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

) documented how a series of three TCs (Florence, Helene, and Leslie), recurving in the North Atlantic between September and October 2000, led to the continuous invigoration of atmospheric blocks over the North Atlantic and to repeated Rossby wave breaking over western Europe associated with extreme flooding over southern Switzerland and northern Italy. It has been hypothesized that a link exists between ET in the western North Pacific and enhanced blocking activity over western North America during the

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Georgios Fragkoulidis and Volkmar Wirth

.1002/qj.2139 . 10.1002/qj.2139 Glatt , I. , A. Dörnbrack , S. Jones , J. Keller , O. Martius , A. Müller , D. H. Peters , and V. Wirth , 2011 : Utility of Hovmöller diagrams to diagnose Rossby wave trains . Tellus , 63A , 991 – 1006 , . 10.1111/j.1600-0870.2011.00541.x Grazzini , F. , and F. Vitart , 2015 : Atmospheric predictability and Rossby wave packets . Quart. J. Roy. Meteor. Soc. , 141 , 2793 – 2802 , https

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

rotation and the sphericity of Earth [for an introductory-level text on Rossby waves see Rhines (2002) ]. Rossby waves are in distinct contrast to other types of waves such as gravity waves or sound waves, which rely on gravity or the compressibility of air, respectively, for their basic restoring mechanism. The atmospheric general circulation cannot be understood without reference to Rossby waves because they transfer energy, moisture, and momentum across large distances. This can generate

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Volkmar Wirth and Christopher Polster

amplitude. It turns out that strong and narrow jets are generally associated with strong waveguidability. To the extent that changes in the atmospheric background flow imply an increase in jet waveguidability, there is less dispersion of Rossby wave energy in the meridional direction, and this may lead to larger wave amplitudes ( Branstator 2002 ) and an increased probability of extreme weather downstream of the wave source. Specific mechanisms that have been proposed in this context are “quasi

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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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Marlene Baumgart, Paolo Ghinassi, Volkmar Wirth, Tobias Selz, George C. Craig, and Michael Riemer

. , 146 , 4099 – 4114 , . 10.1175/MWR-D-18-0068.1 Heifetz , E. , C. H. Bishop , B. J. Hoskins , and J. Methven , 2004 : The counter-propagating Rossby-wave perspective on baroclinic instability. I: Mathematical basis . Quart. J. Roy. Meteor. Soc. , 130 , 211 – 231 , . 10.1002/qj.200413059610 Hohenegger , C. , and C. Schär , 2007 : Atmospheric predictability at synoptic versus cloud-resolving scales

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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

-based observation activities. Special ground-based observations were conducted in Iceland, the United Kingdom, and France ( Fig. 3b ). At Keflavik International Airport, a radiosonde facility was set up by DLR to increase the frequency of the operational soundings. In cases of orographically induced gravity waves (GWs), large balloons were launched to reach altitudes up to 42 km. Also in Keflavik, a Doppler cloud radar [the Bistatic Radar System for Atmospheric Studies (BASTA); Delanoë et al. (2016) ] allowed

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Thomas Engel, Andreas H. Fink, Peter Knippertz, Gregor Pante, and Jan Bliefernicht

characteristics and hitherto poorly documented atmospheric dynamics of these events. Only the Ouagadougou rainfall event is briefly discussed in Cornforth et al. (2017) as an example of an African easterly wave (AEW) breaking event, and a synoptic overview is given in Galvin (2010) . The paper is structured as follows. Section 2 gives a detailed description of the gauge, satellite, and reanalysis data used in this study, including their advantages and limitations. Section 3 describes the methods to

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