Barton, Paraskevi Giannakaki, Harald von Waldow, Clément Chevalier, Stephan Pfahl, and Olivia Martius

.g., Mailier et al. 2006 ; Vitolo et al. 2009 ; Pinto et al. 2013 ) and extreme precipitation in the North American Midwest ( Villarini et al. 2011 ). In those studies, clustering in time was assessed using a one-dimensional homogeneous Poisson process model (i.e., a Poisson process with a constant intensity of events). The temporal clustering of regional-scale heavy precipitation in southern Switzerland has so far not been investigated. To study the processes behind temporal clustering, it is necessary

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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

downstream, and the next downstream ridge builds, which signifies the downstream propagation that arises from the initial local changes in the jet near the site of ET ( Fig. 1c ). Meanwhile, Nuri reintensifies into a strong extratropical cyclone and initiates cyclonic wave breaking over the western North Pacific ( Fig. 1c ). Subsequently, the upper-level wave pattern amplifies farther downstream, establishing a high-amplitude ridge–trough couplet over North America. A heat wave develops in the high

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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

amplified downstream ridge over western North America on 17 January. Cross-isentropic ascent within this WCB leads to a transport and injection of subtropical low-PV air into the mid- to high-latitude upper troposphere and contributed, hence, to the maintenance of the existing ridge. This is reflected by PV anomalies of less than −0.7 PVU (1 PVU = 10 −6 K kg −1 m 2 s −1 ), which persisted in some parts of the ridge for at least five consecutive days ( Fig. 7a ). Fig . 7. (a) Representative synoptic

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

reintensified strongly as an extratropical cyclone about 24 h later ( Fig. 3c ). Choi-Wan approached a preexisting and moderately amplified Rossby wave pattern in the midlatitudes. The interaction between the transitioning storm and the midlatitude flow features led to an amplification and modification of the midlatitude Rossby wave train that affected the entire North Pacific. This amplified wave train evolved into a wave breaking event over North America, around 22 September 2009 and the formation of a

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Gabriel Wolf and Volkmar Wirth

reconstruction for the above wave packet on 7 August 2002. On that date, there is a wave signal over North America and the western part of the North Atlantic Ocean. It is quite apparent from the previous figure that this wave signal should be considered as one single wave packet—any split must be considered as spurious. Figure 7a shows the result using the Hilbert transform. The southerly and northerly wind maxima (red and blue contours) occur in pairs, with larger spacing in between (this feature is also

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