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T. N. Krishnamurti, Mukul Tewari, D. R. Chakraborty, Jose Marengo, Pedro L. Silva Dias, and P. Satyamurty

anticyclone also varies, especially in the southeast flank. b. Planetary-scale aspects Figures 3a–c illustrate, for the June 1994 case, a Hovmöller diagram of 500-mb geopotential heights (meridionally averaged between 25° and 45°S) and its decomposition with its long- (zonal wavenumbers 1–3) and short- (zonal wavenumbers 4–10) wave components, respectively. Here we take the 500-hPa heights around the latitude circles between 25° and 45°S and average them in the south to north direction; thus, we have one

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Steven L. Mullen and Bruce B. Smith

form 15 December 1992) ABSTRACT Sea level cyclone errors for two contrasting planetary-scale flow regimes, a long-wave trough verses a longwave ridge over western North America, are computed for the National Meteorological Center's Nested GridModel (NGM) and "Aviation Run" of the Global Spectral Model (AVN). The study is performed for the1987/88 and 1989/90 cool seasons (1 December-31 March). All available 24- and 48-h forecast cycles areanalyzed for North America and

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Young-Joon Kim and Maria Flatau

1. Introduction Sudden stratospheric warming (SSW) is an abrupt disruption of the stratospheric winter circulation involving a rapid breakdown of the polar vortex. SSW is triggered by anomalous planetary (Rossby) wave activity propagating from the troposphere, and it is characterized by a rapid increase of the polar stratospheric temperature (about 40 K in a week) and a reversal of the stratospheric polar night jet. An SSW event is called “major” if the 10-hPa (or below in height) zonal mean

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Wayne F. Feltz and John R. Mecikalski

. The primary focus of this paper is not on the strong upper-level short wave, but rather is on the rapid thermodynamic destabilization that occurred within the Oklahoma–Kansas area on 3 May 1999, which assisted in initiating severe convection. Previous studies of the preconvective environment highlight the importance of changing planetary boundary layer (PBL) heat and moisture for the initiation of deep convection ( Beebe 1958 ; Carlson 1983 ; Sanders 1986 ; Colby 1984 ; Sanders and Blanchard

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Laura A. Stoss and Steven L. Mullen

Abstract

Forecast errors in the 500-mb geopotential height field over North America and adjacent ocean environs are calculated for the National Meteorological Center's Nested Grid Model (NGM). The eight winters 1985/86-1992/93 are examined. Errors are compared for the time-mean flow and for four recurring planetary-scale flow regimes, and the statistical significance of the differences is estimated.

Overall, the NGM produces very accurate 500-mb height forecasts out to 48 h, with every forecast cycle of the study period exhibiting useful deterministic skill at 48 h when averaged over the study domain. During the first two winters of operational NGM implementation, the spatially averaged errors were noticeably greater than in subsequent winters. NGM error was essentially constant during the 1987/88-1992/93 winters.

The bias in the NGM works to erode the asymmetries associated with the wintertime stationary waves. When the errors are categorized by the initial flow configuration, no significant differences are found among the spatially averaged error statistics for the regimes and the winter-mean values. On the other hand, the distribution of the bias varies notably among the regimes, with none of the regimes exhibiting a tendency to weaken the preexisting anomalies of its initial state.

The rms error (RMSE) is dominated by contributions from the random component at all forecast projections. The random RMSE is typically an order of magnitude larger than the bias, except over the Gulf of Mexico and the Caribbean, where they are of comparable size. The random RMSE at 48 h tends to be locally enhanced along axis of the polar jet stream, where its size approaches the observed rms variance due to synoptic-scale transients. Based on that finding, the authors hypothesize that 48 h may be close to the useful limit of predictive skill for mobile short waves at 500 mb.

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Øyvind Saetra and Jean-Raymond Bidlot

1. Introduction In June 1998, the Ensemble Prediction System (EPS) at the European Centre for Medium-Range Weather Forecasts (ECMWF) was coupled to the ocean wave model. From then on, daily ensemble wave forecasts have been available. Although the positive impact on both the atmospheric and the wave forecasts was the main reason for the introduction of the coupling ( Janssen et al. 2002 ), probabilistic forecasts of ocean waves are also potentially very valuable products by themselves. There

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W. F. Feltz, K. M. Bedka, J. A. Otkin, T. Greenwald, and S. A. Ackerman

1. Introduction Atmospheric turbulence is a major aviation hazard responsible for 609 fatalities, 823 injuries, and an estimated property loss of $134M from 1983 to 1997 ( Eichenbaum 2000 ). Aircraft turbulence is often associated with rapid thunderstorm development (e.g., Lane et al. 2003 ), upper-tropospheric folding events (e.g., Endlich 1964 ; Koch et al. 2005 ), and topographically induced mountain-wave and rotor phenomena (e.g., Reiter and Foltz 1967 ; Clark and Gall 1982 ; Clark et

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Yung Y. Chao, Jose-Henrique G. M. Alves, and Hendrik L. Tolman

1. Introduction There is a critical need to improve the skill of operational forecasts of extreme wind–wave fields associated with intense hurricanes, because of the potentially damaging impacts they can have on coastal settlements and economic activities. Hence, the National Centers for Environmental Prediction (NCEP) have implemented two specialized wave models to provide regional forecasts of hurricane-generated wind waves in the North Atlantic and Pacific Ocean basins on an operational

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Julio T. Bacmeister, Paul A. Newman, Bruce L. Gary, and K. Roland Chan

possible interactions between the wave and thebackground wind. It is likely that waves breaking in astrongly sheared flow will produce stronger turbulencethan those breaking in a uniform wind. Strong shearsare generally thought to be rare in the stratosphere.However, it appears that planetary waves in the stratosphere can occasionally create relatively strong vertica!gradients in on-ridge winds over individual ridges, asis illustrated in Fig. 6. Figure 6a shows NMC analysesof geopotential height for 16

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Stephen D. Eckermann, Andreas Dörnbrack, Harald Flentje, Simon B. Vosper, M. J. Mahoney, T. Paul Bui, and Kenneth S. Carslaw

symmetric vortex circulation, which isolates cold polar air from midlatitudes. Planetary waves episodically propagate upward from tropospheric sources, displacing and distorting the vortex and enhancing descent, which warms the stratosphere adiabatically ( Newman and Nash 2000 ). Global models capture these dynamics quite well: indeed, global NWP models have better forecast skill in the stratosphere than the troposphere during polar winter, permitting relatively accurate predictions several days or more

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