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Peter Vogel, Peter Knippertz, Andreas H. Fink, Andreas Schlueter, and Tilmann Gneiting

). Conventional observations such as surface stations and weather balloons are scarce at low latitudes, particularly over the vast tropical oceans. Consequently, the observing system is dominated by satellite data, which are heavily skewed toward measuring atmospheric mass variables rather than wind (e.g., Baker et al. 2014 ). However, data denial experiments for periods with a much enhanced radiosonde network during field campaigns over West Africa have shown a relatively small impact on model performance

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

from a tropical wave off the coast of West Africa, was declared a tropical depression 1 near the Cape Verde Islands on 14 September 2016, and further intensified to tropical-storm strength on 15 September. Tropical Storm Karl moved mainly westward for several days until beginning to interact with an upper-level low on 21 September. During this interaction Karl weakened to a tropical depression and started to move to the northwest. When the upper-level low moved away toward the south, Karl started

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Andreas Schlueter, Andreas H. Fink, Peter Knippertz, and Peter Vogel

most frequently observed in the central and western Pacific region, where they significantly affect precipitation ( Takayabu and Nitta 1993 ; Holder et al. 2008 ; Kiladis et al. 2016 ). MRG waves occur in a wave domain next to TD. Over the Pacific Ocean, the transition of MRG to off-equatorial TDs has been documented ( Takayabu and Nitta 1993 ; Zhou and Wang 2007 ). Similarly, “hybrid” AEW/MRG waves have been documented over Africa (Y.-M. Cheng et al. 2018, personal communication). However, the

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Andreas Schlueter, Andreas H. Fink, and Peter Knippertz

variability over northern tropical Africa. The Madden–Julian oscillation (MJO) is a wave mode that is not predicted by the shallow-water equations but shares several properties with Kelvin waves ( Madden and Julian 1971 ; Roundy 2012 ). It is a global mode with highest activity over the Indian and Pacific Oceans. The MJO modulates the occurrence of tropical rain clusters on the intraseasonal time scale and its influence has been documented for northern tropical Africa (e.g., Matthews 2004 ; Pohl et al

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Gabriel Wolf and Volkmar Wirth

have the same strength | F| but that differ in their meridional component. Figure 3 indicates a coherent RWP starting west of 135°W at the beginning of August and propagating all the way across the Atlantic Ocean toward Asia. As in Fig. 2 , the troughs still appear as relative maxima in Fig. 3 despite the use of the semigeostrophic coordinate transformation. Fig . 3. Evolution of the RWP associated with the Elbe flooding by the use of the wave activity flux F at 300 hPa. (bottom) The

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Paolo Ghinassi, Georgios Fragkoulidis, and Volkmar Wirth

1. Introduction An important feature of midlatitude atmospheric dynamics is the existence of upper-tropospheric Rossby waves with synoptic- to planetary-scale wavenumbers. Often a Rossby wave is not strictly circumglobal; rather, its amplitude is spatially inhomogeneous with a relative maximum at a specific location decaying to smaller values at larger distances. This gives rise to so-called Rossby wave packets [RWPs; for a recent review see Wirth et al. (2018) ]. A key feature of RWPs is the

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

concomitant momentum flux convergence acts to sharpen the jet ( Chang 2001 , 2005b ). Apparently, much of the mean flow modification occurs in the vicinity of the RWP. Most of the events with significant growth and decay of hemispheric total eddy kinetic energy can be traced back to a single RWP, especially when the RWP propagates across the highly baroclinic region in the south Indian Ocean. The final stage of the RWP life cycle is usually associated with Rossby wave breaking ( Fig. 9 ), that is, the

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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

al. 2004 ; Agustí-Panareda et al. 2004 , 2005 ; Grams et al. 2011 ). The reasons for these differences in wave breaking and whether such large-scale circulation patterns associated with ET also exist in other ocean basins have not been investigated yet. In conclusion, the relative position between the transitioning cyclone and the upstream trough (i.e., phasing) is crucial in determining the reintensification of the transitioning cyclone as an extratropical cyclone, the amplification of the

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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

gravity waves . J. Atmos. Oceanic Technol. , 34 , 1371 – 1386 , https://doi.org/10.1175/JTECH-D-17-0021.1 . 10.1175/JTECH-D-17-0021.1 Wulfmeyer , V. , and Coauthors , 2011 : The Convective and Orographically Induced Precipitation Study (COPS): The scientific strategy, the field phase, and research highlights . Quart. J. Roy. Meteor. Soc. , 137 , 3 – 30 , https://doi.org/10.1002/qj.752 . 10.1002/qj.752 Zhang , F. , N. Bei , R. Rotunno , C. Snyder , and C. C. Epifanio , 2007

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