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P. L. Houtekamer and Fuqing Zhang

investigated in the 1980s and early 1990s (e.g., Ghil et al. 1981 ; Cohn and Parrish 1991 ; Daley 1995 ). One unsolved problem, aimed at an application with a realistic high-dimensional atmospheric forecast model, was how to obtain an appropriate low-dimensional approximation of the background error covariance matrix for a feasible implementation on a computational platform. The use of random ensembles currently seems to be the most practical way to address the issue. The use of Monte Carlo experiments

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Clifford Mass and Brigid Dotson

-level pressure gradient. The strongest surface winds occurred when stability was reduced after passage of the occluded front, thus facilitating the vertical mixing of stronger winds aloft down to the surface. They also noted that the particular track of the storm, paralleling the coast from northern California to Washington State, was particularly conducive to widespread damage ( Fig. 2 ). The storm was poorly forecast, with no warning the previous day. d. 13–15 November 1981 A number of major Northwest

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Andrew Staniforth and Jean Côté

, forcedadvcction, and coupled sets of equations) of increasing complexity, in one, two, and three dimensions. Attentionis focused on its accuracy, stability, and efficiency properties. Recent developments in applying semi-Lagrangianmethods to 2D and 3D atmospheric flows in both Cartesian and spherical geometries are then reviewed. Finally,the current status of development is summarized, followed by a short discussion of future perspectives.1. Introduction Accurate and timely forecasts of weather elementsare

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T. N. Krishnamurti

(see Fig. 13b), ii) An examination of the linear barotropic growthrates as a function of scale following Yanai and Nitta(1968) (see Fig. 13c), iii) An evaluation of the nonlinear barotropic energyexchange from zonal flows to the eddies over the Arabian Sea. (see Fig. 13d), and iv) A barotropic forecast experiment during the formation of the onset vortex.Figure 13a also includes the profiles of the mean zonalflows over the Arabian Sea for this period, which exhibits a marked increase during

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Markus Gross, Hui Wan, Philip J. Rasch, Peter M. Caldwell, David L. Williamson, Daniel Klocke, Christiane Jablonowski, Diana R. Thatcher, Nigel Wood, Mike Cullen, Bob Beare, Martin Willett, Florian Lemarié, Eric Blayo, Sylvie Malardel, Piet Termonia, Almut Gassmann, Peter H. Lauritzen, Hans Johansen, Colin M. Zarzycki, Koichi Sakaguchi, and Ruby Leung

results and conclusions from the experimentation can inform the production runs ranges from “difficult” (results are expected in the form of guidance or informing a choice that needs to be made in the design phase) to “direct” (a benefit can be demonstrated straightaway by producing an improved forecast). The parameterizations are typically organized by processes: for example, cumulus convection and cloud microphysics in the atmosphere and lateral and vertical mixing in the ocean. Some of these

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John Molinari and Michael Dudek

subsequent time stepsusing nonconvective forecast equations and advectionby grid-scale motions. In convectively unstable regions,the hybrid approach is thus partly implicit and partlyexplicit. The hybrid approach should be distinguished frompure cumulus parameterization found in the traditionalapproach. For example, Fritsch and Chappell (1980)and Emanuel ( 1991 ) compute condensate in their entraining updrafts and downdrafts and incorporate theinfluence of evaporation of convectively

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Bogdan Antonescu, David M. Schultz, Fiona Lomas, and Thilo Kühne

efforts at collecting reports on a European level (e.g., Dotzek et al. 2009 ). Thus, we are now able to build more accurate and complete climatologies of European tornadoes. There are three reasons why a review is needed at this point. First, there is not a widespread recognition of the threat of tornadoes to Europe and, as a result, many European meteorological services do not forecast tornadoes. Rauhala and Schultz (2009) showed, based on a questionnaire sent to 39 European meteorological

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Daniel Keyser and M. A. Shapiro

cyclogenesisrequire reconciliation. The opposing views concerning the relationship between upper-level frontogenesis and cyclogenesis maybe related to differing perspectives on the nature of thecyclogenesis process between the observational and 23 The concept of"improved forecast accuracy" takes on a precisemeaning only when the needs or requirements of a particular userare taken into account, and is, of course, open to considerable inoterpretation and debate.494

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William H. Raymond and Arthur Garder

is necessary to accommodate changes inthe forecast model resolution (Raymond and Garder1988) as found when computing with nonuniformspaced grids, or when it is necessary to connect datarich regions with regions of sparse data or data of lowerquality. In the latter case, where filter selectivity is notcritical, the second-order fully two-dimensional implicitfilter described in Raymond and Garder (1988) is appropriate. To maintain consistency with Eqs. (20)-(24), basedon the discussion above, e

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Peter Jan van Leeuwen

certain balances should be preserved (e.g., in atmospheric data assimilation the data-assimilation step should not introduce strong gravity waves since these tend to ruin the weather forecast). Up until recently this problem was solved by projection of the analyzed field on the so-called slow manifold, mainly related to geostrophy. However, as mentioned above, at higher resolution the coherent nonlinear structures follow other balances, which are not well known. In fact, the gravity waves become an

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