ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-3659-2012 The 2009–2010 Arctic stratospheric winter – general evolution, mountain waves and predictability of an operational weather forecast model Dörnbrack A. 1 Pitts M. C. 2 Poole L. R. 3 Orsolini Y. J. 4 Nishii K. 5 Nakamura H. 5 Institut für Physik der Atmosphäre, DLR Oberpfaffenhofen, 82230 Oberpfaffenhofen, Germany NASA Langley Research Center, Hampton, Virginia 23681, USA Science Systems and Applications, Incorporated, Hampton, Virginia 23666, USA Norwegian Institute for Air Research, Kjeller, Norway Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan 19 04 2012 12 8 3659 3675 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/3659/2012/acp-12-3659-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/3659/2012/acp-12-3659-2012.pdf

The relatively warm 2009–2010 Arctic winter was an exceptional one as the North Atlantic Oscillation index attained persistent extreme negative values. Here, selected aspects of the Arctic stratosphere during this winter inspired by the analysis of the international field experiment RECONCILE are presented. First of all, and as a kind of reference, the evolution of the polar vortex in its different phases is documented. Special emphasis is put on explaining the formation of the exceptionally cold vortex in mid winter after a sequence of stratospheric disturbances which were caused by upward propagating planetary waves. A major sudden stratospheric warming (SSW) occurring near the end of January 2010 concluded the anomalous cold vortex period. Wave ice polar stratospheric clouds were frequently observed by spaceborne remote-sensing instruments over the Arctic during the cold period in January 2010. Here, one such case observed over Greenland is analysed in more detail and an attempt is made to correlate flow information of an operational numerical weather prediction model to the magnitude of the mountain-wave induced temperature fluctuations. Finally, it is shown that the forecasts of the ECMWF ensemble prediction system for the onset of the major SSW were very skilful and the ensemble spread was very small. However, the ensemble spread increased dramatically after the major SSW, displaying the strong non-linearity and internal variability involved in the SSW event.

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