ABSTRACT

An extensive package of changes will be made to the NCEP global analysis and forecast system on 15 June 1998 at 1200 UTC. The changes include (1) An increase in horizontal and vertical resolution from T126l28 to T170L42 for the analysis and first 84 forecast hours; (2) physics updates, affecting radiation and clouds, land surface parameterization, cumulus convection and gravity wave drag; (3) analysis and data assimilation updates, including changes in time interpolation, nonlinear interpolation, limits on supersaturation and negative moisture, background error covariance, 3-D ozone, level 1-b polar orbiter data, GOES radiances, and Y2K compliance. During the first two months in which the new package was run in parallel with the operational, the following features were observed: (1) reduction in the wet bias in precipitation over land (Fig. 5.6b); (2) reduction of the cold bias over much of the atmosphere, including the stratosphere and near-surface layers, together with the appearance of a warm bias over land in the lower atmosphere, all reflecting changes in the surface physics and in the radiation, and (3) improvement in the model's ability to maintain transient eddy kinetic energy throughout the forecast (Fig. 5.2a) and (Fig. 5.2b). The objective performance statistics for 1-2 months of testing show clear improvement in precipitation bias, reduction in jet-level rms vector error versus analysis - especially in the short range - (Fig. 5.3), increased accuracy of tropical winds versus analysis (Fig. 5.4), and no significant change in 5-day 500-hPa anomaly correlations for geopotential (Fig. 5.5).

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