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A. Hollingsworth, J. Horn, and S. Uppala

Abstract

We examine the tropical wind field analyses produced by a recent assimilation of the Final FGGE II-b dataset. Our aim is to estimate the effects, on the tropical wind analyses, of biases in the data and biases in the assimilation system. The assimilation system was similar to that used operationally at ECMWF in the first half of 1985. The period studied is the first Special Observing Period (SOP-1).

Important differences occur in the intensity of divergence at upper and lower levels in the western Pacific, as measured by cloud-track winds (SATOBs) and by rawinsondes (TEMPs). There appear to be important biases also in the SATOB estimates of the zonal flow at upper and lower levels in the eastern Pacific. There are substantial biases in the wind directions at some west African stations.

The 6-hour forecasts which provide the background fields for the analyses show important underestimates of the mean intensity of the tropical divergence field, particularly in the equatorial western Pacific. The errors in the background field probably occur because of underestimation of the intensity of tropical convection in the diabatic initialization and in the course of the forecast; the heavy spatial smoothing applied to the convective heating in the initialization probably also plays a role.

Data were available in sufficient quantities that the analysis algorithm corrected the mean errors in the background field to a very large extent. As a result, any residual uncertainty in the mean analyses is within the uncertainty of the observations. The analysis algorithm has a rather poor response to divergent information even on large scales, so the analyzed divergence field agrees best with the observational data showing the weakest divergence, both in the upper and lower troposphere. The mean analyzed divergence field in the west Pacific agrees with the 850 mb TEMP data but is weaker than the intensity suggested by the low-level SATOBs and the SHIPs. In the upper troposphere the analyzed divergence is weaker than that suggested by the TEMPS, but agrees with that suggested by the (probably less reliable) SATOBs. Thus in this important area in the tropics the biases in the new analyses of the mean divergent wind field appear to be within the range of biases in the data, but the divergence is probably still underestimated in the upper troposphere and near the surface.

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L. Bengtsson, M. Kanamitsu, P. Kållberg, and S. Uppala

A presentation of the First GARP Global Experiment (FGGE) Research Programme at the European Centre for Medium Range Weather Forecasts (ECMWF) is given. An excellent data coverage in areas previously practically void of observations has made it possible to analyze synoptic features in the tropics and the Southern Hemisphere in great detail. The studies strongly suggest that the winter circulation in the Southern Hemisphere is more intense than previously assumed. The tropical circulation shows several examples of episodes of very active interhemispheric exchange. The large-scale circulation in the tropics is dominated by a giant ascending cell over the western Pacific having a particularly strong component in the equatorial plane. This circulation is especially pronounced during the Northern Hemisphere summer. Prediction experiments show increased skill, particularly in the Southern Hemisphere and the tropics. Comparison with operational forecasts performed at ECMWF after FGGE, as well as with observing system experiments, shows that this is due to the improved data coverage during FGGE.

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Lennart Bengtsson, Phil Arkin, Paul Berrisford, Philippe Bougeault, Chris K. Folland, Chris Gordon, Keith Haines, Kevin I. Hodges, Phil Jones, Per Kallberg, Nick Rayner, Adrian J. Simmons, Detlef Stammer, Peter W. Thorne, Sakari Uppala, and Russell S. Vose
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