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Ronald Gelaro, Rolf H. Langland, Simon Pellerin, and Ricardo Todling

ships and buoys; and temperature, humidity, and wind from commercial aircraft. Other more-recent observation types, such as the Atmospheric Infrared Sounder (AIRS) and Infrared Atmospheric Sounding Interferometer (IASI) are not included in this baseline experiment, but will be considered in future comparisons. In keeping with the narrow focus of the comparison, it was decided that each center would use its usual assimilation algorithm, including data selection and quality control procedures

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Thomas M. Hamill, Jeffrey S. Whitaker, Michael Fiorino, and Stanley G. Benjamin

Whitaker et al. (2008) . Several changes were made to this EnKF algorithm for the 2009 experiment. Updates to new observations were performed every 6 h and included data from 3 h before to 3 h after the update time, so ensemble forecasts were computed to 9-h lead. The NCEP Gridpoint Statistical Interpolation (GSI) three-dimensional variational system (3D-Var) ( Wu et al. 2002 ; Kleist et al. 2009 ) library of forward operators was used to generate the observation priors for each ensemble member; the

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Warren J. Tennant, Glenn J. Shutts, Alberto Arribas, and Simon A. Thompson

convection parameterization. Numerical dissipation, occurring explicitly through diffusion and implicitly through interpolation in numerical advection schemes, destroys eddy kinetic energy near the grid scale of an atmospheric model ( Shutts 2005 ). Semi-Lagrangian advection algorithms involve the interpolation of fields to the departure point, thus smoothing model fields and removing energy. Additionally, horizontal diffusion employed to explicitly smooth model fields is unlikely to result in a

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John E. Janowiak, Peter Bauer, Wanqiu Wang, Phillip A. Arkin, and Jon Gottschalck

to include the latest improvements for the model physics and analysis algorithms and to assimilate new observations to produce the best initial conditions. (Changes to the GFS and GDAS during the past years are documented online at .) The GFS is integrated at T382 horizontal resolution (∼35-km grid) for the first 180 h, after which a T190 resolution (∼70-km grid) is used. Sea surface temperatures (SSTs) in the GFS are represented as

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